摘要:

Chemical Society Reviews Vol 7 No 2 1978 Page CHEMISTRY AND FLAVOUR I Molecular Structure and Organoleptic Quality By H. Boelens, L. M. van der Linde, D. de Rijke, P. J. de Valois, J. M. van Dort, and H. J. Takken 167 I1 Application of Research Findings to the Development of Commercial Flavourings By W. Schlegel 177 I11 Safety Evaluation of Natural and Synthetic Flavourings By K. R. Butterworth 185 IV The Influence of Legislation on Research in Flavour Chemistry By W. H. Nightingale 195 V The Development of Flavour in Potable Spirits By J. S. Swan and S. M. Burtles 201 VJ The Influence of Flavour Chemistry on Consumer Acceptance By R. Swindells 21 2 Collisional Transfer of Rotational Energy and Spectral Lineshapes By Krishnajiand V.Prakash 219 Contributions of Pulse Radiolysis to Chemistry By J. H. Baxendale and M. A. J. Rodgers 235 The Chemistry of Dental Cements By A. D. Wilson 265 Autocatalysis By G. A. M. King 297 Review of Chemical Education Research and Development in the U.K., 1972-1976 By A. H. Johnstone 317 The Chemical Society London Chemical Society Reviews Chemical Society Reviews appears quarterly and comprises approximately 25 articles (ca. 500 pp) per annum. It is intended that each review article shall be of interest to chemists in general, and not merely to those with a specialist interest in the subject under review. The articles range over the whole of chemistry and its interfaces with other disciplines. Although the majority of articles are intended to be specially commissioned, the Society is always prepared to consider offers of articles for publication.In such cases a short synopsis, rather than the completed article, should be sub-mitted to The Managing Editor, Books and Reviews Section, The Chemical Society, Burlington House, Piccadilly, London, W 1V OBN. Members of the Chemical Society may subscribe to Chemical Society Reviews at 26.00 per annum; they should place their orders on their Annual Subscrip- tion renewal forms in the usual way. Non-members may order Chemical Society Reviews for E16.00 ($33) per annum (remittance with order) from : The Publications Sales Officer, The Chemical Society, Distribution Centre, Blackhorse Road, Letchworth, Herts., SG6 lHN, England. 0 Copyright reserved by The Chemical Society 1978 Published by The Chemical Society, Burlington House, London, W1V OBN Printed in England by Eyre & Spottiswoode Ltd, Thanet Press, Margate

ISSN:0306-0012    

DOI:10.1039/CS97807FP005

出版商:RSC    

年代:1978

数据来源: RSC

摘要:

CHEMISTRY AND FLAVOUR* Molecular Structure and Organoleptic Quality By H. Boelens, L. M. van der Linde, D. de Rijke, P. J. de Valois, J. M. van Dort, and H. J. Takken RESEARCH DEPARTMENT, NAARDEN INTERNATIONAL, P.O. BOX 2, NAARDEN-BUSSUM, THE NETHERLANDS 1 Introduction Flavour is one of the main elements which distinguishes between food and nutrition; however, the total flavour complex usually represents less than 0.1 % by weight of any food. Existing flavourings can be prepared from the following raw materials :1 (a), natural flavourings ;(b), nature-identical substances; (c), artificial flavourings. The approximate numbers of flavouring raw materials mentioned in the literature are shown in Table 1. Table 1 Flavouring raw materials Number Ref.Natural 600 2 Nature-ident ical 4200 3 Artificial 250-350 495 The estimated number of nature-identical substances in use (see Table 2) is still increasing, whereas the number of artificial substances is decreasing. The organoleptic quality of flavouring components of a particular fruit or foodstuff can be divided into :(a)more or less characteristic; (b)not characteristic, but still essential ;(c) neither characteristic, nor essential. This article is concerned with some substances whose organoleptic quality is more or less characteristic of a special kkd of fruit or foodstuff; the scope and limitations of structure-activity relationships in flavour chemistry are discussed briefly and some physico-chemical parameters of flavouring substances which may govern the human organoleptic response are also mentioned.*These papers were originally presented at a ‘Chemistry and Flavour’ symposium, organised by the Chemical Society Food Chemistry Group, and held at the Scientific Societies Lecture Theatre, Savile Row, London W1, on 19th October 1977. J. P. Ostendorf, ‘Flavours’, Food Chemistry Group, London, 1976. * Food Additives and Contaminants Committee, Report on the Review ‘Flavourings in Food’, 1976. F. Rijkens and H. Boelens, Proc. Int. Symp. Aroma Research, Zeist, 1975. FEMA-GRAS lists no. 3-10, Food Techn., 1965,253, 151. F. Grundschober, ‘Flavours’, Food Chemistry Group, London, July-August, 1975. Chemistry and Flavour. Part I Table 2 Volatile nature-identical and artifcialfravouring substances (June 1977) Chemical group Nature- iden t ica I Artificial (FEMA-GRAS lists)4 Hydrocarbons 300 1 Alcohols and Phenols 300 11 Ethers 100 7 Acet als 110 14 Carbonyls (functionalized) 460 43 Carboxylic acids 370 4 Esters 600 151 Lactones 140 5 Furans and Pyrans 160 23 Functionalized isoprenoids 650 34 Sulphur compounds 350 44 Aliphatic N-compounds 220 12 Pyrazines and Imidazoles 200 2 Pyridines and Pyrrols 150 2 Oxazoles and Thiazoles 90 4 Total 4200 357 2 Structures with Fruity Flavour Hundreds of compounds have been isolated from fruits and subsequently identified.The number of publications, the number of identified compounds, and three examples are given in Table 3.Table 3 Fruit Publ. Substances Examples Ref. Apple 83 340 2-Methyl butanal (1) 6 Pear 33 155 Ethyl dodecadienoate (2) 7 Raspberry 30 150 p-Hydroxybenzylacetone(3) 8 It is.difficult to believe that there exists a relationship between the structures of a wide variety of fruit qualities. Study of the organoleptic quality of apples in more detail shows that there are at least three different types of apples,699 with regard to flavour quality, viz. alcohol, aldehyde, and ester type. In Table 4 a substance.characteristicof the main flavour types of apples is shown. The structural aspects of substances with the different apple qualities are not very alike. S.Oishi, Kaseigaku Zasshi, 1976,27, no. 8, 566. W. G. Jennings and R. Tressl, Chem. Mikrobiol.Techn. Lebensm., 1974,3, 52 M. Winter, Helv. Chim. Acta, 1961,44,2110. F. Drawert, Phytochemistry, 1968,7,881. H. Boelens et al. Dodeca-cis-4, trans-Zdienoates (2) 0 Table 4 Alcohol type Aldehyde type Ester type Hex-cis-3-enol(4) 2-Methyl butanal (5) Ethyl non-cis-3-enoate1° (6) The following structures have a raspberry flavour : p-OH-benzylacetone,(7) ; p-OH-benzylacetate, (8); ar-damascone, (9); p-damascone, (10). With some imagination one can recognize a resemblance in these structures; however, there 0 0 I0 0 (9) (10) lo R. Tress], Dissertation, Techn. Hochschule Karlsruhe, 1967. Chemistry and Flavour. Part I are other different structures giving rise to.the perception of similar organoleptic quality.3 Structures with Citrus-like Flavours Many aliphatic and isoprenoid substances have been isolated from the most common citrus fruitsll. Up until now 108 publications have appeared in which 450 substances have been described. A few structures with more or less charac- teristic qualities are shown in Table 5. Table 5 Orange n-Decanal(l1) Lemon Citral(l3) Grape fruit (+) Nootkatone (15) p-Sinensall2 (1 2) 3-Methyl oct-2-enal(l4) (1 5) The substances with an orange quality have quite different structures. 3-Methyl oct-2-enal has been identified in lemon oil and has a typical lemon odour.13 Its structure shows some resemblance to that of the more generally known citral. (+)-Nootkatone has a fresh, green, sour, fruity character, strongly resembling that of grapefruit, with a threshold value of 0.8 p.p.m.14 However its optical l1 E.Kovats, Swiss P., 15 667/1967. la K. L. Stevens, J. Org. Chem., 1965,30, 1690. l5 H. Boelens, in ‘Proceedings of the International Symposium Food Science and Technology’ Madrid, 1974, vol. 1, p. 79. l4 H. G. Haring, J. Agric. Food Chem., 1972,20, no. 5, 1018. H.Boelens et al. antipode, (-)-nootkatone, has no fruity character at all and its threshold value is about 600 p.p.m. These types of stereo-isomers have the same molecular volume, and differ only in optical activity, i.e. chirality. One must conclude from this that there exist almost identical structures (optical antipodes) with completely different organoleptic quality. 4 Structures with Spicy-Aromatic Flavours The structures of substances with bitter almond flavour have been studied in detail.15 Less is known about the structural aspects of compounds with cumin characteristics. Some structural examples of both classes are depicted in Table 6.Table 6 Bitter almond Cumin Benzaldehyde (1 6) p-Isopropyl benzaldehyde (1 9) 5-Methyl furfural(l7) Perilla aldehyde (20) 2-Methyl pent-Zenal(181 2,4-Dimethyl hepta-2,4-diena1(21) a0 In these cases one may conclude that even rather different compounds can have some structural features in common, which define the bitter almond or cumin quality. These features are: an aldehyde function; at least one conjugated double bond; an aromatic nucleus or isosteric group; bulky a-group in cumin.5 Structures with Meat-like Flavour In about 160 publications, 550 compounds have been described which have been isolated from different kinds of meat. When eating one can easily distinguish between beef, mutton and pork; however, it is quite difficult to recognize this specific difference in the organoleptic quality of one single compound. It may be that 4-methyl octanoic acid, which should represent the typical flavour of mutton, is one exception.l6 l5 H. Boelens and J. Heydel, Chem.-Zfg.,1973, 97, 1. E. Wong, New Zealand J. Agric. Res., 1975, 18, 261. Chemistry and Flavour. Part I Many substances, especially sulphur compounds, play an important role in the overall flavour of prepared meat. Sulphur compounds are formed in meat, during preparation, by : (a),degredation of cysteine17 [e.g.3,5-dimethyl-l,2,4-trithiolane (22), 2,4,6-trimethyl-1,3,5-trithiane (23), 2,4,6-trimethyl-5,6-dihydro-l,3,5dithiazine (24) (thialdine)]; (b), degredation of vitamin Blls (e.g thiazole derivatives, thiophen derivatives, furan derivatives); (c), reactions of carbohydrates with S-precursorslg [e.g. hydroxyketofuran derivatives, hydroxy- ketothiophen derivatives Scheme (l)]. The compounds derived from cysteine HO GLUCOSE -Furan derivatives Thiophen derivatives Scheme and vitamin B1 may be essential for the overall meat flavour, however they are not characteristic for a meaty aroma. The substances derived from carbohydrates and sulphur are, rather, characteristic for a meat-like organoleptic quality.6 Structures with Fried Flavour Three structures20~21~22 [2-acetyl tetrahydropyridine (25), 2-acetyl thiazolidine (26), furfuryl methyldisulphide (27)] are shown which have a typically fried bread (crust) flavour. The first two show some structural resemblance but the l7 H. Boelens, J. Agric. Food Chem., 1974, 22, no. 6, 1071. l8 B. K. Dwivedi, Diss. Abs. (B), 1974, 33, no. 10, 4851. l9 E. H. M. Gruell, Chem. Weekblad, 1974, 17. 2o I. R. Hunter, Cerial Science, 1966, 11, 493. 21 C. H. T. Tonsbeek, J. Agric. Food Chem., 1971, 19, 1014. 22 E. J. Mulders, Z. Lebrasm.-Untersuch., 1973, 151, 310. H. Boelens et al. last one has a different structure. It is not understood why such different struc- tures have similar organoleptic qualities.7 Structures with Cocoa Flavour The flavour complexes of roasted foodstuffs like peanut (23 publications, 340 compounds), coffee (55 publications, 650 compounds), and cocoa (27 publica- tions, 380 compounds), have been studied extensively.23 The structural aspects of compounds with a cocoa quality have been published.15 A few structures [(28)-(30)] with a cocoa flavour have been ~elected.2~ 0i-These structures: 2-phenyl-4-methyl-pent-2-enal(28), 2-phenyl-5-methyl hex-2-enal(29), 3-methyl butyl cinnamate (30), are quite similar. One recognizes (Figure 1) : an aromatic nucleus (substituted)-feature A, a branched aliphatic chain (C4-C6)-feature B, a functional group (--OH, -0-, -C=O, C=C-C=O, -COO-, -N=C)-feature C.Figure 1 General structure for cocoa quality. A = aromatic nucleus, B = branched aliphatic chain, C = functional group. The authors studied 75 compounds with a more or less cocoa flavour and found the following structural features: A, B, and C present, 65%; A and B present, 80%; A or B present, 90%; C present, 90%. 23 E. Landschreiber and W. Mohr, in '1st International Congress on Cocoa and Chocolate Research', 1974, p.-124. 24 M. van Praag, J. Agric. Food Chem., 1968, 16, no. 6, 1005. 173 Chemistry and Flavour. Part I 8 Structures with Alliceous Flavour For the organoleptic quality of the allium species, such as 0nion,~5 garlic, and leek,26 aliphatic di- and tri-sulphides are very important. About 100 publications concerning this flavour-type have appeared, describing the isolation and identi- fication of 280 compounds.Characteristics for the flavour complexes of onion and leek are propenyl aikyl di- and tri-sulphides (31), while for the quality of garlic the ally1 derivatives play an important role. The authors studied the possible formation of sulphur compounds in fresh, boiled, and fried onion.25 The most characteristic compound for the fresh onion flavour was found to be propylpropane thiosulphonate, and for boiled onion propylpropenyl di- and tri-sulphides. When 3,4-dimethyl thiophen was prepared by heating dipropenyl disulphide (Scheme 2) the endproduct had a distinct Aavour of fried onion, probably due to minor impurities. -SHs\R Disulfides Thiophen derivatives Scheme 2 The flavour character of dimethyl thiophens has been described by the authors as fried onion-like; however, this has proved to be incorrect.27 Possibly formed as intermediates, 3,4-dimethyl thiophenyl alk(en)yl (di)sulphides28 may be responsible for the fried onion character.9 Structure-Activity Relationships The search for correlations between structures and organoleptic properties of flavour compounds is rather complicated. The aforementioned examples show that one has to allow for: the existence of quite different structures with the same characteristics; the occurrence of structurally almost identical compounds with quite different qualities; the existence of groups of rather similar structures with the same organoleptic quality.It should be noted that the determination of the organoleptic qualities of 26 H. Boelens, J. Agric. Food Chem., 1971, 19, no. 5,984. a6 L.Schreyen, J.Agric. Food Chem., 1976,24, no.2,336. W. G. Galetto and P. G. Hoffman, J. Agric. Food Chem., 1976, 24, no. 4, 852. H. Boelens and L. Brandsma, Rec. Truv. chim., 1972,91, 141. H. Boelens et al. compounds is a more or less subjective matter, Several groups29J0 have tried to ‘objectify’ their results, which makes their findings more reliable. 10 Some Aspects of Chemoreception Chemoreception covers the area of the effects of stimulation of the senses of smell and of taste by chemical substances. A chemical substance (stimulus) may interact with a biological system (receptor), which can result in a response.The way interactions occur, or the mechanism of perception, is still unknown. One can study the physico-chemical parameters of the stimulus and the resulting verbal response of human beings. The authors divide the parameters into : concentration (volatility, partition coefficients); electronicity (polarity, dipole moments) ; stereocity (molecular size, shape, and chirality); flexibility (rotation and vibration). The verbal response has as its main aspects: detection (yes/no); intensity (weak/strong); recognition (quality) ;preference (like/dislike). 11 Calculation of Bitter Almond Quality The authors made an attempt to objectify the relationship between physico- chemical parameters of a molecule and its organoleptic quality by using the following formula :31 Odour quality (one facet) = f(1og P)+ f(E) + f(S)+ C log P = log (Coctan-l-ol/Cwater) (partition coefficient) E = dI/lO00 (I = Kovats index) S = mol volume/100 + width/height A total of 16 compounds were studied, with benzaldehyde as reference material, and by using multiple regression analysis a correlation coefficient of 0.95 and a standard deviation of 0.65 (on a scale of 1-9) were determined.Because these physico-chemical parameters are to a certain extent interrelated and, moreover, other parameters may have their influence it cannot be expected that the given formula gives a complete and absolutely correct description of the correlation between structure and odour quality.Perhaps a quantum mechanical description of the molecule can lead to a more refined relation with odour quality. The first attempts in this direction have been published recently.32133 12 Experimental Assessments Finally some qualitative statements which seem to be evident for odoriferous D. G. Land, in ‘Proceedings of International Symposium Aroma Research’, ed. H. iMaarse and P. F. Groenen, Zeist, 1975, p. 131. 30 E. von Sydow, Inst. Food Sci.Technol.Proc., 1974,7, 190. 31 H. Boelens, in ‘Proceedings ECRO Symposium’, ed. G. Benz, Wadenswil, 1975, Inform- ation Retrieval Ltd, London, 1976, p. 197. 32 A. Eriksson, P. Lindner, and 0. Martensson, private ed. 1977, Department Organic Chemistry, Box 531, Uppsala. Sweden. 33 J. R.McGill and B. R. Kowalski, Analyt. Chem., 1977, 49, no. 4, 596. Chemistry and Flavour. Part I substances are listed: the compound must be volatile; the product must have a certain hydrophilicity and Iipophilicity; the organoleptic quality can change by variation of the concentration; one chemical compound has several different odour qualities ; the number of characteristics increase by increasing the flexibility of the compound; the type, number, and place of the Junctional group(s) influences the organoleptic quality; the size and shape (chirality) of the compound do influence the organoleptic quality; the physico-chemical para- meters of a compound, which govern the organoleptic response, are interrelated.

ISSN:0306-0012    

DOI:10.1039/CS9780700167

出版商:RSC    

年代:1978

数据来源: RSC

摘要:

I1 Application of Research Findings to the Development of Commercial Flavourings By W. Schlegel GIVAUDAN DUBENDORF A.G., UBERLANDSTRASSE 138, ~~-8600DUBENDORF, SWITZERLAND 1 Introduction It is estimated that each year between fifty and one hundred million US dollars are invested in flavour research and the combined efforts in industrial and institutional laboratories must certainly be ranked among the major research efforts in the food field. Since flavour is inseparably linked with food, it is only logical that the research efforts in both fields have very much in common. Some topics of modern flavour research could just as well be filed under food research. Flavour research involves the chemist, the physicist, the biochemist, the microbiologist, the food technologist, the toxicologist, the nutritionalist, and the flavourist, and in order to apply research findings to the development of commercial flavourings, quite often physiological, psychological, and even sociological and ethnological aspects have to be considered.The development of food flavours has many facets. Three problematic areas have been chosen which occupy much of our time. These may help to explain that the development of new and better food flavours is not only a function of analytical capacity. 2 Research and Food Safety In the early days of flavour research new or known chemicals which exhibited more or less pleasant flavour properties were incorporated into commercial flavours without any of today's considerations. Fortunately, the knowledge and the procedures in those early days were rather limited, so that only simple chemicals, mainly esters, were used commercially. Today the situation is different.The large number of known flavour substances already permits an indefinite number of combinations, leading to all sorts of nuances of flavour, yet we still need more insight into the natural pathway of flavour generation, and we need more flavour chemicals in order to develop those flavours which cannot yet be made in reasonable qualities, e.g. meat, vegetables, bread, and roasted products. The analytical part of flavour research today is exclusively geared to detect those food components which are responsible for or which contribute to the specific flavour of a food.Flavour research must however be limited by our concern zbout the safety of our products, Food safety cannot be achieved without flavour safety! The analytical tools which are at our disposal will certainly discover hundreds of new constituents of natural flavours, but our knowledge of possible hazards Chemistry and Flavour.Part I1 requires that none of these substances should enter our flavours if there is even the slightest doubt about their safety. There are only two countries which permit expressis verbis in their food law the utilization of those flavouring substances which are known under the term ‘nature-identical’. Only one country has published a list of permitted synthetic flavouring substances. In all other countries practically no reference can be found as to what is and what is not permitted.As much as the flavour industry welcomes the liberty of using research results without public disclosure, it also realizes the burden and the responsibility of its trade. Responsibility in this context does not mean bearing the burden and the consequences afterwards, but endeavouring to the hest of its knowledge and conscience that no adverse effect will be en- countered. It is obvious that it neither wants to take risks nor can it afford to do so. But only very few official publications or statements indicate how new flavouring substances can be tested and how undue risks can be excluded. The flavour industry finds itself in the situation of having to decide on its own how to handle this problem.The decision as to whether a newly isolated, unique flavouring substance, the result of many hours work of a flavour chemist, should be used by the flavourist, who again invests much time to evaluate the potential of this material before he starts using it in his flavour creations, requires the utmost care and attention. Those who have to take these decisions can receive scientific help from toxicologists and pharmacologists. They can base their considerations on two systems which have been published and which so far have not been proven to be erroneous, the FEMA approach,34 and a paper published by the Food Protection Committee of the National Research Council.35 The following criteria offer a reasonable system for the evaluation of the relative safety of flavouring substances: (a),chemical structure; (b),purity; (c), functional properties; (d), dosage in food; (e),total consumption anticipated; (f), structural relationship with chemicals of known toxicity and/or known metabolism; (g),toxicity data; (h),occurrence in traditional food; (i), toxicological significance.A. Chemical Structure.-The FPC publication differentiates between ‘simple’ chemical structures and complex molecules. In this context ‘simple’ means ‘straight chain or branched chain aliphatic alcohols, acids, and esters, mono- nuclear aromatic compounds containing only carbon, hydrogen, and oxygen and equipped with one or more functional groups that include hydroxyl, aldehyde, and keto’.These substances are recognized as being practically non-toxic. Pyrazines, thiazoles, pyridins, and other more complex chemicals are not regarded as ‘simple’ and they should be evaluated more carefully and with more reservations. 34 R. L. Hall and B. L. Oser, Food Technol., 1965,19, 15 I. 35 Guidelines for estimating toxicologically insignificant levels of chemicals in food. Food Protection Committee, National Research Council, National Academy of Sciences, Washington DC, 1969. 178 Schlegel B. Purity.-This criterion is of little importance in so far as flavouring substances represent only p.p.m. in food and any impurities in the order of a few percent would be present in food in extremely tiny quantities far below any significant concentration.Unless the impurities consist of heavy metals, compounds of heavy metals, or other substances known to be highly toxic, impurities can be neglected for practical purposes, though their chemical character must be known. C. Functional Properties.-This reference is again found in the FPC report and it has obviously been taken over from another source36 where a compilation of all published toxicity tests has shown that only heavy metal compounds and chemicals manufactured for the purpose of the destruction of biological life, exhibit toxic properties in tests with experimental animals at dietary levels below 40 p.p.m. It is very unlikely that any of the known toxic substances which have been developed and which are used by virtue of their toxicity, will ever be incorporated into food flavours.D. Dosage in Food.-It is interesting to observe that practically all of the newly discovered constituents of natural food flavours are active at extremely low concentrations (flavouring substances occurring naturally in larger quantities were found long ago and are publically known in literature). E. Total Consumption Anticipated.-This figure becomes important when approximate per capita consumptions need to be calculated. Whenever this figure surpasses a certain amount, responsible flavour companies will have to reconsider their original decision: they will have to support it by further studies. F. Structural Relationships with Chemicals of Known Toxicity and/or Known Metabolism.-To incorporate these considerations into the evaluation of a new chemical certainly requires experience, and pharmacologists and toxicologists should be consulted.In spite of insufficient knowledge about the relationship between structure and pharmacological properties, quite a number of critical structures are known, which permit the prediction of the possibility of hazards. G. Toxicity Data.-Whenever doubt is cast on the safety of a new substance toxicity studies become necessary. The Ames Test37 may be helpful in verifying the absence of carcinogenic properties. Also, 90 day feeding studies and metabolic investigations are carried out. More stringent studies are economically justified only when the chemical under question is to be used in very large quantities.This, however, applies only to very important and widely used substances. Whenever the economics do not permit such studies, the chemical has to be dropped. 36 J. P. Frawley, Food Cosmet. Toxicol., 1967, 5, 293. 37 J. McCann, E. Choi, E. Yamasaki, and B. N. Ames, Proc. Nar. Acad. Sci U.S.A.,1975, 72, 5 135. Chemistry and Flavour. Part II H. Occurrence in Traditional Food.-This criterion is a very important one for the flavour industry. The well-known argument that whatever is contained in traditional food, the safety of which is beyond any doubt, must be regarded as safe as long as the daily intake of the synthetic material lies in the same order of magnitude as that of the natural constituent, is still the main basis for the safety evaluation of flavouring substances.I. Toxicological Insignificance.-The FPC report covers this aspect widely. A careful extrapolation of its conclusions-the report has not been compiled for flavouring substances-leads to levels of toxicological insignificance of the order of between 1 and 10 p.p.m. in food, depending on the evaluations of the various criteria. To apply the listed criteria in a responsible manner for the evaluation of newly discovered flavour constituents of traditional food is today the most important basis for the decision whether and how research results can be applied in com- mercial flavours. Self-discipline and a highly ethical conscience are necessary. The flavour industry wants to defend its present relative freedom in some countries and it wants to be regarded as honest, safety conscious, and a reliable partner of both the consumer and the food industry.3 Food Science The objectives of modern flavour research have become more complex. While the discovery of new substances in food was the primary objective some years ago, flavour research today contributes to make the work of the flavourist more scientific by supplying him with data about the physico-chemical fate of flavouring substances in food. Modern food technology, the various processes, the very large number of food items, and certainly also the increasing quality consciousness of the consumer require more and more that flavours be tailor made. If the parameters of food technology, heat stress, time, and pH are disregarded, flavours may perform unsatisfactorily.Mostly, it is said that flavour is unstable. If we were able to stabilize our flavours, or if we could predict in which application instability can be expected, we would largely facilitate the development of flavours for specific food and specific applications. Looking closer at the phenomenon of instability, or better incompatibility, we can distinguish between the following possibilities : physical instability; chemical instability (i),innate; (ii),in food; interactions of flavour components with food ingredients. Physical instability is in most cases due to the volatility of flavour components at elevated temperaiures. The poor flavour retention during the baking procedure is a well known difficulty.While it is technologically possible to protect volatiles in such a manner that evaporation losses become negligible, these protective measures normally do not yield products which release the flavour at the time and at the conditions of consumption. A compromise has to be found between the reduction of evaporation losses and ready release properties under con- sumption conditions. Schlegel Research becomes necessary to overcome chemical instability problems. Oxidation of sensitive flavour components is well known to every flavourist. Polymerizations, hydrolyses, and condensations occur as well, but they are only of minor importance. We also know some cases where the natural flavour components themselves are unstable, e.g.those responsible for the flavour of freshly baked bread. A fairly new type of ‘instability’ is caused by interactions of flavour components with food ingredients. It is not simply a case, for example, that a rancid, peroxide-containing fat, which might accidentally find its way into food, rapidly destroys most of the commonly used flavour components, Interactions occur between flavouring substances and such common food constituents as carbo- hydrates, proteins, and fats. The first investigations in this field were concerned with the poor flavour retention in extrusion procedures, especially during the extrusion of soy protein.38 It became evident that soy protein and the extrusion operation were a very complex field for exercises, and valid conclusions could not really be drawn. It could only be established that reversible and irreversible bindings of flavour components to some part of the soy protein occurred.Consequently the behaviour of easily detectable single flavouring substances in model food systems has been studied.39 The concentrations of the flavouring substance under question were determined by different analytical procedures, extraction, steam distillation, high vacuum transfer, head space techniques and by sensory analysis with the help of a trained panel, Table 7 demonstrates one typical result of these studies. Table 7 System Citral content measured by extraction in % of added amount Added amount (p.p.m.): 100 lo00 10 Ooo 5 % Maltodextrin in.HzO 78 - 85 10% Lactose in H2O 81 82 77 10% Casein in H2O 76 65 48 10% Soy isolate in HzO 42 22 7 The significance of such results to the practical work of the flavourist is obvious. We can deduce from this table for instance that carbohydrates can, and soy protein cannot, easily be flavoured with citral.We can also deduce that the increase of the citral concentration in a vegetable protein will markedly affect its sensory level. The stability of flavour chemicals in food depends of course also on time, temperature, and for example, the pH of food. These parameters make the 38 H. A. Gremli,J. Amer. Oil Chem. SOC.,1974,51,95A. 30 B. A. Gubler, H. Gremli, J. Wild, and C.Verde, Proceedings International Union of Food Science and Technology, Madrid, 1974; B. A. Gubler and H. Friedrich, Lecture at the 4th Meeting of the Austrian Society for Nutrition Research, 1977. Chemistry and Flavour. Part I1 investigations quite voluminous, for instance the results of the citral stability as a function of time (Figure 2). 100 1 I I I 1 I I I 1 I 1 ) 100 Days of storage 5 %Maltodextrine in water ---------10% Lactose in water ------10%Butterfat in water -----5 ”/,Soya isolate in water -------5 %Casein in water ---------Water Figure 2 Citral stability in various food models. Storage temperature: 4 “C Quite severe losses of flavour activity can be observed in all tested systems. The’drastic and immediate drop of the citral concentration in the casein sus- pension is the most pronounced.The stability of other important flavouring substances in different systems as well as the influence of various stabilizing materials, food additives or substances naturally present in food have also been studied. The results are still not easy to interpret; however, only the surface of this field has been touched and we Schlegel are confident that ultimately these studies will lead to a better understanding of the complexity of food and also to better flavours by enabling our flavourists to use our raw materials in a more effective and a more economical way. 4 Flavour Creation The development of a new flavour requires teamwork between the researchers, the flavourists, the technologists, and a panel representing the target consumer.The subjectivity of flavour perception requires that the communication between the participants is shaped in such a manner as to overcome this handicap. The flavourist will for instance smell the effluents of the gas-chromatograph in order to pinpoint those notes, that is to say substances, which he requests. Whenever a new substance has been isolated from a natural source, the flavourist has to use all his skill and his experience in order to find out how this material fits into his flavour. Missing the right concentration, or putting the new chemical into a test mixture of flavour chemicals of unsuitable composition might lead to the rejection of a potentially important new raw material.There are only a few flavour chemicals-known as impact chemicals-which can easily and immediately be associated with specific food. The majority of flavouring substances fit into a number of food flavours. To remember at the right time the right association, requires an extensive, abstract, and creative imagination. Semi-mathematical models have been developed to facilitate this task, but a substitute for experience and talent has not yet been found. The communication between the flavourist and the chemist is therefore not only a matter of a common vocabulary, it is a matter of mutual understanding. The creation of a new flavour is of course largely facilitated by available analytical information and literature studies, and seen through the optic of a pure chemist it should be easy to reconstitute a flavour using all this background knowledge. Yet, experience has shown that the task is really much more com- plicated in as far as the reconstituted mixture of analytically detectable substances of a food usually fails to perform as it should.A very critical interpretation of the analytical results is necessary to know which of the single substances are essential and how they have to be dosed in order to achieve the desired result. It is not only the well known threshold value which counts, it is the much more complicated system of flavour strength as a function of concentration and solvent. This relationship is different for each of the known-and also for the so far unknown-flavouring substances.A tremendous memory, experience, and patience are necessary to formulate a flavour to full satisfaction. These talents of the flavourist are quite often referred to as ‘artistic’. But the application of analytical results is only one part of a flavour creation. Market research results as well as legislative and economical aspects have to be taken into account. The following flow sheet of flavour creation lists the most important conditions which have to be considered (Figure 3). As much as the flavourist and the chemist have to guide each other carefully, just as important is the connection between the flavourist and the food techno- logist who has to learn how to use a flavour in food for best results.Today, our Chemistry and Flavour. Part II MARKET RESEARCH Flavour type Target application Flavouring cost etc. 4 RESEARCH RESULTS VARIOUS FACTORS Analytical data Processing stress Literature Food type Experience Food laws U EVALUATION Safety clearance Panel evaluation Application studies Stability tests Figure 3 application laboratories are far from being only ‘home made sweet manufac- turers’. More and more in this field we encounter the limits of traditional craftsmen’s knowledge, which calls for real food research. The utilization of sugar substitutes, the development of cocoa free chocolate and other food novelties are examples of this type of work which in turn usually calls for other specific and suitable flavours. At the finish of the development of a new flavour or a new food stand the last communication barriers, the company panel, and finally the large panel, the consumer. And he, still being ‘the King’, decides whether the research has been worthwhile. 184

ISSN:0306-0012    

DOI:10.1039/CS9780700177

出版商:RSC    

年代:1978

数据来源: RSC

摘要:

111 Safety Evaluation of Natural and Synthetic Flavourings By K. R. Butterworth THE BRITISH INDUSTRIAL BIOLOGICAL RESEARCH ASSOCIATION, WOODMANSTERNE ROAD, CARSHALTON, SURREY SM5 4DS It is well known that the number of flavouring substances which need to be evaluated for safety is large, and flavours are only a small, albeit important, part of the group of chemicals which are employed in modern food technology. In recent years several oficial reports have been published which classify these natural and artificial flavouring substances, for example, that of the Council of Europe (1974)4O and last year the Food Additives and Contaminants Committee (FACC) Report (1 976).41 Table 8 Alode of listing in Council of Europe document on natural and artificial flavouring substances (1974) Categor v Description of artificial flavouring substances No.of materials included in class I Those which may be added to foodstuffs (1-2000) without hazard to public health 692 I1 Those which may be added temporarily to (2001-4000) foodstuffs without hazard to public health 293 I11 Those which are not admissible at present (400 1-6000) because technological and toxicological data are absent 243 IV Those which are not admissible since (6001- ) biological data indicate toxicity 3 In the 1974 Council of Europe ‘Blue Book’ the artificial flavouring substances were classified as shown in Table 8. A number was allocated to each material according to the group into which it fell. When the FACC report (1976) came out the 6000 series was omitted, and now it is expected that a new Council of Europe report will appear soon from which the members of both the 4000 and the 6000 series will be omitted.Even the materials in the 1-2000 series are not sacrosanct as the FACC report (1976) shows, since it requires further studies on several of these flavourings (e.g. propenylguaethol). Jo Natural Flavouring Substances, their Sources, and Added Artificial Flavouring Substances. Council of Europe, Strasbourg, 1974. dl Food Additives and Contaminants Committee (1976). Report on the Review of Flavouringsin Food. HMSO, London, 1976. Chemistry and Flavour. Part ZII Table 9 Studies required on temporarily admissible synthetic flavours (2001 -40oO) to investigate the possibility of upgruding members to the group which may be added to foodstufls without hazard to public health (1-2OOO) 13 Long-term studies 151 90-day studies 24 Metabolic studies 92 Hydrolysis studies 66 Acute studies The figures in Table 9 are based on the original ‘Blue Book’ and show the number of tests required to upgrade the second group of 293 temporarily admissible artificial flavourings to the first group, where it is considered that they may be added to foodstuffs without hazard.The actual details in Table 9 are not important, but what is interesting is that an estimate of the cost to undertake these studies is E3.5 million at present day prices, and if the studies were carried out only at one set of laboratories which are equipped to undertake the work, such as BIBRA, they would take something like 35 years to complete.It is stressed that this is only to investigate the possible upgrading of members of the second group of synthetic flavourings to that of the first, which may be added to foodstuffs without hazard to public health. This list does not include the natural or the nature-identical flavours which have been used for many years and therefore are said to be less likely to have serious harmful effects in man. The testing of flavourings is no different from any other food additive or contaminant. First of all it is necessary to ensure that the material to be inves- tigated has an adequate specification. This has to be realistic, i.e.not so impure that the results of the subsequent toxicity studies are meaningless, and not so pure that it would be too expensive to produce this flavouring for industrial use. Tests performed on an unidentifiable material are completely worthless. Detailed analysis of all the constituents present in the material is essential and, in this context, technical specifications are generally inadequate because they are written for the purpose of selling the material. To check the composition and purity of a material, any of the facilities of a well equipped chemistry laboratory may be required. These can range from simple measurements like melting points to elaborate examinations like mass spectroscopy. A good example is Strawberry Aldehyde, one of three flavourings which are in the Council of Europe (1974) ‘Blue Book’ in the 6OOO+ series, and thus not recommended for use in foods.It is likely that this is because this material has been reported to cause paralysis of the hind limbs of rats.42 Samples. of Strawberry Aldehyde were obtained from five different manufacturing houses and it was found that on gas-liquid chromatographic examination between two and seven peaks could be found in these materials (Table 10). The two peaks were believed to be due to the cis and trans isomers of the parent material, 4a F. Griepentrog, Men. Emuhr., 1969, 10, 89. Butterworth ethyl methylphenyl glycidatc, and the other peaks to contaminants accidentally or deliberately added.43 In long-term studies in rats and other species, Mason and co-workers4 have not been able to detect any neurological disorders and it is probable that the toxic effects reported previously were due to a contaminant rather than to ethyl methylphenyl glycidate itself.It is to be hoped that this important strawberry flavour may soon be upgraded. Table 10 Gas-liquid chromatographic finger-prints of 'Strawberry Aldehyde' samples Are" ( x)ofpenk number I 2 3 4 5 6 7 Retention Supplier rimelmin 1.43 4.00 4.42 5.87 6.53 9.00 9.92 A 1.3 1.6 31.5 2.5 33.0 0.9 28.3 B 0.3 0.1 39.8 0.1 57.6 0 2.I C 0.3 0 39.6 0.1 58.0 0 2.o D 0.2 0 33.5 0.1 57.2 0 9.0 E 0.1 0.2 40.5 0.1 58.2 0 0.9 Most people know what an LD50 value is, and therefore it is only necessary to define it and outline its limitations.'An LDx is the dose which will kill 50% of a group of animals in a specified time (cJ.g.24 hours or 7 days)'. Nowadays the limitations of such an LDSOtest are obvious. For example, usually the material is administered on one occasion to each animal and then the animal observed for only a few hours or, at the most, days. Also, the chief concern is to determine how many animals die, rather than finding out the cause of death and attempting to detect any effects less dramatic than death. Occasionally attempts are made to assess the cause of death by simple observations. These include noting whether the respiratory movements cease before the heart stops, or whether the animal convulses and, if so, what is the type of convulsion.However, even today, it is rare to perform a post-mortem examination of animals in such an LD50 test. The importance of this point can be illustrated by an animal that might have died from, for example, a perforated stomach, which would have caused internal haemorrhage. This in its turn might have shown up as the heart stopping before the respiratory movements. Thus it might have been concluded that this was an effect primarily on the cardio-vascular system which, of course, would have.been wrong. Nowadays the LD5o test has a very limited place in the assessment of toxicity and a much more meaningful assessment of safety of flavourings can be made from short- and long-term studies.The 90-day test in the rat is a typical example P. L. Mason, K. R. Butterworth. I.F. Gaunt. P. Grasso. and S. D. Gangolli, Food Cosuret. Toxicol., in the press. P. L. Mason, K. R. Butterworth, I. F. Gaunt, P. Grasso, and S. D. Gaiigolli, report in preparation. Chemistry and Flavour. Part I11 of such a short-term study. In a typical study of this duration four groups, each of 25 male and 25 female weanling rats are employed. One group acts as a control and the others receive low, medium, or high doses of the materials. Figure 4 shows the routine in such a study. At two and six weeks, interim MONTHS OF FEEDING 0 I 2 3 , tIInterim exam inat ions TermFation HaematologySerum chemistry Urine analysis and renal function tests Necropsy, organ weights, and histology t f Clinical observation Body weight Food and water consumption Figure 4 Progress of a short-term .feeding study examinations of the animals are performed. On these animals as many parameters as appropriate are measured or observed. These include body weight, food and water consumptions, appearance and behaviour, haematology, serum chemistry, and urine analysis.At the post-mortem examination, the macroscopical appearance of the tissues and the weights of the organs are recorded. Sub- sequently a histological examination of the tissues is performed. This is an outline of a typical short-term feeding study and obviously the test can be tailored to a particular flavouring material. For example, extra rats may be required in the study so that a special investigation can be performed.An example of this is the measurement of hormone levels (e.g. insulin or prolactin) in the blood. Also, extra tissue may be required for electron microscopic examination. Such a 90-day study enables a good assessment in the rat of the effects of a material over a wide dose range, and for a period of time which is approximately equivalent to ten years in the life span of man. There is a great lack of experimental data on flavouring materials and, naturally, expert committees are grateful for any information. In 1965, Oser and co-workers45 suggested a ‘short-cut’ which might appear to generate thenecessary data rapidly. They proposed that several materials could be examined in parallel against a single control group of animals. One of each experimental group would receive one of the flavourings at a dose level which was 100 times the 46 B.L. Oser, S. Carson, and M. Oser, Food Costnet. Toxicol., 1965, 3, 563. 188 Butter worth maximum, on a weight basis, which would be consumed by man in a day. This study would continue for the usual 90 days. This method is sound as long as no untoward effects are obtained in the control and appropriate experimental groups. However, this is rare in practice. Usually, in a standard 90-day study there are some anomalous results which can be dismissed on examination of the other treatment levels but, of course, this is not possible if the material is adminis- tered at only one level.Other disadvantages of this ‘short cut’ which must be obvious to Oser are: (a), when more than three flavours are examined at the same time it is necessary to increase the number of animals in th;: control group; (b),the estimate of a no-untoward-effect level for each flavour in the investigation is poor or impossible. It could be much less or much more than the treatment level employed; (c), all the flavourings have to be administered in the same way, e,g. in the diet, or drinking water, or by intubation; (d),the same solvent has to be employed throughout otherwise the control group is not strictly comparable; (e), many flavouring materials in trace amounts can be very pleasant, but in large quantities they are often very unpleasant to the staff and probably to the rats.This was particularly obvious when studying furfuryl mercaptan. While in its stock container the odour which leaks out is a delicious aroma of coffee, but it is quite different when the container is opened. Because of this factor the treatment level which the rat will accept is often self-limiting unless the flavouring is administered by stomach tube; (f), finally, such an investigation only gives short-term toxicity information and this type of experiment is completely unacceptable in place of a long-term study when one is looking also for possible carcinogenic effects. The Oser-type test is an attempt at a ‘short cut’. However, far from reducing the standards, these days the tendency is to make the requirements for the assessment of safety more rigorous.This point can be illustrated by the recent FASEB report (1976).46 Last year a 119-page report of the Select Committee on Flavouring Evaluation Criteria of FASEB (The Federation of American Societies for Experimental Biology) was produced at the request of the FDA. Most of this report is concerned with the ways of determining the priorities for testing flavours. However, it also recommends the type of studies which should be performed and outlines the ways in which they should be carried out. Instead of the 90-day study, the FASEB report suggests that such a study should be carried out in rats from parents which have been exposed to the flavouring material from weaning.Also, some of the animals at the end of the exposure period should have the administration of the test material stopped and the animals should be placed on control diet for a further four weeks to see if any observed effects are reversible. Naturally the introduction of a more rigorous test procedure is to be commended, but it considerably increases the time and cost of performing the study. Also, if the animals do not produce young due to a direct or even an indirect effect of the flavouring then it is obviously impossible 46 ‘Criteria for Evaluation of the Health Aspects of using Flavoring Substances as Food Ingredients’. Federation of American Societies for Experimental Biology, FDA no. 223-75-2002,1976, 189 Chemistry and Flavour.Part III to carry out the main 90-day study. There is something to be said for carrying out separate short-term, reproduction and teratological studies, although this means that the flavouring would not be administered to two generations of animals. In this country the main, generally accepted, method of ensuring that a flavouring material has no long-term toxic or carcinogenic potential is to administer the material at three treatment levels to groups of at least 50 rats of each sex for not less than two years. Such a study resembles the standard short- term study which has been outlined above. However, these days the tendency is to extend the two year period of treatment to a lifetime. In the FASEB report (1976)46 ‘lifetime’ is defined as that time when only 20%of the starting group are still alive.Exposure to the compound is required not just from weaning but from the time of conception. Also, in order to establish negative findings as valid, the report requires that more than half the starting rats should have survived at least 18 months. When indicated, specific tests also may be required on certain compounds for unwanted actions such as the production of cataracts, or the measurement of oxygen consumption as an indication of the basal metabolic rate when a material is suspected of acting on the thyroid gland. In the case of ethyl methylphenyl glycidate, which has been mentioned above as having been reported to cause hind limb paralysis, test animals were placed on a treadmill at intervals throughout the study in an attempt to detect early signs of nerve damage.4 No such neurological lesions were found in this study.Animal tests should usually be performed on more than one species and it is desirable that one of these species should be non-rodent. Many flavours are esters and therefore hydrolysis and metabolic studies are particularly relevant. As far as hydrolysis studies are concerned many esters are capable of being broken down rapidly by enzymes, for example in the intestinal wall, or in gastric juice, or in the liver, to yield the parent acid and alcohol. Butterworth and co-~orkers~~ have considered that by a process of extrapolation it should be possible to reduce significantly the number of studies which need to be carried out on a series of esters, providing that these esters are rapidly hydrolysed in vivo to their parent acid and alcohol and also that there are adequate toxicological data on the acid and alcohol.This principle, if feasible, would save a great deal of time and money in the safety testing of flavours. Butterworth and co-workers47 have been studying a series of allyl esters in order to put this theory to the test. The ally1 esters were selected because they are known to cause a very specific kind of damage to the liver, namely periportal necrosis. Detailed studies on allyl alcohol48 and allyl hexanoate49 have shown that when these materials are given to groups of rats for 90 days, similar toxic effects are produced by equimolar doses.Thus a comparative study of allyl alcohol and six of its esters was designed. 47 K. R. Butterworth, F. M. B. Carpanini, 1. F. Gaunt, P. Grasso, and A. G. Lloyd, Brit. J. Pharmacol., 1975,54, 268P. F. M. B. Carpanini, I. F. Gaunt, J. Hardy, S. D. Gangolli, and K. R. Butterworth, Toxi-cology, 1978, 9, in the press. 49 S. A. Clode, K. R. Butterworth, I. F. Gaunt, P. Grasso, and S. D. Gangolli, Food Cosmet. Toxicol., 1978,16, in the press. 190 Butter worth Table 11 Summary of periportal liver lesions produced by allyl alcohol and its esters Compoiind Dose Cell Cell Fibrosis and (mg/kg/day) enlargc.ment necrosis bile duct hyperplasia Allyl alcohol 5 * 1 1 25 * 1 8 60 * 6 5 Allyl acetate 8 * 2 0 43 * 3 0 103 * 7 5 Allyl propionate 9 0 1 0 49 0 I 0 117 4 5 7 Allyl hexanoate 13 * 1 0 67 * 4 3 161 * 4 7 Allyl isobutyrate 11 0 0 0 55 0 0 0 132 2 0 0 Allyl isovalerate 12 0 0 0 61 0 1 0 149 8 5 1 Allyl 2-ethylhexoate 16 0 0 0 80 0 0 0 192 7 3 0 *Effect masked by other damage Table 11 lists the esters employed in the comparative study.The straight-chain esters are the acetate, propionate, and hexanoate, and the branched-chain ones are isobutyrate, isovalerate, and 2-ethylhexoate. They were administered to rats by oral intubation at three dose levels on an equimolar basis. The top dose was selected as the maximum tolerated dose of allyl alcohol while the lowest dose was chosen as one which was expected to produce no macroscopic effects.After 21 days treatment the animals were autopsied and samples of liver were taken for histopathological investigation. Several effects were observed, such as a reduced rate of gain in body weight, but the results of the microscopic examina- tion of the liver were much more dramatic, The liver lesions fall into three main categories. It is believed that the lesion develops through three stages. The first signs of damage are cells of increased size. It is thought that these then die, forming areas of cell necrosis in the portal region. Then there is an attempt at repair on the part of the liver, which is indi- cated by fibrosis and bile duct proliferation. This latter stage is indicative of the most extreme damage. The figures indicate the number of animals out of ten Chemistry and Flavour.Part III which showed the lesions. Clearly, the results are similar for allyl alcohol and the straight-chain esters. Also there is a dose-response relationship and in all cases there is a high incidence of the most extreme lesions. In fact, in most cases the damage was so great as to obscure any signs of early damage. The difference in the results from the branched-chain esters and those from the straight-chain ones is immediately obvious. There was virtually no effect seen at the two lower levels of treatment and most of the effects seen at the highest level were confined to the early signs of damage. The results of these in vivo experiments are in agreement with it1 vitro hydrolysis studies which indicate that branched-chain esters are hydrolysed about 100 times more slowly to allyl alcohol than are the straight-chain ones.It would appear that the effects observed are due to allyl alcohol which has been liberated by hydrolysis and that the allyl esters should be assessed as a group rather than as single compounds. In this experiment the question being asked was: ‘Is it possible to reduce the quantity of work needed to assess the toxicity of a series of esters if certain biochemical information is available?’ and it would appear to be so, providing that certain requirements are met, namely that: (a),the esters are hydrolysed in the alimentary tract and the rate and degree of this hydrolysis are known; (h), there are well established toxicological data on the parent alcohol and acids; (c), certain limited animal studies are performed, so that it is possible to correlate in vitro hydrolysis studies with those conducted it? vivo.It is not suggested that this is the way in which all esters should be investigated. These were only preliminary experiments with some allyl esters which were in- tended to investigate the feasibility of such an approach to the problem of the vast number of flavouring esters which have to be assessed for safety. Other ester series would have to be investigated before a true evaluation of this approach could be made. Retlirning to the subject of hydrolysis and metabolic studies, it is important to realize that a metabolite of a substance rather than the parent compound may produce a toxic effect.Most metabolic processes in the body detoxify foreign substances, but this is not always true (e.g. acrolein produced by the metabolism of allyl alcohol). Usually what is important is to determine whether the com- pound is metabolized by a similar route in the experimental animal being employed in the toxicity study to that in man. Briefly, metabolic studies are carried out in order not only to determine the metabolites produced, but also to discover the route followed through the body, any possible storage, and how the material is excreted. The metabolites may be determined using any of the multifarious methods employed in modern chemistry, e.g. chromatography and i.r. or U.V.spectrophotometry. Alternatively, labelled materials may be employed both to determine the metabolites and to reveal the route followed through the animal. These days whole body radioautography is becoming a commonly employed method of following the passage of a flavouring material through the animal. It is worthwhile considering the modern trend of trying to replace experimental Butterworth animals by cells. The cells may be isolated animal cells such as human fibroblasts, or HeLa cells, or they may be strains of bacteria. Alterations in genetic material in cells are of increasing interest these days because of their relationship to teratogenicity and possibly to carcinogenicity. In this sense the in vitro tests which follow are possible ‘short cuts’ for animal carcinogenicity studies which are both time-consuming and expensive.The use of cells may be illustrated by the Ames testso, a DNA repair test,sl and a cell transformation test.52 The Ames test is a bacterial mutation test which is intended to establish the mutagenicity of the test material (e.g. a flavouring substance) in mutant strains of Salmonella tjphimurium. Histidine-dependent bacterial strains are suspended in histidine-free agar to which is added the test or control substance. After two days incubation the colonies are counted. With this system only back-mutated bacteria are able to grow in the absence of histidine so that the number of colonies gives an indication of the mutation rate and an answer is obtained in a matter of days.The second method,51 DNA repair, employs either an autoradiographic technique or may involve evaluation by scintillation ~0unting.j~ The relevance of this technique depends on the observation that carcinogens produce damage of the DNA. Repair of this damage can be demonstrated by the uptake of 3H-thymidine by human fibroblasts but only after suppression of scheduled DNA synthesis by growing the cells in an arginine-deficient medium or by blocking the cells chemically by hydroxyurea. Again, using a DNA repair method it is possible to obtain an answer in a matter of a few weeks. The last method is a cell transformation test. In a living animal, one important characteristic distinguishes cancer cells from normal cells, namely their capacity for uncontrolled proliferation. This property is reflected in the ability of malignant cells to form colonies when inoculated into soft agar ;non-malignant cells survive under these conditions but do not multiply to form colonies.Such transforma- tions of normal cells can be brought about by chemical carcinogens in vitro and therefore cell transformation in vitro has been suggested by Purchase and co-~orkers~~as a means of detecting potential chemical carcinogens. Here again we have a simple screening test for carcinogenicity which can be completed in a matter of days. Of course, there are many limitations to these ill vitro tests, but in view of the large number of flavouring materials which require safety assessment such tests could permit a rapid screening of a large number of them.In the long run, however, it is still necessary to carry out animal studies in order to assess the risk to man, but in vitro tests may provide an early warning of materials which might give rise to concern. 50 B. N. Ames, J. McCann, and E. Yamasaki, Murut. Res., 1975, 31, 347. 51 R. H. C. San, and H. F. Stich, Ititcrnat.J. Catic.er,1975, 16, 284. 52 I. F. H. Purchase, E. Longstaff, J. Ashby, J. A. Styles, D. Anderson, P. A. Lefevre, and F. R. Westwood, Nature, 1976, 264, 624. 53 J. E. Troska, and J. D. Yager, Exp. Cell Res., 1974,88,47. Chemistry and Flavour. Part III Summary An adequate assessment of safety of a flavouring material must be based on a series of inter-related investigations which are invariabiy time-consuming and range from simple chemical tests at one extreme to elaborate biological studies at the other. The object of the present paper is not to suggest definitive methods, but rather to contrast the present accepted methods with the possible procedures of the future. However, such ‘short cuts’ are unproven and require much further evaluation before it is likely that expert committees will accept them.

ISSN:0306-0012    

DOI:10.1039/CS9780700185

出版商:RSC    

年代:1978

数据来源: RSC

摘要:

IV The Influence of Legislation on Research in Flavour Chemistry By W. H. Nightingale FOOD lNDUSTRlES LTD., RR0MBOROUC;H PORT, WIRRAL, MERSEYSIDE, L.62 4SU The proposition that the sale of adulterated food should not be allowed appears to be a statement of the obvious. However, it is apparent that one man’s adul- teration is another’s amelioration and that which some consider to be a garden pest others prize as an edible snail. It is necessary, therefore, to elaborate on the sentiment, defining the meaning of the words used; in short it is necessary to draft laws and initiate legislation. The Chemical Society too has it’s rules. Few chemists know them well but despite their ignorance they usually conform to the rules because they match the common interests of chemists.In many ways societies like this model the state, e.g.,the rules are devised and perfected by a select few who, on behalf of the majority, take great pains to match the words to the sentiments. In the United Kingdom, in the European Community and elsewhere, the members of these macro societies employ professional law writers- the civil servants. Although the authority for the law of the land derives solely from our democratically elected parliaments and hence from us, the structure of our states is such that in the writing of law relating to food. the effective power is in the hands of the men from the Ministry. Although the Ministries are not without some effect on academic research, the following remarks are confined to the effect of some postulated legislation on flavour research which is commercially funded.The ultimate source of those funds is the purchaser of the end product, the consumer. The consumer is not a special variety of Homo sopiens; everyone is a consumer and each time a person chooses to buy or not to buy he very effectively criticizes the goods on offer. Hence research into flavour chemistry is undertaken with the firm intention of providing an incentive to the purchaser to choose the product which has benefited from the results of that research. Research, therefore, is also an investment made in the expectation of achieving a counter balancing financial reward. This is the most critical area where legislation may affect research into flavour chemistry. In order to depict the consequences of legislative action it is necessary to examine the financial description of a research project and a model situation is shown in Figure 5.Imagine that an idea, a product concept, has been tabled. Research estimates that two years work at f 10,OOOa year is needed and Marketing calculate that if the idea can work then sales can be such that the net income shown in the table will be realized. The solid lines show these basic facts. The accountants 195 Chemistry and Flavour. Part IV Year -51 Actual Present day value at lo:/,’, ------Figure 5 Model sesenrch project then rightly point out that a promise of &14,000in year eight is not the same as E14,OOO in the bank today.They teach the necessity of working in present day values and the dotted line in Figure 5 shows the value today of the expectations discounted at I0 %. A research project in these terms is indeed a projection. This is not an historical record but a prediction. One further factor has to be tabled. Research is the exploration of the unknown. It can, therefore, fail. For this simple model it is assumed there are only two possible results, total success and total failure. At this point the relative probabilities of the outcomes must be weighed. Assume that the probability of success decided on is 0.7. The project is now completely described in financial terms; an investment of E23,OOO at today’s values for a 70% chance of receiving &55,000,again at present value.In fact one looks at the converse, the 3004 chance of investing &23,000 with no return at all. The project has a mean expected income of E55,000 x 0.7, that is &38,500but this is not one of the two possible outcomes. It is vital to reduce the chance of loss. This is achieved by having more than one project, in fact by having a research programme. Consider a very simple model programme consisting of five projects each with the financial description which has just been detailed. Table 12 shows the possible results of such a programme and the probability of each result. There is a I /SO0chance of no success at all. There is only a 3 % risk that E5,OOO or more will be lost. This might be an acceptable situation. However, these are very simplified models put up solely to illustrate what can go wrong if an unfortunate legislative act is committed or indeed threatened.The following can go wrong: (a):Research and Development costs can be increased; (b), the chances of success can be reduced; (c),the timing of sales can be put back; (d), the magnitude of sales can be diminished. Some of the proposed legislation can achieve all of these changes in the financial picture at one stroke. However, for the present the gross effects of these distortions are considered. Essentially they diminish the incentive to commit Nightingare Table 12 Model research programme Five financially identical projects Research costs E23,OOO Projected net income if successful E55,000 Probability of success 0.7 Probability of outcome equal to o r not worse Possible results than result shown 1 .OOo Lose E115,OOO 0.998 Lose < f60,000 0.97 Lose f5,000 0.84 Gain E50,OOO 0.53 Gain f105,000 0.17 Gain f160,OOO Programme mean expectation : Gain E77,500 resources to Research and Development. It may become more profitable to improve the distribution system or to indulge in more advertizing than to engage another flavour chemist.Successful businesses are successful because they adapt rapidly to their environment. Very successful businesses anticipate the environ- mental changes and plan their adaptation in advance. However, the major victim will as usual be the consumer. The gross effect of ill-considered legislation must be to deny to the consumer the improvements which he might otherwise have enjoyed.One of the most extraordinary distortions of the truth is put about by those who relentlessly insist on doing good for us. They claim that all their efforts are aimed at restraining the wicked manufacturer or the trader. This claim does not sustain close examination since usually the restraints do not apply to manufacture for export. The restrictions bear entirely on the domestic consumer. In effect the legislators and the do-gooders are saying to the consumer, we will tell you what you may eat and what you may not eat, in what unit quantities you will be permitted to buy it, and so on. The analogy with the censorship of the written word is obvious.The offence is committed by the publisher but the objective is to prevent public access to kinds of literature which authority deems to be harmful. The Codex Alimentarius and the Codex Expurgatorius have a great deal in common. Another analogy is relevant in this connection. In the seventeenth century the witches of Salem were hung. This particular set of trials is well recorded but there were many more victims in Europe at that time. The important factor is that, at that time, throughoat the world there was universal belief in witchcraft. Every- one concerned, accused as well as accusers: the judges, the people and their leaders, sincerely and honestly believed that they dealt with a very real problem- witchcraft. Today, thanks to the activists of the consumerist movement, to the delight of the media in predicting doom, and of course to authority which converts all Chemistry and Flavour.Part IV things to its own sustenance, the people at large are persuaded that they are in peril from food additives and flavours. It is ironic that the Codex Alimentarius draft code of ethics for the Internation Trade in Food is graced by a remarkably appropriate quotation, dating back some 3,500 years. It reads: ‘Thou shalt not bewitch thy neighbours fat’ (FAO/WHO CY/GEN 77/1 June 1977). Little has changed in thirty-five centuries except that the flavour chemists are now cast in the role oft he witches. It is in this context of almost superstitious fear that legislation is being for- mulated.It is not a context that admits rational argument and always one is finally confronted with the two ultimate absurdities; prove that it is safe an4 prove that regulatory action is unnecessary. In this context the possibility of the legal control of the use of flavourings by total positive listing cannot be ignored. Suppose one of the research projects was aimed at establishing the identity of the more important substances con- tributing to the delightful aroma of butter fried mushrooms and assume that this work leads to the identification of just one novel compound and its synthesis. The R & D money has now been spent but the discovery cannot be exploited since the substance is not on the positive list! Presumably there will be some mechanism for petitioning the authorities for it to be added to the list.One has to make a guess about the delay this will entail: assume, for example, that it is one year. Immediately this diminishes the present day value of the predicted income by 10%. This is the most optimistic view. A more likely result of such a petition will be a demand for biological work costing, for a modest programme, perhaps another &lO,OOO and a further delay of one year. An additional factor to take account of is that biological work can, on an innocuous substance, incur a risk of an adverse verdict arising by chance. The original picture of a R & D investment at present day values of E23,OOO with a 0.7 probability of a return income of E55,OOO has now altered to an investment of around .€31,000 and a 0.60 probability of an income of &44,550.That becomes a mean expectation of an income of about &27,000 and that is a mean expectation of a loss of &4,000.If the expectation of success had been left at a probability of 0.7 it would have meant a mean expectation of just about breaking even. Why does the probability of success fall even though it has been assumed that our substance is quite innocuous ? In biological testing a randomly selected group of test animals is subjected to large doses of the test substance and their responses are compared with a similar, randomly selected, control group. Two kinds of quantified observations are possible, typified on the one hand by weights of an organ and on the other hand by counts of injuries (e.g.tumours). It is well known that individuals of a species vary naturally one from another and in this circumstance the methods of statistics seem appropriate. The-logic of the usual analyses is worth careful consideration. In effect one makes the hypothesis that the test substance is without effect. Hence the two groups are each a sample of the composite whole. The question posed is then what is the probability of getting by chance a result equal to or more disparate than that observed. If the answer is less frequent Nightingale than 1 in 20 times then, as a reflex, out comes the cry-a significant result. When the probability of success in the model project was lowered from 0.7 to 0.6 this was equivalent to assuming that only three comparisons of this nature would be made.Thousands of pounds have been spent and animals have been sacrificed (a beautifully appropriate word) to obtain a verdict which could be as validly obtained in 30 seconds with the aid of a pair of dice, or by tossing a coin, or if one really wanted to be scientific, by using a table of random numbers! The author might cheerfully condone this game of roulette if he were tolerably certain that a harmful substance would be detected and condemned along with the unlucky innocent. Regrettably it is not common practice to consult a statis- tician in advance of fixing the protocol and to ask him how many experimental animals would be necessary, say, to detect a 10% increase in the incidence of a cancer in a given strain with only a 5% risk of not detecting an increase of this magnitude.It would be convenient to state the answer in terms of kilo rats. One more effect must be noted. The law is a public matter; hence the contents of the positive list must be published and each addition to the list will be scrutinized carefully by everyode in the flavour industry throughout the world. It follows that the breakthrough in identifying a new substance is shared with the world before it is possible for the discoverer to begin to exploit it and therefore it is doubtful if it is possible to maintain any confidence in the sales forecast on which the expected income was based. And incidentally it would be illegal to have a gentlemen’s agreement with ones competitors not to take advantage of this situation.It is a curious quirk of this positive list scenario that it favours work on substances which do not occur naturally. The patent system gives virtually no protection to the discovery of a novel naturally occurring substance of the kind that has been discussed. This is because it is extremely difficult to police a patent. The problem is essentially the same one which makes enforcement of a total positive list by analysis an impossibility. However, a truly artificial flavour substance can be usefully patented because it can be identified by analysis. Thus the sales potential can be better protected. Clearly the control of the use of flavour by means of a positive list must seriously diminish the commercial incentive to conduct research into flavour chemistry.Authority has recently considered other extraordinary possibilities. The first draft of an EEC directive concerning the labelling of food would have required at least the qualitative disclosure of the composition of flavours, A rather old fashioned strawberry flavour composition was tabled to show that its disclosure would require rather a lot of additional label space to list each of its thirty or so components. It was suggested that thirty components was rather excessive. This line of discussion was closed by the submission of six foolscap sheets listing the known volatile components of natural strawberry juice.Nor must attention be confined to legislation specifically directed at chemists or at flavours. The proposed EEC directive on Product Liability must encourage Chemistry and Flavour. Part I V insurance companies to consider demanding higher premium for the risks associated with innovation which is again a disincentive for R & D work in any field, including flavour research. One must pause and ask does the consumer really benefit by putting progress into the deep freeze to await more enlightened authority. There is one small ray of hope. The U.K. law will evolve in common now with the emerging patterns of E.E.C. directives. These stem from article 100 of the Treaty of Rome which reads: ‘The Council shall, acting unanimously on a proposal from the Commission, issue directives for the approximation of such provisions laid down by law, regulation or administrative action in Member States as directly affect the establishment or functioning of the common market.The Assembly and the Economic and Social Committee shall be consulted in the case of directives whose implementation would, in one or more Member States involve the amendment of legislation,’ Thus to be legal a directive must approximate existing laws and the differences between national laws must be a technical barrier to community trade. There is no authority in the Treaty for a directive demanding an extrapolation of existing national laws. No member state has a total positive list system of controlling the use of flavours. Should the E.E.C. issue such a directive, action to have it voided by the European Court could be initiated within 60 days of its promulgation. It is a sad picture of impeded progress which is evoked by the title of this paper; the effect of legislation on research in flavour chemistry. I hold that as chemists and as scientists, we have a broad duty to our fellows to press for rational legislation founded on fact and not on fostered fears, and if the law persists in being an ass, we must not cease from calling attention to its foolishness.

ISSN:0306-0012    

DOI:10.1039/CS9780700195

出版商:RSC    

年代:1978

数据来源: RSC

摘要:

V The Development of Flavour in Potable Spirits By J. S. Swan and S. M. Burtles PENTLANDS SCOTCH WHISKY RESEARCH LTD., 84 SLATEFORD ROAD, EDINBURGH Ell IQU 1 Introduction Potable spirits are distinct from most other food products in so far as they are homogenous solutions, thus presenting a medium for flavour studies which is free from the problem of flavour release due to the intricacies of texture. Even the study of aroma in carbonated beverages may be influenced by the effect of C02 release. However in common with most other products the number of components present is large and their low concentrations render accurate analyses very difficult. Kahn5* reported in 1969 that 226 compounds had been observed in whisky and since then at least another 30 have been reported.In the case of beverages matured in wood, e.g. whisky, brandy, and rum, the total level of flavour com- ponents rarely exceeds 1 % of the total weight (and is normally very much lower) in a base of 40-45 % ethyl alcohol. This high concentration of ethyl alcohol presents particular problems in both sensory and analytical studies. Furthermore the many analytical studies reported since the advent of gas chromatography show that most potable spirits contain the same components. It follows therefore that the nuances of flavour are essentially attributable to small differences in the relative proportions of these components. In 1966, Guadagini and co-workers made an important contribution to the interpretation of flavour in food products by the introduction of the term odour unit55 where : Odour Unit (Ou) = concentration presentlthreshold concentration.Further, if the sum of the odour units from each fraction in a mixture equals, or nearly equals the whole, then the contributions of the individual fractions may be considered additive and each contribution may be expressed as a percentage: 100 x Ou fraction/Ou whole = % odour contribution from each fraction. If the sum of the fractions is substantially less or greater than rhe whole the presence of suppressive or synergistic effects may be postulated.56 Using this type of approach Salo, Nykanen, and Suomalainen57 studied the odour thresholds and relative' intensities of volatile aroma components in an artificial beverage imitating wnisky.The beverage contained 67 ingredients 64 J. H. Kahn, JAOAC, 1969,52, (6), 1168. 55 D. G. Guadagni, R. G. Buttery, and J. Harris, J. Sci. Food Agric., 1966, 17,142. 56 P. Salo, Proceedings International Symposium Aroma Research, Zeist, Pudoc Wageningen, 1975, 121. E.' P. Salo, L. Nykanen, and H. Suomalainen, J. Food Sci., 1972,37, 394. Chemistry and Flavour. Part V consisting of acids, alcohols, esters, and carbonyl compounds. From a knowledge of the concentrations and odour thresholds of all the substances present they determined that the alcohol group contributed 4% of the total aroma, the acids contributed 3.5 %, the esters 26%, and the carbonyl group 58 % (Figure 6). 1mass fraction/ ”/, 0odour contribution/ total odour units = 1I30 Alcohols Acids Esters Carbonyls Fcgure 6 Mass fraction % of alcohols, acids, esters, and carbonyl compounds and their cont.-/ibutionsto odour units of a whisky imitation56 Normally the components present in this artificial beverage derive from the fermentation stage of the process as a result of the action of yeast upon the sugar containing substrate.Whilst the choice of yeast strain and the fermentation conditions undoubtedly influence the composition of the volatiles produced58 and control of these features are of vital importance in the brewing industry, such precise control is of lesser importance in potable spirits production owing to the modifying influence of the distillation, maturation, and blending stages which follow. Traditionally at least, potable spirits with characteristic and repro- ducible flavour have been produced with little deliberate control of fermentation conditions.In potable spirit production it is desirable to produce the maximum yield of alcohol and consequently the wort is not sterilized prior to fermentation, in order to preserve enzyme activity, thus permitting the production of volatile compounds from competing bacterial fermentations. Whilst the production of flavour volatiles during yeast fermentation has been extensively studied, less attention has been given to those other aspects of the process and their overall contribution to flavour. It is proposed to do so here, and it is suggested that the odour construction of the artificial whisky might be used as a model for assessing the nuances of flavour between different spirits brought about by the influence of other stages of the process in contributing additional classes of compounds or their modifying role upon the relative proportions.O8 H. Suomalainen and M. Lehtonen, Kemia Kemi, 1976,2,69. Swan and Burtles Scheme 3 shows the production process for the manufacture of Scotch whisky and with certain modifications it may be used for other whiskies as well as rum and brandy. Thus, whisky in some countries is produced without the malting process by the action of enzyme products on the cereals, whereas if the malting and mashing stages are replaced by the crushing of grapes or the dilution of molasses the scheme suitably represents brandy or rum production.Malting.1 Enzymatic conversion of starch to fermentable sugars Mashing Fermentat ion .1 i Solubilization of the fermentable sugars Production of alcohol and secondary products by the action of yeast Distillation I Fractionation in continuous (equilibrium) stills or in pot (batch) stins. Always in copper J. Maturation Flavour modification in oak wood Scheme for the production of Scotch whisky Scheme 3 Examples of other classes of compounds which may contribute to aroma are the phenols, organonitrogen compounds, and organosulphur compounds. In practice the reason for scarcity of knowledge of their role in spirit flavour is of course due to the difficulties of accurate analysis and of obtaining pure samples for sensory studies.2 Production and Influence of Volatile Phenols As long ago as 1895 Zwaardemaker59 listed phenol as an example of an empyreumatic or burned aromatic. For even longer, Scotch malt whiskies have been identified as possessing smokey aromas and this has been attributed to the traditional process of drying the malt over peat smoke.60 Whilst the measurement of adsorbed phenols is taken as an indication of the degree of peating, Deki and Yoshimura61 identified some 80 aroma components in peated malt. Bathgate and Taylor62 recently grouped these into three major divisions namely: (a), hydro-carbons.; (b), furfurals; (c), phenols; and pointed out that it is still not certain that the phenolic constituents are the major contributors to the characteristic smokey aroma.Studies on the analysis of phenols in peated malt samples and the odour potency of phenols in whisky63 indicate that these compounds could 59 H. Zwaardemaker, ‘Die Physiologie des Geruchs’, Engelmann, Leipzig, 1895. 6o J. A. Nettleton, in ‘The Manufacture of Whisky and Plain Spirit’, Cornwall and Sons, Aberdeen, 1913, p. 308, 310, 319. M. Deki and M. Yoshimura, Chem. Pharm. Bull., 1974, 22, 1748, 1754, 1760. 62 G. N. Bathgate and A. G. Taylor,J. Inst. Brew, 1977, 83, 163. 63 J. S. Swan, S. M. Burtles, and D. Howie, in preparation. 203 Chemistry and Flavour. Part V provide a significant contribution to the aroma of the spirit. Table 13 shows the analysis of the major phenols present in a peated malt sample along with the odour units which could result in the spirit produced, assuming an average distillery yield.These studies also indicate that the odour potency of such phenols is approximately doubled when combined into a group of this nature, thus providing a potential of 81 odour units. When compared to the total odour units in the artificial modeF7 this represents a contribution of around 7 % derived entirely from the peating of the malt. It is probable however that owing to the contribution of the hydrocarbon and furfural groups, whatever the magnitude, that a true peated aroma can only be obtained at present by the conventional process of kilning. Table 13 Phenol fraction in a typical peated malt and possible impact in malt whisky Phenol Conc. in malt Potential conc.Odour Odour mg/kg -l in spirit threshold units p.p.m.at 40Xalc. p.p.m.* Phenol 3.1 3.3 40 0.08 o-Cresol 0.25 0.26 0.3 0.9 m + p-Cresol 0.60 0.64 0.06 10.6 p-Ethyl phenol 0.22 0.23 0.6 0.38 2,3-Xylenol 0.06 0.06 0.5 0.12 pEthy1 guaiacol 0.03 0.03 0.05 0.6 Eugenol 1.32 1.40 0.05 28.0 Total 40.681. *determined in 20 % ethanol-water base using the ascending series method twhen adjusted for observed synergism the total becomes 40.68 x 2 = 81.36 units Phenols have also been detected in rumM@ and other types of whisky66b67 and it appears that they can be produced during the mashing, fermentation, and maturation stages.In the late 1950s Whiting and Ca1~~89~9 demonstrated that volatile phenols could be produced by the action of Lactobacillus pastorianus on phenolic acids which are present in many natural products. Steinke and Paulson70 in 1964 demonstrated that p-vinyl phenol and p-vinyl guaiacol are formed by thermal decarboxylation during mashing, and microbiological decarboxylation during fermentation, from p-coumaric and ferulic acids. Subsequent yeast and bacterial action converts these intermediates into p-ethyl phenol and p-ethyl 64 P. Dubois, G. Brule, and J. Dekimpe, Industr. Aliment. Agric., 1972, 89, no. 1, p. 7. 65 P. Dubois and J. Rigaud, Ann. Technol. Agric., 1975,24,.no. 3-4, p. 307. 66 K. Nishimura and M. Masuda, J. Food Sci., 197 1,36,8 19.67 J. H. Kahn, P. A. Shipley, E. G. La Roe, and H. A. Conner, J. Food Sci., 1969,34, 587. 68 G. C. Whiting, and J. G. Carr, Nature, 1957, 180, 1479. 69 G. C. Whiting, and J. G. Carr, Nature, 1959, 184, 1427. ‘O R. D. Steinke and M. C. Paulson, J. Agr. Food Chem., 1964,12, no. 4, p. 381. Swan and Burtles guaiacol, and p-methyl guaiacol may be similarly produced from vanillin (Scheme 4). OH OH HC=CH-CO,H HC=CH, H,C-Me ferulic acid Cvinyl guaiacol 4-ethyl guaiacol OH OH OH HC== CH-C02H HC=CH, H,C-Me p-coumaric acid p-vinyl phenol p-ethyl phenol OH OH HC=O Me vanillin 4-methyl guaiacol Transformation of precursors to steam volatilephenols70 Scheme 4 Volatile phenols also appear to be produced during maturation by the action of ethanol on oak.Soumalainen58 and co-workers demonstrated that guaiacol, phenol, m-cresol, vanillin, and particularly eugenol can be formed in this way. It may be concluded that the presence of volatile phenols in potable spirits is largely attributable to the breakdown of naturally occurring phenolic acids deriving from a variety of sources, e.g. peat, corn, and oakwood. 3 oc and B Ionone The ionones have intense violet-like aromas and a threshold of 7 x 10-6 p.p.m. (0.007 p.p.b.) for p-ionone in water has been reported.71 La Roe and Shipley72 have detected the presence of these compounds in American, Canadian, Scotch, and Irish whiskies as well as grape brandy. They showed that these compounds can result from the thermal decomposition of /%carotene. in grain under the conditions prevailing during the mashing or cooking process.It may therefore 'l R. G. Buttery, R. M. Seifert, D. G. Guadagni, and L. C. Ling, J. Agr. Food Chem., 1971, (19)3,524. 72 E. G. La Roe and P. A. Shipley,J. Agr. Food Chem., 1970,18, 174. Chemistry and Flavour. Part V be anticipated that small variations in operating conditions between distilleries may result in the production of varying quantities of these components. 4 The Influence of Distillation on Congener Composition Distillation of potable spirits may take place in either continuous, multiplate fractionating columns, or in simp1epot:stills by two or three successive distillations. In continuous stills the system operates under equilibrium conditions and has been extensively studied elsewhere, particularly in the petrochemical industry.The degree of sophistication of continuous stills in use varies widely fom the two column Coffey-type still most commonly used in Scotch grain whisky produc- tion (Figure 7) to more elaborate 2, 3, or even 4 column units which provide more fractionation and improved extraction of feints. Spent Feints wash Figure 7 Cofley still used for Scotch grain whisky distillation Reviews of continuous still distillation of Scotch grain whisky73 and Califor- nian brandy74 show that the various flavour components distribute themselves fairly sharply around particular plates in the column according to their boiling point. Accordingly the level of components in the spirit produced depends upon the choice of plate (i.e.spirit strength) at which the spirit is removed. In Scotch grain whisky production the product is drawn off at 94%alcoholicN strength and is therefore much ‘lighter’ than Californian brandy N 84% (maxi-mum). 73 M. Pyke, J. inst. Brewing, 1965, 71, 209. 74 J. F. Guymon, in ‘Chemistry of Winemaking’, ed. A. Dinsmoor Webb, American Chemical Society, Washington, 1974. 206 Swan and Burtles By contrast pot still distillation is a batch process where equilibrium conditions are not achieved and mathematical treatments are more complex. Generally speaking however, spirits produced by this procedure, e.g. pot still distillation of malt whiskies in Scotland, Cognac brandy produced by the ‘Methode Charen- taise’ and the ‘Heavy Continental’ rums are preferred by connoisseurs and enjoy a price advantage over continuous still products, and it must be observed that the relative proportions of flavour components is different with pot still distillates compared to continuous still distillates even when drawn at the same strength.Pot stills are always constructed in copper, are rarely lagged, and the reflux area of the neck is usually void. A diagram of a typical Scotch whisky still is shown in Figure 8. Consequently the heat input substantially exceeds the actual Figure 8 Typical iraditional Scotch malt whisky still. Either direct heating by gas or coal or indirect heating by steam coils niay be irsed heat required to volatilize the fermented- liquors and the degree of fractionation approximates to a single theoretical distillation stage.In terms of flavour the pot still may be regarded as a reaction vessel and the following examples can be cited to illustrate the point. (1) Recently Nemoto75 in a study of rum-making utilizing butyric acid bacteria reported substantial increases in the levels of ethyl acetate and ethyl butyrate during distillation of the corresponding acids in 10% ethanol solution. They also determined that the extent of ester formation was very pH dependent. (Table 14). (2) During fermentation the long chain esters of ethanol and monocarboxylic acids, i.e. ethyl decanoate (caprate) to ethyl hexadecanoate (palmitate) are 75 S.Nemoto, Ann. Technol. Agric., 1975,24, no. 3-4, p. 397. Chemistry and Flavour. Part V Table 14 Production of esters during the distillation of organic acids in 10% ethanol solution Solution to be distilled Distillate Concentration pH Ester Acid mg/ 100 ml mg/ 100ml mg/100 ml Acetic acid 2.o 6.2 48 106.2 3.5 4.4 43.2 5.O 0.9 48.6 2.0 15.8 93 204.2 3.5 4.4 88.8 5.0 2.6 52.8 2.0 23.8 131.4 301.1 3.5 5.3 131.4 5.O 5.3 87.6 Butyric acid 2.0 5.8 165.4 96.8 3.5 2.3 157.5 5 .O 1.1 94.2 2.0 17.4 287 201 .o 3.5 5.8 308 5.O 5.8 178 2.0 18.6 442 302.4 3.5 9.3 451 5.O 8.1 278 bound to the yeast cell wa1176 and consequently are absent from non-distilled beverages (e.g.beer) after separation from the yeast During distillation in pot stills however it is normal practice to charge the dead yeast cells into the stills whereupon the fatty esters are liberated and pass into the distillate. The low odour thresholds of ethyl caprate and la~rate~~ enable them to make a significant contribution to the odour of the distillate whilst the longer chain esters contribute to the mouthfeel effects. These compounds are however the major contributors to chill haze and current commercial pressure requires that a proportion of these are removed by cold filtration prior to bottling. (3) When a typical fermented liquor for whisky making is distilled in glass, the product has a strong sulphur aroma normally absent in the commercial product.Traditional distillers claim that the presence of copper is necessary to ‘fix’ organosulphur compounds. Studies in our laboratories77 have shown that dimethyl sulphide in the distillate from a single stage distillation in a copper pot still is reduced by 70% compared to all-glass distillation. This explains why DMS (which has a reported threshold of O.OOO3 p.p.m.ls in water) is less trouble- some in spirits production compared to ale or lager production. ’13 K. Nordstrom, J. Insr. Brewing, 1964,70, 233. ”J. S. Swan and L. Hill, unpublished work. Swan and Burtles During the distillation period of the pot still only a narrow fraction of distillate is collected as spirit (‘coeur’ in cognac production) and the composition of flavour components present obviously depends upon the choice of fraction.Williams78 prepared vapour-liquid equilibrium data for many of the components in spirit distillate and Guym0n7~ presented volatility curves for alcohols and esters from these data. The optimum distillation conditions for each component can therefore be calculated. It is however, traditional practice to recycle that part of the distillate which contains alcohol other than the spirit fraction. Those flavour components which are rejected initially by the choice of cut eventually must pass to the spirit fraction thus reducing the apparent influence of the physico- chemical data in predicting the composition of the final product. 5 The Influence of the Maturation Stage The maturation stage may last from one to fifty years depending upon the type of spirit concerned.In whisky production four years to twelve years are usually allowed depending upon the type and quality of the products. Maturation always takes place in oakwood and both American oak (generally Quercus alba) and European oak (Quercus robur and Quercus sessilis) are used. During maturation additional materials are derived from the cask and re-equilibration of components already in solution also occurs. Generally, three distinct processes are believed to take pla~e,~g namely: (1) Chemical interactions among substances in solution (2) Oxidation reactions (3) Extraction and breakdown of substances from the wood.It is believed that the levels of esters, carbonyl compounds and acids generally increase during the period whereas the level of alcohols remains relatively static. In a recent study of bourbon maturation a small amount of 14C-labelled ethanol was added to a test cask and Reazin80 and co-workers were able to confirm these general trends. Earlier Baldwin and Andreasens1 from the same organization had proposed that the term TBDM (total barrel derived materials) which they defined as the sum of the volatiles and the non-volatiles in an aged product, could be a useful parameter. The increase in the quantity and multiplicity of esters which takes place is perhaps the easiest to study in flavour terms. Since the constituent alcohols and acids have lower odour thresholds57 an overall increase in aroma must result.In a study of malt whisky maturations2 isoamyl acetate increased from 7.2 p.p.m. (at bottling strength) to 9.0 p.p.m. in 5 years, an increase of 8 odour units or 0.7 % compared to the artificial ‘base’. The increase in the multiplicity of esters is believed to influence the quality of the aroma, producing a more blended effect. This can be demonstrated by the well 78 G. C. Williams, Alner. J. Enol. Viriculr., 1962, 13, 169. 7y A. I. Liebmann and B. Scherl, Inrl. Eng. Chem., 1949,41, no. 3. p 534. G. H. Reazin, S. Baldwin, H. S. Scales, H. W. Washington, and A. Andreasen, JAOAC, 1976, 39(4), 770. S. Baldwin and A. Andreasen, JAOAC, 1974, 57, no. 4, p. 940. 82 J. D. Gray and J.S. Swan, unpublished work. 209 Chemistry and Flavour. Part V known technique where individual components are represented by triangles whose areas are proportional to their odour potency. The nearer the outline approximates to an arc then the more blended the aroma will become (Figure 9). 3 2 1 0 Figure 9 Combination of‘individual aroims to for-w? u blended complex Recently, in a study of acetal formation in rum, Misselhorn83 has shown that acetals have lower odour thresholds than the corresponding aldehydes from which they are formed. In any event it was suggested that most acetals detected in rum are artefacts and are without importance in rum flavour. Turning to the influence of oxidation reactions Petersen84 in a study of wine maturation has recently shown that a vacuum builds up inside casks daring maturation presumably due to the evaporation of liquid out through wood pores without a corresponding ingress of air.The amount of oxygen available to the product depends therefore on the physical soundness of the cask. Singleton85 believes that oxygen does not pass through the timber and that an impermeable layer is formed at the interface of the oxygen and barrel contents. For the most part the third process, i.e. interaction of the ethanol and wood, results in an increase in relatively non-volatile compounds such as phenolic acids, tannins, etc. which influence the astringency and bitterness of flavour rather than the aroma. Guymon and Crowells6 identified diethyl succinate, 5-methyl furfural and P-methyl-y-octalactone as substances deriving from the oak cask during brandy ageing.This latter compound, often known as oak lactone has an intense coconut-like aroma but was considered by Suomalainenj’ to have little overall impact owing to the low concentration present in matured spirits. Singleton87 83 K. Misselhorn, Ann. Techno/. Agric., 1975, 24, no. 3-4, p. 371. R. G. Petersen, Amer. J. Enol. Viticult., 1976, 27, no. 2, p. 80. 85 V. L. Singleton, Paper presented at a Maturation Symposium, Long Ashton Research Station, Bristol, 21 January 1977. 86 J. F. Guymon and E. A. Crowell, Amer. J. Enol. Viticiitt., 1972, 23, no. 3. p. 114. V. L. Singleton, in ‘Chemistry of Winemaking’, ed. A. Dinsmoor Webb, American Chemical Society, Washington, 1974. 210 Swan and Burtles has shown that European oak contributes more extract and more tannin during maturation of wine but American oak contributes more flavour, although it is possible that the differing techniques for preparing the wood may play an important part in this difference. 211

ISSN:0306-0012    

DOI:10.1039/CS9780700201

出版商:RSC    

年代:1978

数据来源: RSC

摘要:

VI The Influence of Flavour Chemistry on Consumer Acceptance By R. Swindells BEECHAM PRODUCTS, BEECHAM HOUSE, BRENTFORD, MIDDLESEX TW8 9BD 1 Introduction Any discussion of flavour is concerned mostly with food, which at its most primitive is the fuel with which we fire our bodies to provide energy and with which we maintain the body in a healthy state. There are still some parts of the world where this prime function of food is dominant, resulting in a simple concern that enough familiar food should be available. In most communities however, an element of choice has developed and choice results in preference- which is where flavour comes in; for flavour is the main factor, along with texture and colour, which will influence consumer choice-consumer choice which is the lifeline through which all in the food and flavour industry survive.Food, whether of animal or vegetable origin is alive in the biochemical sense even after it has been cropped, and will continue to change through enzymic reactions until or unless something is done to stabilize it. Flavour changes result from these enzymic reactions and also from simple chemical reactions which may continue before, during, and after any stabilization. These flavour changes may be attractive or the opposite. In either event they are a major area of interest for the flavour scientist who is concerned to optimize desirable flavours and avoid those which are unpleasant. The history of flavour variations and hence choice begins with primitive man competing with animals for the natural crops; this clearly did not work too well for man who was weaker and less mobile than many of the other animals, and in any event the crops were highly seasonal.Eventually man started to grow his own crops-a major step forward particularly for the ones fortunate enough to be in fertile areas-but still suffering from extreme seasonalfluctuations, which led to alternating glut and famine. From this situation evolved several ways of preserving glut crops, and it is interesting to note that the basic methods of maintaining food in a fit condition to eat have not really changed over several thousand years. One can dry food, originally in the air and sun, or desiccate it, e.g. with sugar, salt, etc.The food may be heated, which we now know destroys both micro- organisms and food enzymes; and this often results in a degree of concentration which, like drying or desiccation, reduces the amount of active water: An alternative approach is to preserve chemically, originally with spices, hops, etc., or with burnt sulphur, to which have now been added other chemical preserva- tives. Another method discovered very early is to allow the food to decompose 212 Swindells (ferment) which leads to products which are attractive in flavour, and in which the alcohol or acetic acid provides a stabilizing effect. Finally, it should be remembered that the concept of animal husbandry is based, at least in part, upon keeping the animal alive ‘on the hoof’until it is required. This is still the preferred method of distributing chickens in tropical markets, ensuring they are still fresh when purchased.It may fairly be said that food technology commenced with the discovery of these processes, and food scientists and flavourists are still today very much concerned with this area-seeking to improve the efficiency of these processes as regards yield and nutritional quality and to optimize and stabilize the flavour. 2 Flavours-Selected Examples The following specific examples are intended to draw attention to a number of facets which should influence the activities of the flavour scientist. It is sometimes tempting to liken flavours to perfumes and indeed the Flavourist Society has many joint meetings with its perfume counterpart.In many ways the development of flavours is technically similar to that of perfumes but they are rather different creatively, for the target is more visible and definable for the flavourist, and the consumer is comparatively well able to judge objectively how well the product has been formulated. There are, however, two types of flavour or flavour project, the simpler being the flavour of a defined material, which is the easier to describe and is comparatively static. More intriguing is the question of flavouring for a usually bland material. This includes more licence and is dynamic with respect both to time and to different consumers, and it is to this area that most of the following remarks are directed.The former, however, should not be neglected for the supply of real food commodities is finite and will easily be over-run as more of the Third World populace aspire to an adequate and more varied diet. The recent cocoa and coffee shortages are a reminder of how close this problem may be and this raises the question of how to cope with the situation. Agricultural efforts to grow more will, of course, continue but beyond this it will be necessary to produce synthetic foods, and in doing so to decide whether to imitate the natural and familiar commodity, or to offer essentially new types of foods. The question of providing adequate protein is an interesting one. Vegetable proteins have been developed to supplement the limited and costly animal proteins, but technologists, who when formulating for the Western markets have targeted to make it like a familiar foodstuff, when developing vegetable protein products for the emerging nations have totally ignored texture and flavour or have formulated them to the Western diet.In either event, the resultant end product has been so totally unfamiliar to the target audience as to achieve only very limited success. There is a major opportunity here for food and flavour scientists to study more carefully the ways in which such products would be utilized within existing diets and to give attention to the types of flavours which are likely to appeal. A. Fruit.-Fruit is an interesting foodstuff and includes a vast array of variously Chemistry arid Flavorrr.Part VI preserved products which have evolved from essentially perishable material. The flavour characteristics of these various products-fruit juices, canned fruits, jams, wines, etc. are often profoundly different; and particularly for those fruits which are not indigenous the flavourist and formulation man must carefully review which version of the flavour will have the highest appeal in the particular new product he may be developing. Another factor which can be discussed against the backcloth of fruit (although it is also relevant in many other areas, e.g. cooked meats), is the complex balance between volatile aroma with its associated taste, and the non-volatile taste factors including mineral salts and non-volatile organic materials.Moreover, these flavour considerations are intimately meshed with a colour, texture, and human environment matrix, so that an apparently simple project may indeed be quite complex. Supposing it is wished to formulate an orange powder drink. First it is necessary to decide who it is for, and then what it should taste like. Perhaps it is required to compete with low priced squashes and carbonates or maybe the higher quality fruit juice market. Assuming that it is the latter, the nutritionist will want to have his say, desiring a balance of all the nutritional factors which might have been present in the real orange juice. The flavourist, knowing that he must get as near to ‘the real thing’ as possible, must still decide whether the fresh or the canned juice flavour is his target.The importance of non-volatile taste factors has been mentioned. The General Foods product, ‘Tang’, is an excellent example of the way in which a very realistic orange juice taste can be achieved without the use of orange juice by a combination of high quality flavoirr with cloud and texture ingredients, and a deliberate rebalancing of mineral constituents towards those found in orange juice, In rather similar vein it has been found that for fruits which are relatively high in astringent tannins or polyphenolics, the inclusion of low levels of tannin or other bitter constituents can add realistic breadth of taste to a low juice or juice-free product. B. Sweetness.-From fruit to sweetness is but a small leap.Sweet things can be delicious, and long before the development of a significant sugar-cane industry, honey had achieved an almost unique reputation; but if sweet things are delicious, people can eat too much of them as the high incidence of obesity in western civilization testifies. If people like sweet things which are bad for them, what is to be done? For many years synthetic, non-calorific sweeteners have been available, notably saccharin. But the sweet taste of saccharin is not perfect, being marred by a bitter taste and an aftertaste, and we can see the influence of the flavour scientist in achieving modifications, for example with ethyl n:altol, monosodium glutamate, ribotides, etc. to extend and soften the sweetness character. Even if an adequate sweetness is achieved there remains a physiological question about satiety, or satisfying the appetite.A formula with a reduced, but finite carbohydrate level can probably achieve satiety at lower calories, and achieve a far superior taste, especially if a very sweet natural sugar (e.g. fructose) Swindells is chosen. The Beecham low calorie products marketed under the ‘Bittersweet’ brand are good examples of this approach. Ethyl maltol is just one example of a compound not itself sweet in the true sense but extending the overall impression of sweetness. Certain fruit juices have this sort of characteristic. For example, pineapple can increase the impres- sion of sweetness, beyond that which can be shown analytically, when blended with orange; whilst the Florida orange, apart from analysing as sweeter or less acid, also has a sweeter aroma than typical mediterranean oranges.Whenever it is necessary to eke out natural sweetness, whether for low calorie or cost considerations, more use could be made of these natural examples and indeed, perhaps more fundamental information would come to hand from a closer scientific study of ingredients which seem to have this characteristic. In foods the use of sugar for desired sweetness mainly involves problems of cost and nutrition, but for some more compact products it is simply not possible to get enough sugar in to achieve the desired sweetness rating. Toothpaste is a very good example which is thus dependent upon high strength artificial sweet- eners, and if as seems likely, at least in North America, saccharin and cyclamate are banned, the achievement of adequate sweetness is a significant challenge to the flavour scientist.Obviously he will search for new artificial sweeteners, recognizing more contemporary ground rules in which safety is crucially important, although cost can within reason be accepted. Some of the flavour submissicns now coming forward show that significant progress can be achieved by bringing together ingredients having a synergistic tendency to enhance the sweet character. Often, however, these flavours are rather less conventional, at least for use in toothpaste. So the questions of consumer acceptability and perhaps even re-education will have to be tackled.C. Bitterness.-Although bitter things tend to be regarded as nasty, there are clear exceptions, particularly for the adult palate in which the non-sickly character of a bitter product may have high appeal. The active ingredients of medicines, however, often have intense bitterness, presenting the formulation and flavour team with the dilemma of whether to wrap up the bitter factor (e.g. by encapsulating), to mask it, or to complement it. This last approach is one which holds great promise and indeed, there are a number of recently introduced medicinal products in which this approach has been successful. Another approach to this problem is the development of loose chemical derivatives, often chelate compounds which have reduced bitterness but readily dissociate before absorp- tion (e.g.guaiphenesin, chlorhexidine). D.Milk.-From ancient times milk has had an almost unique nutritional status and aura; the perfectly balanced food on which babies can and do grow. Even beyond breast feeding, mothers recognize its nutritional value and surely every mother tries to get her growing children to take their daily quota. Also, it is one of the few foods the farmer produces without actually losing the source! Milk, however, does not keep well, and this has led to the development of the Chemistry and Flavour. Part VI whole gamut of methods of preservation involving pasteurization, sterilization, concentration, and lactic fermentation. In the U.K.we are still accustomed to find our milk on the doorstep, but distribution problems have necessitated a search for better stabilization whilst retaining the well-known character of fresh milk, e.g. UHT sterilization, and more delicate spray-dried treatments for producing dried milks. Another question which arises is whether all children actually like milk. If not, how can the flavourist help? Traditionally mothers sought to lose the milk, in milk puddings or in strongly flavoured beverages such as cocoa, but in more recent years a variety of other flavours have been offered to improve the palat- ability and acceptability of fresh milk and of milk-like beverages, e.g. soft fruits, malt, caramel, etc. More recently still, cultured milks (yoghurts) have become very popular, both plain and in a variety of flavours; these are limited shelf-life products with special dynamic flavour problems.We worry about whether milk will still be on the doorstep as years go by, but for the world population at large, a more profound question of absolute avail- ability arises; and if milk will not be available in adequate quantity, what alternatives may arise? Among these could be soya milks, i.e. based on vegetable rather than animal protein, which raises yet again the question for the flavour scientists-‘should he eliminate or mask the characteristic soya flavour, or seek to complement it so that an acceptable, if new, taste is developed?’ In view of its very good nutritional image it is ironical that nutritionists in recent years have been asking whether milk is really so good for us after all.In the United States and Scandinavia particularly, the development of filled milks having vegetable fats in place of the animal fat constituent has occurred. As yet no wide movement of milk-based products on to filled milk bases has occurred but perhaps, in the future, there will be cheese based on filled milk, or maybe the cow will disappear as a source of milk (whole or filled), and force the food technologist to create a whole range of vegetable cheeses! 3 Agronomic Evolution Most basic foods whether animal or plant based are undergoing a continuous process of development targeting for new strains which are more economically grown, resist diseases, and have better ‘consumer characteristics’. The consumer characteristics which are chosen are often related to marketing and distribution, e.g.uniform size, shape, good colour, and good shop-keeping qualities. Flavour, however, is often neglected and here is yet another opportunity for the flavour scientists to play a part. The opportunities are not so obvious in fresh foods if their natural integrity is to be preserved, but easy opportunities occur during processing whether at home or in the factory. Even in fresh foods injections of tenderizers, water, etc. into meat, chickens, and so on are not unheard of; why not flavours ? This means that as fast as the agronomist is breeding flavour out of our foods, so the food scientist and flavourist should be studying the best flavoured species to capture their analytical secrets for grafting on to inferior flavoured products. Swindells Much of this flavour identification does of course occur, but not always with a differential slant on the flavour quality of different strains.How often have the assurances of the pundits been heard that white shelled eggs and battery eggs are just as good, taste just as good, as brown, free range eggs? The agronomic evolution must of course proceed since food is needed in ever increasing quantities. But the flavourist can work alongside to ensure that flavour is protected and even developed as the food yield advances. 4 Other Countries Many foods which grow elsewhere, especially those which travel well and to which we were introduced during England’s colonial pre-eminence are already familiar, and many of these foods now seem as natural in the U.K.as elsewhere for they can be grown sometimes with a little early protection from the winter frost. In recent times, however, less stable products from more tropical regions which will not grow here have been introduced, sometimes successfully, and perhaps the flavour scientist can help in capturing these as well as the long-familiar flavours for use in manufactured products here. Another aspect is consideration of foods and flavours for overseas territories, including the Third World, for virtually all our normal products have filtered through to white colonial areas, e.g. Bowyers and Walls sausages are as con- spicuous in Australian, South African, and Far Eastern supermarkets as they are in Tesco.However, in less developed communities this is not so and these people will often require somewhat different products and flavours from those with which they are more familiar, as selected and screened by religious and cultural taboos. Profound differences arise between regions, e.g. Far Eastern territories such as Malaysia and Singapore, dominated by a culturally advanced but different Chinese people having most delicate palates and with a high health food interest. It is evident from observing these marketplaces that the products enjoying high success do not conform closely to those in more western communities. There can be exceptions, however, where by conscientious effort a product at first absolutely unfamiliar can gradually succeed if it has intrinsic merit, e.g.‘Ribena’ is based on blackcurrants which simply do not grow in that part of the world, yet Hong Kong has the highest per capita consumption of ‘Ribena’ in the world. For territories having a shorter cultural history, rather different problems arise and here one may cite Nigeria where a high proportion of the mass popu- lation is emerging from a much simpler, primitive past. With less entrenched views it might be thought they would be more easily led, but new ideas are treated with suspicion, and new products and flavours need to be compatible with basic staples and life styles until or unless these change. Communication is important in any project but requires special care on development work for overseas markets.Typically, information will have to be obtained, expressed and communicated as follows: (a),Local market studies lead to definition of consumer product requirement ;(b),Product Brief relays this information to Product Development team, usually in U.K./Europe or U.S.A. ; Chemistry and Flavour. Part VI (c), Flavour Brief is passed to selected Flavour Houses, from which specially tailored flavours arise; (d), Product(s) arising from ‘informed selection’ by development team and specialist panels is passed to overseas market for consumer testing; (e), Consumer findings, and any Product revision needed are relayed back to Product Development team; etc. It will be very obvious that the opportunities for error or misunderstanding are high; and in the author’s experience there is a need for regular face-to-face discussions to reinforce and elaborate on the essential written briefs. Also, people concerned with overseas projects need to involve themselves in these markets; and similarly the flavourist will need to have a feel for the particular market, which can often be achieved by regular feedback from Regional Flavour House personnel.

ISSN:0306-0012    

DOI:10.1039/CS9780700212

出版商:RSC    

年代:1978

数据来源: RSC

摘要:

Collisional Transfer of Rotational Energy and Spectral Lineshapes By Krishnaji and V. Prakash DEPARTMENT OF PHYSICS, UNIVERSITY OF ALLAHABAD, ALLAHABAD, INDIA 1 Introduction Molecular collisions are an unavoidable occurrence in the gaseous state of matter. They influence greatly a wide range of observable properties of the gas. These properties include pressure-induced absorption of radiation,l-3 induced optical birefringence,* pressure-broadening of spectral line^,^-^ ultrasonic relaxation,s nuclear-spin relaxation,g dielectric properties,lOIl‘ transport properties,12 tempera- ture variation of virial coefficient~,129~3 abnormal rotational distribution of molecules in the interstellar space,l4 etc. A satisfactory explanation of these properties requires a thorough understanding of the collisions.Attempts to study the collisions experimentally have developed along two lines. In the first one, any of the properties mentioned above is studied experimentally and a back calculation is made to derive certain parameters of the forces of collisional intera~ti0n.l~ In the other, specially devised experiments16-21 are M. F. Crawford, H. L. Welsh, and J. L. Locke, Phys. Rev., 1949, 75, 1607. a A. A. Maryott and G. Birnbaum, J. Chem. Phys., 1962, 36, 2026, 2032. N. H. Rich and A. R. W. McKellar, ‘A bibliography on collision-induced absorption’, National Research Council of Canada publication number 15, 1975, p. 145. 4 A. D. Buckingham, J. Chern. Phys., 1959, 30, 1580; see also Adv. Chern. Phys., 1967, 12, 107.G. Birnbaum, Ah. Chem. Phys., 1967, 12, 487. a Krishnaji, J. Sci. Inn. Res., India, 1973, 32, 168. 7 H. Rabitz. Ann. Rev. Phys. Chem., 1974, 25, 155. a T. L. Cottrell and J. C. McCoubrey, ‘Molecular Energy Transfer in Gases’, Butterworth. London, 1961. * M. Bloom and I. Oppenheim, ‘Intermolecular Forces’ ed. J. 0. Hirschfelder, Wiley, New York, 1967, p. 549. lo D. R. Johnson, G. J. Oudemans, and R. H. Cole, J. Chem. Phys., 1960, 33, 1310. l1 T. K. Bose and R. H. Cole, J. Chern. Phys., 1970, 52, 140. l2 J. 0.Hirschfelder, C. F. Curtiss, and R. B. Bird, ‘Molecular Theory of Gases and Liquids’, Wiley, New York, 1954. l3 S. Kielich, Phj..sic.c., 1962, 28, 51 I. T. Oka, MPm. Sor. Roy. Science Liege 1972, 6, 111, 37. l5 Krishnaji and V.Prakash, Rev. Mod. Phys., 1966, 38, 690. A. P. Cox, G. Flynn, and E. B. Wilson, jun., J. Chem. Phys., 1965, 42, 3094. l7 T. Oka, J. Chem. PhJps., 1966, 45, 752, 754. Is T. Oka, ‘Advances in Atomic and Molecular Physics’ ed. D. R. Bates and I. Estermann, Academic Press, New York, 1973, Vol. 9, p. 127. lDT.Carrington, J. Chem. Ph~is.,1959, 31, 1418. 2u J. P. Toennies, Discuss. Faraday Soc., 1962, 33,96; Z. Phpsik., 1957, 182, 257; Z. PhJ*sik., 1966, 76, 193. See also J. P. Toennies, Chem. Soc. Rev., 1974, 3, 407. 21 M. Cavallini, M. G. Dondi, A. Scoles, and U. Valbusa, Chem. Phjis. Lett., 1971, 10, 22. Collisional Transfer of Rotational Energy and Spectral Lineshapes performed to study the collisions in a more direct manner.Experiments of the first kind have been successful in yielding the desired parameters within reason- able limits of uncertainty, but they do not give any information about the precise effect of a collision on the state of a molecule. Experiments of the second kind were devised to extract this information but they have, as yet, been only partially successful. What most of these experiments observe, is the over-all effect of a large number of collisions producing various changes in the state of the molecule; it is difficult to devise an experiment which will single out one collision or one type of collision and study its consequences in detail. This situation is also no better from the theoretical point of view. A theoretical investigation of collisions depends on a knowledge of the forces of interaction between molecules and on a subsequent formulation of a rigorous theory which relates to the experimental results. Unfortunately, the finer details of interaction potential are only partially understood.22 The formulations of rigorous theories for most of the collisional phenomena have been done, but they have the draw- back that they are not in an easily computable form so as to make them more meaningful.The theories which have been put in computable form, involve certain approximation methods which have limited validity. The problem is further complicated due to the possibility of simultaneous collision between three or more molecules. Thus molecular collisions present a challenging problem to experimentalists as well as to theoreticians.Discussion in this paper will be restricted to binary or two-body collisions and to the way they influence lineshapes in pure rotational spectra at low pres- sures. The term collision will be used to refer to the subtle encounters where two molecules pass each other with a fairly large distance between them and while doing so, they undergo a change either in their translational state of motion or in their internal energy state, which is expressed in terms of the electronic state, the vibrational state, and the rotational state. Most molecular collisions at room temperature or moderate temperatures are very low energy collisions and are not able to produce any electronic or vibrational excitation of the mole- cule.They do, however, induce changes in the rotational states of colliding molecules and are sometimes accompanied by small changes in their translational motion. Rotational changes occur in two forms: (i) the rotational phases of the molecules may be changed while their ro- tational energies may remain unchanged; these collisions are known as elastic or adiabatic collisions. (ii) the rotational energy may change as a result of a transition from one rotational state to another; these collisions are known as inelastic or non-adiabatic collisions. In general, both the collision-induced phase shifts and the collision-induced transitions influence various gaseous properties mentioned in the beginning, although their relative importance varies in each case.There has not been any J. 0.Hirschfelder, J. Chem. Phys., 1965, 43, S199. Krishnaji and Prakash direct study of collision-induced phase shifts. Direct studies of collision-induced transitions have, of course, been attempted in recent years. A brief description of these experiments and their important conclusions is given in the following section. 2 Collision-induced Transitions Consider an assembly of gaseous molecules. The molecules are constantly undergoing collisions and, in general, making transitions. However, the system as a whole is in a steady state having an equilibrium distribution of molecules among various rotational states. As such, these transitions can not be monitored directly.However a disturbance can be introduced into one of the rotational levels, where its effect on the neighbouring levels can be monitored and ascribed to collision-induced transfer of molecules. Two such methods have been devel- oped. They are based on four-level double resonance studiesl6.17 and optical fluorescence studies.lS Besides these, another very elegant method involving molecular beams20.21 has been used for a direct study of collision-induced transit ions. A. Four-level Double Resonance Experiments.-In a double resonance experi- ment23124 resonance absorptions due to two different transitions are observed simultaneously. Two types of double resonance experiments have been carried out: a three-level double resonance [Figure l(a)], in which the two transitions have one energy level in common, and a four-level double resonance [Figure l(b)] in which the four levels involved in the two transitions are all different.1 (a) (b) Figure 1 Energy level scheme used in a double resonance experiment (a)Three-level double resonance (6) Four-level double resonance In both cases, the experiment consists in pumping one of the transitions with high power radiation and observing its effect on the intensity of absorption due to the other transition. In the three-level double resonance experiments, the common energy level provides the necessary coupling between the two transitions. In the four-level double resonance experiment, however, the two 23 A. Battaglia, A. Gozzini, and E. Polacco, Nuovo Cimenfo, 1959, 14, 1076.*I T. Yajima and K. Shimoda, J. Phys. SOC.Japan, 1960, 15, 1668. Collisional Transfer of Rotational Energy and Spectral Lineshapes transitions do not have any common level and the only way they may be coupled is by means of collision-induced transitions. Pumping the transition 2 -1 of frequency, say vp, results in a highly non-Bdtzmann population in levels 1 and 2. In the extreme case of saturation, the populations in levels 1 and 2 are equalized. This non-Boltzmann population is partially transferred to the neighbouring levels by way of collision-induced transitions until a new steady state is reached. Under these conditions, populations in levels 3 and 4 become (n3 + 6n3) and (n4 + 6n4)respectively where n3 and 124 are the normal Boltzmann populations and an3 and 6n4 are the deviations introduced by the pumping.The intensity Z of the signal transition 4 -3 which depends on the population difference be- tween levels 4 and 3, also changes by an amount dZsuch that I n4 -n3 (1) Although the deviations an4 and 6n3 are small compared to n4 and n3, the value of (dZ/Z)can be relatively large because the denominator in equation (1) is also small. An attempt to measure this ratio was first made by Cox, Flynn, and WilsonlG in 1965, but it was unsuccessful because of an unsuitable choice of the levels and the low sensitivity of the instrument. The first successful experiment was done by Oka17 in 1966 for the CzH40 molecule, and was followed by a series of experiment~l89~~-~8for a very wide variety of systems.Figure 2 shows the block diagram of the apparatus used in Oka's experi- v* > Frequency Filter meter Power .meter+ Figure 2 Block diagram of a double resonance apparatus T. Oka, J. Chem. Phys., 1967, 47, 13. T. Oka, J. Chem. Phys., 1967, 47, 4852. T. Oka, J. Chem. Phys., 1968, 48,4919. as T. Oka, J. Chem. Phys., 1968, 49, 3135. a0 R. M. Lees and T. Oka, J. Chem. Phys., 1969, 51, 3027. 30 P. W. Daly and T. Oka, J. Chem. Phys., 1970, 53, 3272. 31 A. R. Fabris and T. Oka, J. Chem. Phys., 1972,56, 3168. 33 J. M. Roger, S. M. Freund, K. M. Evenson, and T. Oka (private communication). 33 A. M. Ronn and E. B. Wilson, jun., J. Chem. Phys., 1967, 46, 3262. 34 G.Roussy, J. Demaison, and J. et Baniol, Compt. rend., 1969, 269, 1080. 36 M. Takami and K. Shimoda, Japan. J. Appl. Phys., 1971, 10, 658. 36 P. J. Seibt, J. Chem. Phys., 1972, 57, 1343. 37 L. Frenkel, H. Marantz, and T. Sullivan, Phys. Rev. (A), 1971, 3, 1640. 38 J. B. Cohen and E. B. Wilson, jun., J. Chem. Phys., 1973, 58,442; 1973, 58,456. Krishnaji and Prakash ment~.l~9~5A high power microwave source provides the pumping radiation. Signal radiation is fed in the opposite direction from one of the standard low power microwave sources. The signal detector directly records the relative change in the intensity of signal line. It might be mentioned that effective pumping and good filtering of signal power from pump power are crucial to the success of the experiment.For further details of the experiment, reference can be made to the original papers, cited previously. However, the important results and conclusions are summarized below. Table 1 Observed values of (AIlI)for some systemsa Molecule Pumping transition Signal transition AIlI ( %) H2CO 1029 -1028 929 -+ 928 -30.7 f 2 827 -+826 -14.5 f 2 183,~3 183~5 173.15 3 173,14 -22.0 f 2 163~4-+ 163~3 -8.8 f 2 HDCO 153~3-+ 153~2 143~2+ 143~1 -21.3 f 2 --t 133,io -16.9 -t 2133~1 224~94224,~ 214,ia -214,17 -14.4 f 2 204,17 --t 204,16 -5.7 f 2 HCN J = 12 doublet J = 11 doublet -31.6 f 3 J = 10 doublet -21.0 f 3 J = 9 doublet -19.4 +_ 3 J = 8 doublet -14.0 +_ 3 DCN J = 13 doublet J = 12 doublet -26.4 k 2 J = 11 doublet -19.3 k 2 J = 10 doublet -17.7 f 2 J = 9 doublet -14.5 k 2 HzCCO 121,12 -121,ll -20.7 f 2 111,ll --f 111,lO -13.8 If: 2 101,lO -1019 -7.6 f 2 919 --t 918 -5.4 k 2 ORef.25 Experimentally measured values of the relative change in intensity (&/I) for some of the systems studied by this method are given in Tables 1 and 2. An examination of these and other values listed in the original papers leads us to the following conclusions : (i) collisions do provide substantial coupling between molecular rotational levels as is clear from the large values of (AIII)for many systems; (ii) collisional transitions are not random, but have definite preferences ; (iii) the preference varies from system to system resulting in a positive A1 for some systems and negative for others; 223 h, p” Table 2 Calculated transition-rates and (AIII)for the four-level systems in ammonia System(4 qro‘, Transition-rate (MHz/Torr)a us -ka t kd J.k, ky t kY’J. k, 1 (4,114391) 14.75 11.50 23.37 4.51 5.44 6.62 9.83 8.22 k 0.5 $, (5, 2)-(4,2) 13.91 14.28 37.76 7.09 9.80 3.06 4.87 4.62 k 0.5 (6,3)-(5,3) 10.83 15.39 48.52 7.31 12.96 1.66 2.17 1.91 & 0.4 % (5,1)--(4,1) 17.06 18.76 16.47 3.79 5.75 3.73 13.14 8.81 k 0.5 8 (8,2)-(7, 2) 9.41 22.58 22.16 2.97 8.02 0.89 6.24 3.75 +_ 0.5 f. (8,1)47¶ 1) 11.03 27.00 9.90 1.64 4.38 I .01 11.95 7.40k 1 Q rc t? (3, 1)--(2, 1) 11.84 7.40 36.81 5.83 6.18 10.53 5.01 4.4 k 0.2 s (4,2143, 2) 12.72 8.64 54.83 8.93 8.55 5.23 2.34 2.5 k 0.2 (5,3)-(4,3) 12.06 10.78 64.65 10.16 11.87 2.66 1.01 2.0 k 0.2 Q (6,4)-(5,4) 9.60 11.77 72.59 9.10 13.89 1.45 0.24 1.6 k 0.2 (7,5)-(6,5) 8.46 11.01 79.48 8.60 13.44 0.86 -0.06 0.9 k 0.2 (8,61479 6) 5.90 10.83 85.95 6.29 13.64 0.55 -0.17 0.6 k 0.2 25 b I(2,1)-(L 1) 10.77 74.56 9.00 -13.22 0.95 1.5 k 0.2 3.(3,2)-(2, 2) 11.00 -94.29 11.06 -8.22 -0.03 1.2 k 0.2 s (4,3)-(3,3) 12.14 -104.50 13.36 -4.38 -0.49 1.1 & 0.2 e (5¶4)44¶4) 11.95 -112.28 13.90 -2.36 -0.72 0.8 k 0.2 (6,5)-(5,5) 9.92 -1 19.08 11.87 -1.35 -0.69 0.63 k 0.2 (79 6)-(6,6) 8.77 -124.32 10.74 -0.87 -0.67 0.35 f 0.2 aRef. 39, bRef. 27. 39 V. Prakash and J. E. Boggs, J. Chern. Phys., 1974, 60, 2163. Krishnaji and Prakash (iv) collisional transitions with lAJl > 1 can also occur even though their probabilities may be small; (v) probabilities of collisional transitions depend on the collision-partners- rotational states involved etc.These experiments establish the existence of certain selection rules for collision, induced transitions and indicate the dependence of transition matrix elements on the collision-partners, rotational states etc. In order to gain more specific information regarding these selection rules and the probabilities of allowed transitions, experimental studies must be supplemented by some theoretical calculations. The general procedure followed is to assume an interaction potential and a set of selection rules, calculate the values of (LlI/l),and compare them with the measured values.These calculational methods have been developed and used by Prakash and Boggs.4O The energy level scheme commonly employed for such experiments consists of symmetrically split levels (e.g. Z-type doublets in HCN and DCN, k-type doublets in HKO, HzCCO etc., inversion doublets in ammonia, rotation- tortion doublets in CH30H etc.) as shown in Figure 3. Assuming that the colli- Parity J +2 T* J +1 J J -1 J-2 Figure 3 Energy level scheme relevant to a four-level double resonance experiment. Cob lisional transitiom are shown by wavy arrows, while radiative transitions by solid arrows ‘O V. Prakash and J. E. Boggs, J. ChPm. Phys., 1972, 57,2599. 225 Collisional Transfer of Rotational Energy and Spectral Lineshapes sion-induced transitions obey the same selection rules as the radiation-induced iransitions, the allowed collisional transitions for dipolar interaction are shown as at, a'$and 18 corresponding to AJ = + 1, -1, and 0, parity + --.The relative intensity change (dI/I)can be expressed in terms of the rates at which these transitions occur and is given by25 where k's represent various transition rates. These rates can be calculated theoretically40 provided that the transition matrix elements are known. The values of (dI/I)derived from equation (2) using theoretical values of transition rates totally disagreed with experimental values. This suggests the possibility of additional transitions which might be induced by a collision perhaps as a second order effect.The second order transitions that have been considered are those with AJ = If: I, parity f -+ & and those with AJ = k 2, parity k ---+ i-. These transitions are shown in Figure 3 as y?, 7'4,[I and 62, respectively. Considering these additional transitions, equation (2) is modified to The mechanism for second-order transitions is not completely known. They can occur by one or more of the following processes: (i) two successive dipolar transitions occurring in quick succession so as to take place within the duration of one collision; (ii) one single transition caused by the quadrupolar interaction; (iii) one single transition caused by an induction interaction. In general, all the three processes will contribute to the second-order tran- sitions.Using a second-order perturbation theory, Rabitz and Gordon41 have estimated for HCN that the first process is the dominant one. It has been assumed that this is also a valid assumption for other molecules. Thus, for example, a collision-induced transition from a state J = 4 to a state J = 6 is treated by using a matrix element which is a product of the dipole matrix elements for J = 4 -5 and for J = 5 -6. The values of transition rates and (dI/I)calculated from equation (3) for four level systems in pure ammonia are given in Table 2 along with the experi- mental values of (dI/I).It is seen that the agreement is very good in some cases, satisfactory in others while completely unsatisfactory in others still. An overall review of the results suggests the following conclusions.The collision-induced transitions do not seem to obey very rigid selection rules. H. A. Rabitz and R. G. Gordon, J. Chem. Phys., 1970, 53, 1831 Krishnaji and Prakash Those transitions which are allowed by the selection rules for relevant radiation- induced transitions, are preferentially induced. For example, if collisional interaction involves the dipole moment of the molecule, the dipolar transitions AJ = 0, k 1, parity k -+ T have a preference. However, if the collision is strong enough, other transitions, e.g. AJ = k 1, parity k -k and IdJ I > 1 may also be weakly allowed. The transitions with I AJ I > 1 have been experimentally ob- served using the methods of triple resonance42 and multiple res0nances.2~ Theoretically also the matrix element for a multistep transition n --f m -+ k etc.is not negligibly small in many cases. B. Optical Fluorescence E~perirnent~~~~~-~~---Itis possible to produce molecules in a single quantum state with specified electronic, vibrational, and rotational quantum numbers by absorption of monochromatic radiation which could be either a sharp emission line or a laser line that happens to coincide with the molecular absorption line. These excited molecules undergo a series of collisions with each other and with the molecule of their thermal bath. At each collision, they make transitions to other vibrational and rotational levels within the same electronic state. The sequence is terminated when the molecule loses its electronic excitation in a strong collision or by spontaneous emission giving fluorescence radiation.The whole process is shown in Figure 4. The wavelength of fluorescence radiation will depend on the rotational and vibrational state from which the emission takes place and its intensity will depend on the population in that state. Thus from an analysis of the fluorescence spectrum, it is possible to determine the population distribution among various rotational and vibrational states of the excited electronic state. It has been found that the probability of collisional transfer of rotational energy is much more than that of vibrational energy even in the excited electronic state. The observed distribution of molecules among various rotational states has to be attributed to collisional transitions and it can be interpreted in terms of various transition rates.This, in general, is a very difficult task, mainly because transitions are not restricted to nearest neighbour jumps. The practical approach followed in these studies is to assume a set of collisional transitions, calculate the rotational distribution on this basis and compare it with the observed distribution. It was found that the observed rotational distributions are not consistent with the optical selection rule AJ = 0 -t 1 and transitions with lAJ 1 > 1 are also ‘z R. M. Lees and T. Oka, J. Chem. Ph~rs.,1968, 49, 4234. ‘3 T. Carrington, J. Chern. Ph~.s.,1961, 35,807. u H. P. Broida and T.Carrington, J. Chem. Phj,s., 1963, 3&, 136. 4i J. I. Steinfeld and W. Klemperor, J. Chenz. Ph~,s.,1965, 42, 3475. ’6 K. M. Evenson and H. P. Broida, J. Cheni. Phjss., 1966, 44, 1637. 4i C. Ottinger, R. Velasco, and R. N. Zare. J. Chent. Phjvs., 1970, 52, 1636. C. Ottinger and D. Poppe, Chem. Phj.s. Letters, 1971, 8, 513. 4g D. L. Akins, E. H. Fink, and C. B. hloore, J. Chern. Phys., 1970, 52, 1604. E. H. Fink, D. L. Akins, and C. B. Moore, J. Chem. Phys., 1972, 56, 900. 51 K. Sakurai, S. E. Johnson, and H. P. Broida, J. Chern. Phys., 1970, 52, 1625. 227 Collisional Transfer of Rotational Energy and Spectral Lineshapes Rotational structure of -excited state Monochromatic exciting 1radiation spec tr urn Figure 4 Monochromatically excited fluorescence spectrum.Collisional transitions are shown by wavy arrows while radiative transitions by solid arrows important. In some systems, transitions with 1d.J I as large as 5 are also indicated. More definite selection rules are difficult to derive from these studies. These studies are complementary to the studies described in the previous section in the sense that they enable us to study the collisional transitions in the excited electronic state and also to study non-polar molecules. Practical difficult- ies in these experiments arise due to the exact coincidence required between the molecular absorption line and the monochromatic exciting radiation. With the advent of tunable lasers, it should be possible to overcome this difficulty.The other drawback in the method is that the lifetime of the excited state can- not be controlled. If this is too long, some of the molecules might undergo many collisions before giving out fluorescence radiation. Further, all molecules do not undergo the same number of collisions. In such cases, the detailed features of the collision-induced transitims do not show up in the fluorescence spectrum. C. Molecular Beam Experiments.20,21-In this method, the collision cross-section for a single specified rotational transition is measured in a molecular beam apparatus. The schematic arrangement is shown in Figure 5. An inhomo- geneous elxtrostatic field set up by a four pole20 acts as a state selector. With Krishnaji and Prakash State State Source Figure 5 Schematic diagram of a molecular beam apparatus for the study of collision-induced rotational transitions this field, molecules in a specified rotational state are separated out of a molecular beam.These are then focused into a collision chamber containing another gas. Molecules which are scattered by a small angle are then collected in a second inhoniogeneous field and are analysed for their rotational states. The main advantage of this method is that the analysis yields a single transition probability. However, the experimental difficulties involved in the technique limit the applic- ability of this method. 3 Rotational Lineshapes at Low Pressures Out of the various collisional phenomena mentioned in the introduction, spectral lineshapes have received wide attention because practical engineering applications as well as basic physical interests are involved.The subject has been reviewed from time to time.5-7 This section will, therefore, emphasize only the recent developments with brief critical remarks about the previous work. Rotational lineshapes under most laboratory conditions are attributable to molecular collisions. The collisions, in general, induce rotational transitions as well as rotational phase shifts. However, since the average kinetic energy of molecules at room temperature exceeds the rotational splittings in the micro- wave region, collisional transitions in microwave region are highly probable and should account for the dominant contribution to the microwave linewidths. This implies that the selection rules for collision-induced transitions must be known before any specific calculation of lineshapes can be made.Theories show that the linewidth of any transition i -f does not involve the probabilities of individual transitions i --+ i‘ orf+ f’,but the sum of probabilities of transitions from i andfto all possible levels i’ andf’, i.e.,the sum 2((Pii,) + <Pr,,) }. Thus i’ f’ only the total probability of transition out of a certain level is needed. Under these conditions, the assumption of optical selection rules for collisional tran- sitions is a reasonable approximation as explained below. The various transitions are competitive; one occurs at the cost of the other.As such, considering only the first-order transitions would amount to neglect of higher order transitions while overestimation of the first-order transitions; the total transition probability out of the level might not be substantially altered. Therefore, considering only the first-order transitions has been the universal practice in linewidth calculations. Collisional Transfer of Rotational Energy and Spectral Lineshapes The theories for low pressure lineshapes can be divided into two groups: (i) perturbation theories, (ii) non-perturbative theories. Perturbation theories have the advantage that they can be put in an easily computable form. The two theories which have been mostly used are the Ander- son and the Murphy-Boggs the~ry.~~,~~ The details of these theories can be seen either in the original papers or in the previous reviews.5-737 Anderson's theory has the advantage of incorporating collision-induced transi- tions as well as collision-induced phase shifts in the theory.The width is given as7 where p is the number density of perturber molecules, ot and ofare the total cross-sections for transitions out of the levels i and f, respectively, and ay is related to the phase shifts. This theory, however, has a drawback that an arbitrary interpolation scheme has to be used for the interruption function S2(b) in the regions where perturbation theory is not valid. Anderson suggested three different schemes: (1) (1 -cos[2 Sz(b)]*}to be used in place of the function Sz(b), (2) The perturbation theory expression for S4b) is used for b from 00 down to bo defined by Sz(b0) = 1. From bo to 0, S3(b) is assumed to be unity.(3) (1 -exp [-2 Sz(b)]}is used in place of the function Sz(b). The second scheme has been almost always used even though none of the three has any rigorous justification. An analysis of the expression will show that it can exceed unity at small b values and is likely to oscillate in that regi~n.~ With this in mind, scheme (1) is qualitatively more correct. Johri and Srivastava58 have recently suggested an alternative interpolation scheme in which the function (1 -exP[-Sz(b)Il is used in place of the function Sz(h). They used this scheme to calculate the foreign gas broadening of OCS and CH3*lBr lines by nonpolar perturbers5Q and obtained better agreement with experiments.Their scheme is better in the P. W. Anderson, Phys. Rev., 1949, 76, 647. 63 C. J. Tsao and B. Curnutte, jun., J. Quant. Spectroscopy. Radiative Transfer, 1962, 2, 41. Krishnaji, Research Report No. 3, Microwave Laboratory, Department of Physics, University of Allahabad, India (December 1964). 56 J. S. Murphy and J. E. Boggs, J. Chem. Phys., 1967,47,691; 1967,47,4152; 1968,49,3333; 1969, 50, 3320; 1969, 51, 3891 ; 1971, 54,2443. O6 J. E. Boggs, 'Molecular Spectroscopy: Modern Research' ed. K. Narahari Rao and C. Weldon Mathews, Academic Press, New York, 1972, p. 49. 67 P. R. Berman, Appl. Phys., 1975, 3, 283. 68 G.K. Johri and S. L. Srivastava, Chem. Phys. Letters, 1976, 39, 579. G. K. Johri and S. L. Srivastava, Ind. J. Pure Appl. Phys., 1976, 14, 917. 230 Krishnaji and Prakash sense that a theoretical justificationm has also been obtained whilst the three schemes suggested in Anderson's original paper were quite arbitrary. The Murphy-Boggs theory,55 although not a conventional perturbation method, is a first-order treatment. The width is given as The effects of collision-induced phase shifts are thus ignored. This has been considered as a reasonable approximation for pure rotational linewidths in the microwave region of the spectrum. However, a recent calculations1 and several experimental observations of pressure-induced shifts in the microwave region clearly indicate that the effects of collision-induced phase shifts even in the microwave region are not completely negligible.The Murphy-Boggs theory has a clear advantage over the Anderson theory in that no arbitrary interpolation schemes are needed and a smooth transition from large b to small b is obtained as a natural consequence of the theory. The theory has recently been extended by Mehrotra and Boggs62 to include the higher order multipole interactions and also to be applicable to the rotational lines in the excited vibrational states in the infra-red region. Neither of the two theories discussed above is able to explain satisfactorily the quantum number dependence of rotational linewidths.63 Pressure-induced shifts of microwave spectral lines have received much less attention than widths, primarily because they are expected to be small and their measurement is difficult.Until 1971, the only measurements reported were those by MatsuuraU and Shim~da~~ for the ammonia inversion line and these were detected while working with the ammonia maser. In 1971, Story et aZ.66reported systematic measurements of the shifts of (J,K) = (12, 12) inversion line of NH3 perturbed by NH3, He, Ar, OCS, C02, N2, CH3CN, CHBF, and CH4. They used a highly sensitive microwave spectrometer designed for the purpose and obtained negative shifts ranging from 1-25% of the linewidth in the various cases. The shifts of rotational lines were first measured by Luijendijke' for J 0 --+ 1 line of CH3C1 and J 1 -2 and 2 -+ 3 lines of OCS.Hewitt and Parsons68 also mea- sured the shifts of ammonia inversion lines* and CH3C1 and OCS rotational lines. * Shifts of ammonia inversion lines have also been measured (R. K. Kakar and R. L. S. C. Mehrotra, and J. E. Boggs, J. Amer. Chem. SOC.,in press). Poyntor, J. Mol. Spectroscopy, 1975, 54, 473, and calculated theoretically (K. Tanaka, 6o G. K. Johri and S. L. Srivastava, Chem. Phys. Letters, 1977, 45, 364. J. Jarecki and R. M. Herman, J. Quant. Spectroscopy. Radiative Transfer, 1975, 15, 707. S. C. Mehrotra and J. E. Boggs, Indian J. Pure Appl. Phys. (in press). 63 See the earlier reviews on the subject or, for example, G. P. Srivastava and D. Kumar, J. Phys. B; Atom. Molec.Phys., 1976, 9, 651. " K. Matsuura, Y. Sugiura, and G. M. Hatoyama, J. Phys. SOC. Japan, 1957, 12, 314. 65 K. Shimoda, J. Phys. SOC.Japan, 1957, 12, 558. 66 I. C. Story, V. I. Metchnik, and R. W. Parsons, J. Phys. B; Atom. Molec. Phys., 1971, 4, 593; Phys. Letters (A), 1971, 34, 59. O7 S. C. M. Luijendijk, Ph.D. dissertation, University of Utrecht, The Netherlands (Decem- ber 1973). P. L. Hewitt and R. W. Parsons, Phys. Letters (A), 1973, 45, 21. 231 Collisional Transfer of Rotational Energy and Spectral Lineshapes MacGillivrays9 has very recently modified the spectrometer used by Story et aLs6 and has measured shifts of rotational lines of CH3C1, OCS, CHJ, CHsCN, and CH30H. Parsons et al.70attempted to explain the observed shift of the ammonia inversion line on the basis of Anderson’s theory.They found that this theory allows no contribution for lineshifts from multipolar interactions. The induction, dispersion, and exchange interactions do make a finite contribution provided that the dipole moment and/or polarizability of the ammonia molecule change when the molecule makes a transition.* Even after an approximate estimate of this change, experimental agreement is not good. Frost7l has recently extended Anderson’s theory to permit calculations of shifts of spectral lines arising from multipolar interactions. The theory has been used to calculate the widths and shifts of several ammonia inversion lines broadened by NH3, OCS, and CHsC1 and of self-broadened CHsC1 JO -1 and OCS 1 --f 2 rotational lines.The agreement with measured widths and shifts is not good, though the sign and the order of magnitude of lineshifts have been correctly obtained. The disagreements have been attributed to the cut off procedure used for the interruption function Sz(b). A significant advance in the theoretical formulation for the lineshape problem has been made very recently by Mehrotra and Bogg~.~~ This is a more general theory in which the effects of collision-induced transitions as well as collision- induced phase shifts have been considered in a more rigorous way. It was shown that the two earlier theories are limiting cases of this theory. This theory has been applied very successfully to the rotational transitions in OCS.Its success in explaining the quantum number dependence of OCS linewidths is particularly noteworthy. Further, the lineshifts have also been predicted to the correct order of magnitude. The non-perturbative theories of lineshapes can be fully quantum-mechanical,73 semiclassica1,74~75 or classical.76 The quantum-mechanical theory is obviously the more appropriate one, but the calculations become much more involved and are not feasible except for the simplest systems. On the other hand, fully classical treatments have the disadvantage that any quantum effects do not show up. The semiclassical treatments constitute a compromise between the two. In all of these, the translational degrees of freedom are treated classically while the internal degrees of freedom are treated quantum mechanically.The recent treatment by Mehrotra and Bogg~~~reveals important information * This was later found to be incorrect by Frost (ref. 71). as W. R. MacGillivray, J. Phys. B; Atom. Molec. Phys., 1976, 9, 251 1. 70 R. W. Parsons, V. I. Metchnik, and 1. C. Story, J. Phy~.B; Atom. Molec. Phys., 1972, 5, 1221. 71 B. S. Frost, J. Phys. B; Atom. Mofec. Phys., 1976, 9, 1001. 7a S. C. Mehrotra and J. E. Boggs,J. Chem. Phys., 1977, 66, 5306. 73 C. 0. Trindle and K. H. Illinger, J. Chem. Phys., 1968, 48, 4415. 74 H. A. Rabitz and R. G. Gordon, J. Chem. Phys., 1970, 53, 1815. 75 S. C. Mehrotra and J. E. Boggs, J. Chem. Phys., 1975, 62, 1453. 76 H. J. Liebe, M. C. Thompson, jun., and T. A. Dillon, J. Quant. Spectroscopy.Radiative Transfer, 1969, 9, 3 1. 232 Krishnaji and Prakash regarding the transition probabilities during a strong collision. In a strong collision, as the molecules move closer, the transition probability out of the level increases rapidly until the population in the coupled states has increased sufficiently and the molecules are then transferred back into the initial state. Thus the transition probability out of the level, averaged over the entire trajectory of the molecule, could be small in a strong collision, contrary to the widely held assumption that it is unity. This could have a strong impact on the calculated linewidths and could perhaps explain the discrepancies between experimental and theoretical widths where strong collisions are involved.Among the recent experimental developments for studying rotational line- shapes, mention may be made of microwave refraction spectro~copy,~~ balanced bridge spectrometry,66 lineshape fit method,77 and transient nutation spectro~copy.~* In refraction spectroscopy, the linewidth is derived from pressure-dependence of the real part of the refractive index of the gas. This method has several ad- vantages6 over the conventional absorption spectroscopy and has been used for finding the self- and foreign-gas broadened widths of H2O lines.76~79~80 In the balanced bridge spectrometer used by Story et aZ.,66 a higher sensitivity is achieved by balancing out the effects of the standing waves in the absorption cell.Radiation from the klystron is passed through four identical waveguide cells in parallel. The cells are arranged in two pairs, one member of each pair being a dummy whose function is to balance out the effects of standing waves. In one of the other cells, the gas is introduced at a fixed pressure while in the fourth cell the pressure of the gas can be varied. The width is measured by the usual derivative method. The method is particularly suited for lineshift measure- ments which require high sensitivity. MacGillivray69 has recently improved upon this spectrometer. Rectangular waveguide cells were used instead of the circular waveguide cells. This reduced the standing waves and the two dummy cells could be avoided without adversely affecting the sensitivity.Further, a double phase-lock-loop was used for stabilizing the klystron frequency. It has been claimed that with these improvements, shifts as small as 500 Hz could be measured at an operating frequency of 100 GHz. The lineshape fit method approaches the problem in a more fundamental way. Most of the other methods assume a Lorentzian lineshape and then make use of one or two points on the observed line to compute the width. However, for a Lorentzian line, all the points on the line contain information about the lineshape parameters and it would be more appropriate to make use of all the points for deriving these parameters. The line can, therefore, be recorded point by point and fitted digitally to the Lorentzian lineshape function using the peak intensity and the linewidth as adjustable parameters for obtaining a least square fit. Some of the important sources of error in other measurements are the un- "I D.S. Olson, C. 0. Britt, V. Prakash, and J. E. Boggs, J. Phys. B; Atom. Molec. Phys., 1973, 6, 206. " A. H. Brittain, P. J. Manor, and R. H. Schwendemann, J. Chem. Phys., 1973, 58, 5735. 7s H. J. Liebe and T. A. Dillon, J. Chem. Phys., 1969, 50, 727. T. A. Dillon and H. J. Liebe, J. Quant. Spectroscopy. Radiative Transfer, 1971, 11, 1803. 233 CoIIisional Transfer of Rotational Energy and Spectral Lineshapes detected overlap of the observed line with neighbouring lines and with their stark lobes, excessive distortions due to the applied modulations etc.All these errors distort the lineshape and are readily detected if the entire line is fitted to the theoretical Lorentzian function though they might escape detection if only two points are being measured. The only difficulty with this method is that absorption has to be measured at a very large number of discrete frequencies. This is very time-consuming, however with the development of a computer- controlled spectrograph,s1 it should be possible to overcome this difficulty. The experiments described earlier for lineshape studies are all in the frequency domain. A time-domain experiment has also been devised which can give informa- tion about the collisional relaxation. This is the transient-nutation spectroscopy in which the spectral line is monitored after passing a strong radiation pulse or after rapid stark switching of the spectral line through the frequency of observation.In either case, a damped ringing signal is observed. The damping is due to the collisional relaxation. It has been suggested that the transient- nutation decay rate is essentially the same as the linewidth although the measure- ments on OCS78 and NH382 show some discrepancy with the corresponding microwave determined timewidths. It needs to be ascertained whether the relaxa- tion mechanisms involved in the two cases are the same. Note added in proof. The good agreement obtained between perturbation theories and experimental results in several cases should be considered mainly as fortuitous. The perturbation theory involves underestimation of the function S2(6,v) for large impact parameters, the assumption of unity Sz(6,v) is an overestimation as is clearly shown in the Mehrotra-Boggs theory.75 Therefore, the two errors may cancel when averaging over all impact parameters. This may lead to very good agreement but the J-dependence and the temperature- dependence may still remain unexplained. The authors are thankful to Dr. S. C. Mehrotra for bringing out this point and for making his preprints avail- able prior to publication. S. C. Mehrotra, Ph.D. dissertation, University of Texas, Austin, Texas, U.S.A. (May 1975). 8z J. M. Levy, J. H. S. Wang, S. G. Kukolich, and J. 1. Steinfeld, Phys. Rev. Letters, 1972, 29, 395.

ISSN:0306-0012    

DOI:10.1039/CS9780700219

出版商:RSC    

年代:1978

数据来源: RSC

摘要:

Contributions of Pulse Radiolysis to Chemistry By J. H. Baxendale CHEMISTRY DEPARTMENT, MANCHESTER UNIVERSITY, MANCHESTER M13 9PL M. A. J. Rodgers CENTER FOR FAST KINETICS RESEARCH, UNIVERSITY OF TEXAS, AUSTIN, TEXAS 78712, U.S.A. 1 Introduction The understanding of the primary chemical changes which occur when high- energy radiation (X-and y-radiation, fast electrons, etc.) is absorbed in many pure liquids and gases is now fairly well developed, and the primary species which are responsible for the ultimate products are known for a variety of simple molecules, so much so that there is now much more activity in the use of these radiations to generate such species for chemical studies than in the effects of the radiation per se. As with photochemistry there are two experimental approaches, viz.the continuous-irradiation or steady-state technique and the very short high-intensity pulsed-radiation technique. In this review we aim to give a broad account of how the latter has been applied in various areas of chemical research and has made valuable contributions. We shall be concerned with liquid-phase and a little with gas-phase systems and in almost all cases we shall be dealing with the chemistry of the solute in dilute liquid systems or as a minor component in a gas mixture. The reason for this is that most applications of the technique depend mainly on the reactions of the ions, atoms, free radicals, and excited states which are formed extremely rapidly on the absorption of the radiation. In a dilute mixture of two components such as a dilute solution, these species derive almost entirely from the major component, i.e.the solvent, since the radiation is absorbed by each component approximately in proportion to the mass fraction of the component. Hence, for example, in a 0.1 mol dm -3 solution composed of average-sized molecules the solvent will absorb about 99 % of the incident energy and will pro-vide about the same proportion of the primary reacting species which can react with the solute. Thus any chemical effects on the solute are produced indirectly. The primary species are themselves short-lived and their reactions frequently give short-lived products. The pulse radiolysis technique, which incorporates rapid monitoring methods, is used to follow both.2 Primary Processes1 Excitation and ionization (usually approximately equal amounts per unit of A more detailed treatment of the background radiation chemistry including the primary processes involved is to be found in (a) A. J. Swallow, ‘Radiation Chemistry’, Longmans Green, New York, 1973; (6) 5. W. T. Spinks and R. J. Woods, ‘An Introduction to Radiation Chemistry’, 2nd Edn., Wiley, New York, 1976; and in the specialist articles in (c) ‘Advances in Radiation Chemistry’, ed. M. Burton and J. L. Magee, Wiley, New York, Vols. 1-6, 1959-1976. Contributions of Pulse Radiolysis to Chemistry energy absorbed) follow absorption of energy and for simple gaseous molecules the details of the ionization process can sometimes be found using high-pressure mass spectrometry and the total amounts of ions formed per unit of energy absorbed can be measured using ionization chambers. In general for y-or fast- electron radiation 3-4 ion pairs are produced per 100 eV absorbed in gases, and to a first approximation this generally applies also to liquids but cannot always be assumed.Frequently these ions react with the parent molecules and the nature and rates of such reactions can often be established by mass spectrometry. The approximate time-scale of the events which occur in an aqueous solution between the absorption process and the involvement of a minor component in chemical reaction is as follows : Ionization by photon 10-18 s Excitation or ionization by fast electrons 10-l6-10 -15 s Ion-molecule reactions with solvent 10 -14 s Electrons thermalized 10-13 s Electrons and cations solvated 10-11 s Chemical reactions by diffusion begin 10 -10 s The details of the chemically important final stages would be: HzO -+H20++ e-H20++ H,O+ H,O+ + OH H20+ H + OH The major reactive species available for reaction with solutes are found to be OH and e -which is rapidly solvated to ea,.Minor amounts of H, H2, and H202 are also detectable at the early stages, the molecules being formed largely by dimeri- zation of eag and/or H and of OH. In acid solution eag rapidly gives H : eag + H,O+ -+H + H,O; k = 1.2 x 1Olo dm3 mol-l s-l (4) It is clearly essential to simplify a system by restricting reactions to those of H, OH, or ea, only.Reactions of H atoms can be isolated by using acid solutions to remove ea; quickly and adding t-butyl alcohol, which reacts with OH to give an unreactive butanol radical. The latter usually disappears by dimerization or dismutation. Reactions can be restricted to those of egi by using neutral solutions containing t-butyl alcohol. A small amount of H atoms remains but can usually be ignored or accounted for. If a solution saturated with N2O is used ea, is rapidly removed : N,O + H,O + eag+ N, + OH + OH-; k = 6 x 10edm3mol-1s-1 (5) and OH reactions can be isolated. Thus in irradiated aqueous systems there is available a very strong oxidizing species in the radical OH (E" = 1.9 V), an equally potent reducing species in eG (E" = -2.9 V), and H atoms, any one of which can be produced in nanoseconds using a pulsed electron beam and whose reactions with a large variety of organic Baxendale and Rodgers and inorganic solutes have been studied on time-scales of this order.The elucida- tion of processes involved at initial and subsequent stages is facilitated by a knowledge of the initial concentration of the primary species. These can be calculated from the energy input per electron pulse and the yield per unit energy, usually expressed by the symbol G, or sometimes g,which indicates the number of species per 100 eV absorbed. Thus in neutral water G(0H) = 2.7 radicals per 100 eV G(H) = 0.55, and G(e,-) = 2.7. The processes described above for water have analogies in other polar liquids and it may be generally stated that in liquids with a moderately high static dielectric constant the products of initial ionization exhibit separate chemical existence.With low dielectric constant liquids, however, ion recombination is usually very rapid and although added solutes do show evidence of electron- capture or charge-transfer processes with primary ions observed ion-yields are lower than in water, and need much higher solute concentrations for complete scavenging. Nevertheless the pulse radiolysis technique is a very convenient way of generating and following the reactivities of molecular cations and anions in, for example, a hydrocarbon liquid. In the case of liquid aromatics (benzene, toluene, etc.) initial ion recombination leads to the formation of molecular excited states of the solvent in high yield.These serve as convenient sensitizers for production and observation of solute excited states. The transfer of charge to a particular solute from primary ions, or excitation from primary excited solvent states, is governed, where thermodynamically feasible, only by the solute concentration. It is therefore possible, for example, to populate one solute anionic species and observe electron transfer to another low- concentration solute of higher electron affinity. In this way pulse radiolysis studies allow observation of the approach to equilibrium in electron-charge or energy- transfer reactions. 3 Experimental Methods2 A.Radiation Sources.-Although pulsed X-ray sources have been used, because of the higher deposition energies possible, the most common and convenient sources are pulsed electron accelerators. Three types are in current use, viz. Van de Graaff, microwave, and Febetron field emission machines. The time range over which investigations are possible is determined by the shortest pulse duration available which can deposit sufficient energy to produce measurable changes in a system. All these machines can be designed to deliver usable pulses of a few nano- seconds and in the particular case of the microwave linear accelerator (LINAC), because such pulses have a microstructure corresponding to the microwave Fuller descriptions of machines and pulse radiolysis techniques are given in (a) M.S. Matheson and L. M. Dorfman, ‘Pulse Radiolysis’, M.I.T. Press, Cambridge, Mass., and London, 1959; (b)L. M. Dorfman, ‘Investigations of Rates and Mechanisms of Reactions, Part 11’, in ‘Techniques of Chemistry’ VI Series, ed. A. Weissberger, Wiley-Interscience, New York, 1974; and in articles in (c) ed. G. E. Adams, E. M. Fielden, and B. D. Michael, ‘Fast Processes in Radiation Chemistry and Biology’, Wiley, Bristol, 1975. Contributions of Pulse Radiolysis to Chemistry frequency, pulses of tens of picoseconds duration are available. Energy input per pulse, and therefore product concentration, is determined by the electron-beam current and the effective acceleration voltage. Van de Graaff machines commonly operate at 2 MeV and 2 A, LINACs at 4-12 MeV and 2-12 A, and the Febetron field emission machine at 2 MeV and up to several thousand A, although at a longer pulse length of ca.30 ns. Energies much lower than 2 MeV become inconvenient since the electrons are not absorbed uniformly in liquids or solids over an appreciable depth of material. This introduces problems into the optical monitoring process and can complicate kinetic analysis owing to lack of homo- geneity in the distribution of species. Usually the machine output is chosen so that a single pulse gives 1-10 x 10-5 mol dm-3 products, the particular value being chosen to suit the aims of the experiment and the monitoring equipment being used. B. Detection and Measurement of Species Produced.-The three principal tech- niques used to investigate the species and reactions which occur following the absorption of an electron pulse are optical spectroscopy (emission and absorp- tion), conductivity, and electron spin resonance.Of these, optical absorption spectrometry is by far the commonest. The optical detectors in current use for fast monitoring cover the wavelength range from 3 to 0.2 pm. Photomultipliers are used from 0.2 to 0.8 pm, silicon photodiodes from 0.4 to 1.0 pm, germanium diodes from 0.7 to 1.5 pm and an indium arsenide diode from 1 to 3 pm. Apart from the last of these, the response time of these detectors with their associated amplifiers can be as low as a few nanoseconds.3 The signal/noise characteristics of the solid-state detectors are such that meaningful measurements can be made on absorption signals as low as 0.1 % absorption even though the signal decays in tens of nanoseconds.For measurements in the picosecond time range special and ingenious experimental arrangements have been devised which do not require the use of exceptionally fast detector^.^ Conductivity is used in two areas. One is the study of the kinetics of reactions and mobility characteristics of the ions-electrons and cations-produced in hydrocarbon systems. The other is in aqueous systems where, in addition, the ionization characteristics of transient species are followed. In the former a straight-forward d.c. method is used in which potentials up to several kV are applied to a simple conductivity cell and the current changes which follow the absorption of an electron pulse are monitored.In hydrocarbons, where electrons have mobilities up to lo5 times as great as in water, this is a very fast and sensitive monitoring system which can be used to follow the behaviour of ca. 10-8 M electrons on nanosecond time-~cales.~ In aqueous systems the background conductivity and problems associated with polarization have led to the use of the a.c. bridge. The time response is therefore limited but response times approaching 0.1 ,us have J. H. Baxendale, C. Bell, and J. Mayer, Internat. J. Radiat. Phys. Chem., 1974, 6, 117. J. W. Hunt, Adv. Radiat. Chem., 1976, 5, 185. J. H. Baxendale, J. P. Keene, and E. J. Rasburn, J.C.S.Faraday I, 1974,70, 718. Baxendule and Rodgers been attained and micromolar concentration changes can be followed.6 Another approach to conductivity measurements has been the use of microwave absorp- tion.7 A recent development has been the application of time-resolved polaro- graphy8.9 to investigate the transient species produced following electron absorption. In this way information on the oxidation-reduction properties of free radicals, transient ions, etc. has been documented. A technique designed to observe degradation in macromolecular systems utilizing changes in intensity of scattered light as a consequence of electron-beam irradiation has been de~cribed.~ The time dependence of chain fission and frag- ment separation has been followed with this technique both for synthetic poly- mersg and for DNA solutions.1° The signals generated in the detection circuits described above are displayed and recorded either with a fast oscilloscope plus Polaroid camera combination or, more recently, using a digital signal analyser which converts the analogue wave- form into a set of amplitude-time co-ordinates which can be directly read into an on-line computer for analysis.In addition to the long-standing application of e.s.r. in the identification of very long-lived free radicals produced in solids or in frozen liquid systems at liquid nitrogen temperatures, it is now possible to apply the technique to transient species having lifetimes greater than a few microseconds.11 Using repetitive pul- sing and sweeping the field incrementally, spectra of the species can be built up, and in addition by observing a signal at a fixed field concentration changes can be followed with time.This technique has the obvious advantage of being able, in general, to identify positively the free radicals observed. 4 Inorganic Chemical Reactions and Species In inorganic chemistry the technique of pulse radiolysis has been applied mainly to aqueous systems and in particular to oxidation and reduction reactions. As outlined above the species produced in neutral water by high-energy radiation are OH and eag in about equal amounts together with a smaller amount of H atoms and hydrogen peroxide. Figure 1 shows a composite absorption spectrum of the transient species formed in aerated water.In acid solutions eag is rapidly transformed into H by reaction with H&. Reactions of OH, eag, and H with a wide variety of inorganic solutes have given information on reaction rates, new reactions, and previously unknown species. Many of the latter have been characterized by their optical absorption spectra, lifetimes, and e.s.r. spectra and some can now be seen to be intermediates in reactions previously investigated by less incisive techniques. K.-D. Asmus, ref. 2c, p. 40.’P. R. Infelta, M. P. de Hass, and J. M. Warman, Internat. J. Radiat. Phys. Chemt., in press. (a) A. Henglein, Electroanalyf. Chem., 1975, 9, 163; (b) M. Gratzel, K. M. Bansal, and A. Henglein, in ‘Radiation Research : Biomedical Chemical and Physical Perspectives’, ed.0. F. Nygaard, H. I. Adler, and W. K. Sinclair, Academic Press, New York, 1975, p. 493. G. Beck, J. Kiwi, D. Lindenau, and W. Schnabel, European Polymer J., 1974, 10, 1069. lo D. Lindeman, U. Hagen, and W. Schnabel, Z. Naturforsch., 1976, 31c, 484. l1 R. W. Fessenden, ref. 2c, p. 60. Contributions of Pulse Radiolysis to Chemistry 105 104 -i lo3-E Imz 4 lo2 1Y C Q).C1u8i Q)8 10’ C .-0 c,uC .m c,3 loo 10-lo-; 200 300 400 500 600 700 h/nm Figure 1 Absorption spectra of the species involved in water radiolysis (Reproduced by permission from ‘Pulse Radiolysis’, ed. M. S. Matheson and L. M. Dorfman, M.I.T.Press, Cambridge, Mass., and London, 1959) A. Reactions of OH.12-This is a well established species whose existence in aqueous solution was deduced many years ago from the behaviour of systems containing hydrogen peroxide and whose reactions have been studied using Fenton’s reagent or photolytic techniques. The more powerful pulse radiolysis technique has considerably extended this field. The OH radical itself can be observed in absorption (A,,, = 230 nm), as can its ionized form 0-(Amax = 240 la L. M. Dorfman and G. E. Adams, ‘Reactivity of the Hydroxyl Radical in Aqueous Solution’, Nut. Stand. Ref. Data Ser., Nut. Bur. Stand., 1973, Vol. 46. Baxendale and Rodgers nm). For the ionization pKa = 11.9 at 298 K and the heat of ionization is 40kJ mol-1.Both species are very reactive in electron or hydrogen abstraction reactions but in general 0 -is less effective than OH, owing, no doubt, to its lower electron affinity.l (i) Metal ions. The oxidation of most simple aquated metal ions whose next higher valency states are stable have been followed,l3 e.g. Fe2+, Co2+, Ag+, Mn2 +, Ce3+,Eu~+, and Sm2 +.Oxidation of Cu2 + to the much less stable CuIII has also been observed,l* and the acid-base equilibria involving [CuOHI2 +, [Cu(OH)2]+,and Cu(OH)3 have been identified by spectral and conductivity measurements.15 Ions such as T1+ and Cr3 + requiring two or more equivalents to produce the next stable state give transient intermediate states TlII and CrIV which decay by dismutation to the upper and lower states.16 All these reactions occur with rate constants ca.lo8 dm3 mol-l s-l. (ii) Metal ion co-ordination complexes. Oxidation to the next higher state almost invariably occurs by outer-sphere electron transfer, giving a product which may be stable, e.g. [Fe(CN)6I3-or [Ag(NH3)2I2+, or unstable to various degrees. In the latter case the subsequent reaction is frequently dismutation again, as with17 the transient [Ru(NH3) +,various NiIII ammine complexes, and [Pt(en)3I3 +.Some-times the higher state is not stable with the ligand configuration of the lower state, so that ligand loss or rearrangement rapidly follows the oxidation step. This apparently occurs1* when [Au(CN)2] -is oxidized to the AuII state and [PtC14I2 -to the PtIII state and is very obvious when the stable [Ru(NH3)5(N2)I2+ is oxidized to [Ru(NH3)5(N2)I3 +.Here the RuIII state decays over about 10 ms, the dinitrogen ligand being replaced by OH -.An alternative reaction path to electron transfer from the metal to OH is the reaction of OH with a ligand. A typical example is benzoatopenta-ammine-'Colll, which gives an intermediate formed by the addition of OH to the benzoate ring- a reaction which also occurs with benzoate itself.lg Following the addition, the intermediate apparently may react either in a second-order dismutation step to give presumably new CoIJ1 complexes which have oxidized and reduced benzoate ligands or undergo intramolecular electron transfer to the metal to give a CoT1 l3 (a) G.G. Jayson, B. J. Parsons, and A. J. Swallow, J.C.S. Faraday I, 1973, 68, 2053; (b)G. V. Buxton, R. M. Sellers, and D. R. McCracken, ibid., 1976, 72, 1464; (c) J. Pukies, W. Roebke, and A. Henglein, Ber. BunsengesellschaftphI..s. Chem., 1968,72,842; (d)D. M. Brown, F. S. Dainton, D. C. Walker, and 5. P. Keene, 'Pulse Radiolysis', Academic Press, London, 1965, p. 221 ;(e) J. C. Muller, C. Ferradini, and J. Pucheault,J. Chim.phys., 1966, 63,232; (f)A. K. Pikaev, G. K. Sibirskaya, and V. F. Spitsyn, DokfadyPhys. Chem., 1973, 209, 339. l4 J. H. Baxendale, E. M. Fielden, and J. P. Keene, ref. 13d, p. 217. l5 (a) D. C. Barker and P. Fowles, Trans. Furuduy SOC.,1970, 66, 1661 ; (b) D. Meyerstein, Inorg. Chem., 1971, 10, 638. (a)B. Cercek, M. Ebert, and A.J. Swallow, J. Chem. SOC.(A),1966, 612; (b)J. Barrett and J. H. Baxendale, unpublished results. l7 (a)J. H. Baxendale, M. A. J. Rodgers, and M. D. Ward, J. Chem. SOC.(A), 1970, 1246; (b)J. Lati and D. Meyerstein, Inorg. Chem., 1972, 11, 2393, 2397; (c) D. K. Storer, W. L. Waltz, J. C. Brodovitch, and R. L. Eager, Internut. J. Radiat. Phys. Chem., 1975, 7, 693. (a)A. S. Ghosh-Mazumdar and E. J. Hart, Adv. Chem. Ser., 1968, No. 81, p. 193; (b)G. E. Adams, R. K. Broszkiewicz, and B. D. Michael, Trans. Faraduy SOC.,1968,64, 1256. lS M. Z. Hoffman and M. Simic, J. Amer. Chem. SOC.,1970, 91, 5533. Contributions of Pulse Radiolysis to Chemistry complex with an oxidized benzoate ligand. This rapidly dissociates to Co2 + and free ligands.Aliphatic ligands with abstractable hydrogens may also be attacked, as are ethylenediamine complexes of CuII and other metals. 2O The subsequent reactions here lead to the deamination of ethylenediamine. (iii) Non-metallic anions. Oxidation of NO2 -, N3 -,As02 -,and Se032 -by OH occur readily, but unexpected oxidations which have been shown to occur are those of SO42 -, HC03 -, acd C032 -, which by simple electron transfer12 give SO4 -, HC03, and CO3 -. These transient species absorb in the visible and have themselves been studied as oxidizing radicals with a range of potency.21 Halide- ion oxidations have been shown to produce the species Cla -,Br2 -,and I2 -which were observed previously using flash photolysis, but the more recent pulse radio- lysis work has shown that they are formed through intermediates of the type ClOH -.The analogous products from CNS -oxidation,22 (CNS)2 -and SCNOH -,had not been observed previously, nor had mixed ions23 of the type ICNS-. All exist in equilibrium with the free radical and ion, and equilibrium constants have been determined for most of them. The reaction of the ionized form 0-with 02, 0-+o,+o,-(6) has proved a very convenient way of producing the ozonide ion for studies of its stability and reactivity. 24 The corresponding reaction of OH is not known. B. Reactions of Hydrated Ele~trons.~~-These may give three types of product, viz. anions by addition, e.g. to oxygen to give 02-, free radicals by a bond- scission reaction as with hydrogen peroxide, HzOz+ e&+OH-+ OH (7) or lower oxidation states as in the reactions of many metal ions (including Np and Pu ions) and their co-ordination complexes.Most interesting of the latter are the many transient aquated oxidation states made observable for the first time.26 (i) Metal ions. Reactions having diffusion-controlled rate constants occur with the fairly easily reducible ions such as Ag +,Cr3 +,or Cu2 +. A detailed study of the reduction of Ag + to Ago has shown that the latter subsequently gives Ag2 +, Ag3 +, and Ag2 as intermediates in a reaction sequence leading to the formation of colloidal silver.27 T1+ also givesz8 TIo, which with T1+ gives TI:! +.Electron 4o D. Meyerstein, Znorg. Chem., 1971, 10, 2244. 21 P.Neta, V. Madhavan, J. Zemel, and R. W. Fessenden, J. Amer. Chem. SOC.,1977, 99, 163. 22 D. Behar, P. L. T. Bevan, and G. Scholes, J. Phys. Chem., 1972,76, 1537. 23 M. Schoneshofer and A. Henglein, Ber. Bunsengesellschaft phys. Chem., 1970, 74, 393. 24 B. H. I. Bielski and J. M. Gebicki, Adv. Radiat. Chem., 1970, 2, 177. 25 (a)M. Anbar, Adv. Chem. Ser., 1965, No. 50, p. 55; (6) M. Anbar, Adv. Phys. Org. Chem., 1969, 7, 115; (c) M. Anbar, M. Bambenek, and A. B. Ross, ‘Selected Specific Rates of Reactions of Transients from Water in Aqueous Solution: 1, Hydrated Electron’, Nat. Stand. Ref. Data Ser. Nat. Bur. Stand., 1973, Vol. 43, and Supplement, 1975. 2E 5. H. Baxendale, E. M. Fielden, and I. P. Keene, Proc. Roy. SOC.,1965, A286, 320. 27 J.Pukies, W. Roebke, and A. Henglein, Ber. Bunsengesellschaftphys. Chem., 1968,72,842. z8 B. Cercek, M. Ebert, and A. I.Swallow, J. Chem. SOC.(A), 1966, 612. Baxendale and Rodgers reduction is a very convenient way of preparing Cu + and Cr2 +. In the former case it has been used29 to study the formation of Cu+ complexes with acrylamide, maleic acid, and fumaric acid, and in the latterso the reaction of Cr2+ with oxygen, where an intermediate CrO2 + was observed and characterized. Electron reduction can be used to give the aquated forms of unstable lower oxidation states of many metals which cannot be made using conventional reduction methods. These are short-lived species but can be prepared and investigated by pulse radiolysis. The univalent Co +,Mn +,Zn +,Cd +, and Pb + were the first to be observed26 and have received most attention, but Cr +,Tm2+, and Sm2 + have also been characterized31J2 spectroscopically and most of the other lanthanides have been shown to give the bivalent ion.The above univalent ions all absorb strongly in the near U.V. and this has been used to compare reactivities and follow their reactions with a variety of oxidants.33J36 In these it is found, for example, that M + forms complexes with olefins and free radicals, gives MO2+ with oxygen, and MO+ + N2 with N2O. (ii) Metal ion co-ordination complexes. When these compounds react with e& they almost invariably do so at diffusion-controlled rates. 25 The immediate products, which are very often the lower oxidation state of the metal, may be stable, as with [Fe(CN)6I3-+ [Fe(CN)6I4 -and [Ru(NH3)6I3 + + [Ru(NH3)6I2+,but frequently have only transient existence.Their reactions to the final stable state have been followed in a variety of cases and have revealed some interesting reactions. The simplest perhaps is that of eta-reduced [Ru(dipy)# +.The corresponding RuII complex is produced in an excited state which undergoes a fluorescent transition to the ground state34 and is the same species as has been formed and investigated photochemically. The unstable [Ru(NH3) 5C1] + formed from the corresponding RuIII compound initiates a chain reacti~nl~~ involving the latter which leads to its complete transformation into [Ru(NH3)5(H20)I3 +: [RU(NHJ,CI]~+ + eG +.[Ru(NH,),Cl] + + H,O -+ [RU(NH~)~(H,O)]~+ + C1 [Ru(NH~),(H,O)]~ + + [Ru(NH~),CI]+ + [Ru(NH,),Cl]z+ + [Ru(NH~)~(H,O)]~ + etc. (8) Many complexes simply shed the ligands from the reduced metal as in the case of the multitude of CoIII ammines which have been investigated.25 An interesting development here is that the rate at which some of the ligands are lost can be measured using the conductivity technique.35 Thus from [Co(N&)6l2 + three NH3 ligands are lost too rapidly to be measured, but the rest come off with rate constants 6 x 104, 1 x lo4,and 1.5 x 103 s-l.[Ru(NH3)5(N2)l2+, one of the first dinitrogen complexes to be prepared, is stable in solution but ea, readily reduces it to RuI. The latter dismutes to the original RuII and RuO, which rapidly D.Meyerstein, Znorg. Chem., 1975, 14, 1716. so R. M. Sellars and M. G. Simic, J. Amer. Chem. SOC.,1976, 98, 6145. a1 H. Cohen and D. Meyerstein, J.C.S. Dalton, 1974, 2559. sa M. Faraggi and Y. Tendler, J. Chem. Phys., 1972,56,3287. 8a D. Meyerstein and W. A. Mulac, Znorg. Chem., 1970, 9, 1762. J. E. Martin, E. J. Hart, A. W. Adamson, H. Gaffney, and J. Halpern, J. Amer. Chem. SOC.,1972, 94, 9238. as M. Simic and J. Lilie, J. Amer. Chem. SOC.,1974, 96, 291. Contributions of Pulse Radiolysis to Chemistry loses its ligand~.~~ [Ni(CN)4I2-gives [Ni(CN)4I3 -, which after losing CN -dimerizes37 to the stable [Ni2(CN)6I4 -. Compounds with aromatic ligands present another possibility, viz. the addition of e& to the ligand prior to ultimate intramolecular transfer to the metal. This occurs with CoIII and CrIII tri~bipyridyls~~ and with nitrobenzoatopenta-ammineC~~~~.~~Such reactions provide useful background information on intra- molecular electron transfer 1.vhich is useful in the interpretation of electron- transfer reactions in general.Some biological metal complexes, e.g.Fe-cytochromes and haemoglobins, have also been investigated and although there are discrepancies between the observa- tions from different laboratories it would appear that intramolecular electron transfer and slow conformational changes can be seen following the initial e& react ion.40 (iii) Non-metallic compounds. In aqueous solutions an important reaction of eG is with the solvent, e~,+ HzO-+ H + OH-; k = 16 dm3mol-l s-l (9) which determines the maximum lifetimes of eag in the absence of other reactions.41 It is also important in conjunction with the reverse reaction, whose rate constant is also known, as a basis for calculating thermodynamic parameters for e~3.42 Simple addition of ea< occurs with 02 and C02.The former reaction to give 02-(and HO2 at lower pH) has proved a very convenient source of HO2 for study by pulse radiolysis. Its acid-base properties (pKa = 4.5), heat of ionization, and spectral characteristics have been measured and its reactivity investigated with a variety of compounds. 24 It is of interest biologically since specific enzymes, super- oxide dismutases, exist to catalyse its decomposition to 02 + HzOz, which is normally rather slow.These dismutases are Cu-, Fe-, and Mn-containing com- pounds. Pulse radiolysis studies have shown that they react by being alternately oxidized and reduced by 02 -in cycles of reactions involving the metal in two or more oxidation states.43 The ion C02 -although also readily prepared by electron addition to C02 is more conveniently produced by the oxidation of formate ion with OH. It absorbs in the u.v.44 and has had many applications as a reducing agent, in which role it 36 J. H. Baxendale and Q. G. Mulazzani, J. Inorg. Nuclear Chem., 1971, 33, 823. 37 Q. G. Mulazzani, M. D. Ward, G. Semerano, S. S. Emmi, and P. Giordani, Internat.J. Radiar. Phys. Chem., 1974, 6, 187.38 (a) J. H. Baxendale and M. Fiti, J.C.S. Dalton, 1972, 1995; (b) M. Z. Hoffman and M. Simic, J.C.S. Chem. Comm., 1973, 640; (c) M. Z. Hoffman and M. Simic, J. Amer. Chem. SOC., 1972, 94, 1757. 3s M. G. Simic, M. Z. Hoffman, and N. V. Brezniak, J. Amer. Chem. SOC., 1977,99,2166. lo (a)A. Shafferman and G. Stein, Biochim. Biophys. Acta, 1975,416,287; (6) E. J. Land and A. J. Swallow, Biochem. J., 1976, 157, 781. 41 E. J. Hart, S. Gordon, and E. M. Fielden, J. Phys. Chem., 1966, 70, 150. 42 (a)J. H. Baxendale, Radiat. Res. Suppl., 1964, 4, 139; (6) ref. la. 43 (a)E. M. Fielden, P. B. Roberts, R. C. Bray, D. J. Lowe, G. N. Mautner, G. Rotilio, and L. Calabrese, Biochem. J., 1974, 139,49; (b)M. Pick, J. Rabani, F. Yost, and I. Fridovich, J.Amer. Chem. SOC., 1974, 96, 7239. 40 J. P. Keene, Y.Raef, and A. J. Swallow, ref. 13d, p. 99. 244 Baxendale and Rodgers is not as potent as efi but is sufficiently so to reduce Cd2+ and, for example, [Ni(CN)4I2-to [Ni(CN)4I3 -among many other complex ions.37 Reaction of N2O is the most commonly used of the bond-fission reductions brought about by eag because it is a source of OH. In the same way S20a2-and other peroxy ions react S20s2 -+ eag -,so4-+ so42-and have been used to study SO4 -and analogous species. C.Reactions of Hydrogen Atoms.-The hydrogen atom is much less reactive than eag which probably accounts for the less extensive attention paid to its reactions. Although like eag it reduces metal ions28 such as CelV, T1+, and Cu2 + it cannot produce the univalent species from Co2 +,Ni2+,Mn2+,Cd2+, and Pb2fnor is it capable of reducing complex ions such as the cobalt(n1) ammines.However, an interesting series of reactions uncovered using pulse radiolysis involves its behaviour as an oxidizing agent towards certain reducing ions such as Fez +, Cr2 +, and Ti111.45 Here the overall reaction is H,O + Fez++ H+ Fe3++ H, + OH-(10) but intermediate hydrides such as FeH2 + have been detected through their absorption spectra and these presumably react FeH2++ H++ Fe3+ + H, (1 1) [Ni(CN)4I2-,which is reduced to [Ni(CN)4I3 -by eag, gives H[Ni(CN)4I2 -with H, which then by a bimolecular reaction gives the original product and H2.37 5 Electrons in The suggestion that solutions of ‘free’ or ‘excess’ electrons could be prepared in some liquids is almost as old as the first observation of electrons in the gas phase.47 The study of alkali-metal-ammonia solutions on which the suggestion was based did not progress very quickly beyond the work of Kraus until magnetic susceptibility measurements48 established beyond doubt that, when dilute, these were indeed fairly stable solutions of electrons and metal ions.However, even now examples of this type of system are limited to a few alkali and alkaline earth metals in ammonia, some amines, and a few ethers. A recent review testifies to the current interest in this field.49 By definition, ionizing radiation produces free electrons in all liquids but in most cases, unlike the ammonia systems, recombination with the concomitant cations or reaction with the liquid itself makes their existence transient. Only in 45 G.G. Jayson, J. P. Keene, D. A. Stirling, and A. J. Swallow, Trans. Faraday SOC.,1969,65, 2453. 40 (a)‘Solvated Electron’, ed. R. F. Gould, Adv. Chem. Ser., 1965, No. 50; (6) E. J. Hart and M. Anbar, ‘The Hydrated Electron’, Wiley-Interscience, New York, 1970; (c) W. F. Schmidt, ‘Electron Migration in Liquids and Glasses’, Hahn-Meitner Institute, HMI-B156, 1974; (d) M. S. Matheson, ‘Physical Chemistry’, Vol. 7 ‘Reaction in Condensed Phases’, ed. H. Eyring, Academic Press, New York, 473, 1975; (e) F. S.Dainton, Chem. SOC.Rev., 1975, 4,323. 47 C. A. Kraus, J. Amer. Chem. SOC.,1908, 30, 1323. S. Freed and N.Sugarman, J. Chem. Phys., 1943,354, 11. 4s M.C.R. Symons, Chem. SOC.Rev., 1976,5, 537. 245 Contributions of Pulse Radiolysis to Chemistry 1958 was good evidence produced, in the form of specific chemical reactions, for their presence and importance in aqueous systems. Since 1962, when the pulse radiolysis technique was introduced, it has been possible to observe them by optical absorption and conductivity in a wide variety of liquids. Pulse radiolysis studies have shown that in pure polar liquids, such as water and alcohols, the optimum electron lifetime is determined by its rate of reaction with the solvent, and that this varies from tens of milliseconds in water to a few microseconds in alcohols.50 In aliphatic hydrocarbons, which are inert to electrons, the lifetime depends on the electron concentration since it is governed by the rate of recombination with the concomitant cation.However, it also varies considerably with the hydrocarbon, since electron diffusion (or mobility) is strongly influenced by the liquid structure.46 Thus in n-hexane the ion recombi- nation rate constant is ca. 5 x 1013 dm3 mol -l s -l at room temperature and in iso-octane it is 5 x 1015 dm3 rnol s -l, so that to persist for several hundred nanoseconds concentrations must be kept below ca. 10-7 rnol dm -3 in n-hexane and ca. 10-9 mol dm -3 in iso-octane. The study of electrons in liquids by pulse radiolysis has been mainly in the areas of chemical reactions and optical properties, although measurements of mobili- ties in various liquids have also received attentiong6 (i) Chemical reactions of electrons.Not surprisingly aqueous systems have received most attention and a c~mpilation~~ of reaction rate constants lists more than 600 compounds which have been studied. A much smaller collection of similar data on reactions in alcohols is also a~ailable.5~ These reactions are easy to follow optically since, in polar media, electron absorption is high at wavelengths which are readily monitored, e.g. [e~]= 1.06 x lo3 mol m -2 at 578 nm. Also diffusion coefficients, e.g. 5 x 10-5 cmz s -l in water, are not much greater than those of normal ions, so that even when reacting at diffusion-controlled rates (k z 1010 dm3 mol -l s -1) reaction times can be arranged to be in the range of microseconds.Reactions are of two types, viz. simple electron additions to solutes and addi- tions with bond scission.25 Examples of both of these in inorganic systems have been given above. With organic solutes additions occur to ketones, quinones, aromatic hydrocarbons and substituted derivatives, and nitro-compounds. They give the corresponding anions-ketyl, semiquinone, etc.-whose spectral proper- ties and subsequent reaction behaviour have usually been reported. Addition with bond scission invariably occurs with aliphatic halides to give the halide anion and the organic free radical. This is a very convenient method of prepara- tion of the latter for subsequent investigation. Diperoxides, mercaptans, and alcohols react in this way, giving respectively the oxyradical and oxyanion, the hydrosulphide anion and hydrogen atom, and the oxyanion and hydrogen atom.Disulphides, important in biological systems, react analogously to diperoxides. In polar solvents these reactions almost invariably have rates which are of the tio J. H. Baxendale and P. Wardman, Chem. Comm., 1971,429. 61 ‘Selected Specific Rates of Reaction of the Solvated Electron in Alcohols’, Nar. Stand. Ref. Data Series, Nut. Bur. Stand., 1972, Vol. 42. Baxendale and Rodgers order expected for diffusion-controlled processes as calculated from the diffusion coefficients. Many of the latter have been determined from conductivity changes following pulse radiolysis.52 The rate constants are found to follow the Debye equation when charged solutes are involved, and also the Brransted-Bjerrum application of the Debye-Hiickel equation to such reactions when they occur in the presence of inert electrolytes. In water the reaction with the solvent and its reverse, which can be observed by pulse radiolysis in alkaline solution, viz.H,O+eG+H+OH-(12) have been used to calculate42 the equilibrium constant for the above system, and hence the solvation energy of the electron in water (1.7 V) and the E" for ea, (-2.9 V). The potent reducing capacity consequent upon the latter has already been referred to above. Reactions of electrons in hydrocarbons are technically more difficult to follow optically since lifetimes are short at concentrations which are measurable and absorptions are only appreciable beyond 10oO nm.Liquids in which electron mobilities are low offer the best opportunity and some rates have been obtained in n-hexane using this technique.53 Those in other hydrocarbons have been measured using conductivity.*6 The results reveal a range of very high rate con- stants from ca. 1012 dm3 mol -1 s -1 in n-hexane to ca. 1014 in tetramethylsilane. Electron mobilities, Pe, are correspondingly high but for a particular solute in different hydrocarbons the rate constants do not change as would be expected from the changes in mobility.54 Thus for n-hexane, iso-octane, and neopentane Pe is 0.09, 7, and 7 cm2V s respectively, whereas for reaction with SF6 the rate constants are 2 x 1012, 5.8 x 1013, and 2.1 x 1014 dm3 mol-1 s-l, i.e.a smaller range than for pe. In some cases, e.g. CzHsBr, the rate constants actually pass through a maximum along the series. The reason for this behaviour is not yet completely understood but an import- ant factor appears to be VO,the electron binding energy of the liquid, or in other words the energy required to eject an electron into a vacuum from the liquid. VO has been determined for many hydrocarbons and may be positive or negative. In the above series the values are +0.04, -0.18, and -0.4 V re~pectively.5~ (ii) Spectral characteristics of electrons in liquids. The strong optical absorption of electrons in liquids in the red or near infrared has provided a convenient method of observing them, but in addition the absorption itself has been the subject of much theoretical work aimed at understanding the state of electrons in Absorption spectra in a range of liquids-water, ammonia, alcohols, amines, ethers, hexamethylphosphoramide, and hydrocarbons-have been obtained.46 They are all very broad: e.g.in water the band half-width is about 200 nm, and A,,, = 700 nm. In general A,,, moves to longer wavelengths (see Figure 2) with 6a (a)G. C. Barker, D. Fowles, D. C. Salmon, and B. Stringer, Trans. Faraday SOC.,1970, 66, 1498; (b) G. C. Barker and P. Fowles, ibid., p. 1661;(c) K.-D. Asmus, ref. 2c, p. 40. 6s J. H. Baxendale and E. J. Rasburn, J.C.S. Faraday I, 1974, 70, 705. 64 A. 0.Allen, T. E. Gangwer, and R.A. Holroyd, J. Phys. Chem., 1975,79,25. 6b R. A. Holroyd and M. Allen, J. Chem. Phys., 1971, 54, 5014. Contributions of Pulse Radiolysis to Chemistry Wavelength/nm Figure 2 Transient absorption spectra of solvated electrons in liquids of varying polarity (Reproduced by permission from L. M. Dorfman, ‘Investigations of Rates and Mechan- isms of Reactions, Part II’, in ‘Techniques of Chemistry’ VI Series, ed. A. Weissberger,Wiley-Interscience, New York, 1974) decreasing solvent polarity although there are exceptions to this. The energy of Amsx is generally assumed to be a measure of the depth of the potential well formed by the oriented solvent molecules. Theoretical treatments of the geometry and energy of such trapping and the optical transitions possible have been partially successful*^ in the case of water and ammonia, but an understanding of the large bandwidths of the absorptions is still lacking.The solvated electron system has also provided a means by which the solvation process itself may be observed and quantified using the pulse radiolysis technique. This possibility was first demonstrated in liquid alcohols at low temperature^,^^ where spectra due to solvated electrons were found to take time to develop to the usual broad absorption with a maximum in the red. Immediately (i.e. nano-seconds) after production in the liquids, the electron absorptions are much fur- ther into the i.r., with maxima beyond 2000 nm. The spectral changes with time are attributed to the transformation of the electron environment from a random distribution of polar molecules to an oriented configuration of the molecular dipoles, brought about by the field of the electron.This change from a shallow to a deeper potential well causes the spectrum to shift towards the blue. The time- scales for this process have been related to those for solvent relaxation obtained J. H. Baxendale and P. Wardman, J.C.S. Furaduy I, 1973, 69, 586. Baxendale and Rodgers from dispersion measurements, and with the extension of the observations to room temperature using picosecond methods57 a comparison can now be made over a large temperature range. An analogous use of differences in absorption spectra between polar and non- polar environments has been used to indicate the molecular structure of mixtures such as alcohols and hydrocarbons.Spectral and conductivity measurements support the ideas of clusters of the polar component and can give an idea of their ~ize.5~ Interestingly, no absorption spectra due to trapped or solvated electrons in aromatic liquids have been reported although observations over the range 300-1600 nm have been made. Since the yield of escaped electrons in, e.g.,benzene is not much lower than in n-hexane where transient spectra have been clearly observed,53 their non-appearance must be due to lower extinction coefficients in the observation region. Pulse radiolysis studies on electrons in benzene and toluene have been performed59 using conductivity detection.Kinetic parameters and mobilities resemble those in hexane. 6 Solution Chemistry of Radical Ions At some time after ejection and thermalization, an electron may encounter a substrate having favourable cross-section for capture to occur : eii, + X+ X- (13) or esoG + X -+ X- (14) Many chemical substances act as acceptors of electrons, ranging from 02, H30 f, and hydrated transition-metal ions through carbonyls, aromatic hydro- carbons, quinones, and nitro-compounds to metal complexes and biological macromolecules. Meanwhile the positive species, if stable towards fragmentation and the ion- molecule reaction with a neighbour, can relax to a solvated parent cation and undergo charge-transfer reactions with added solutes having lower ionization potential : S++X-+X++S (15) Substances which commonly react in this way are condensed-ring aromatic hydro- carbons, amines, and inorganic ions which are easily oxidized (e.g.I -or SCN -).In some liquids (H2O and aliphatic alcohols) the product of the ion-molecule reaction also has strong oxidizing properties : H2O+ + H2O + H,O+ + *OH (16) .OH + I-+ I-+ OH-(17) 67 N. J. Chase and J. W. Hunt, J. Phys. Chem., 1975, 79, 2835. (a) J. H. Baxendale and P. H. G. Sharpe, Chem. Phys. Letters, 1976, 41, 440; (b) J. H. Baxendale, Canad. J. Chem., 1977, 55, 1996. A. J. Robinson and M. A. J. Rodgers, J.C.S. Faraday I, 1975, 70, 378. 249 Contributionsof Pulse Radiolysis to Chemistry These processes, which can be made to occur within the duration of the electron pulse, allow the production cf oxidized or reduced species whose properties with respect to reaction with other added substances can be studied.In the remainder of this section we detail some investigations which have used the pulse radiolysis technique to good effect in this way. A. Radical Anions in Organic Liquids.-The study of the chemistry of radical anions of aromatic hydrocarbons has been pursued actively over the past twenty years.60 These stable but reactive species are produced when the appropriate molecule is added to a solution of an alkali metal in an ether or other inert solvent. Na +Ar +Na+ +Ar-(1 8) This technique has been used extensively but suffers from the disadvantage that the alkali-metal cation produced concomitantly modifies the properties of the anion.61 Using electron capture in a pulse radiolysis experiment, however, an ‘infinite-dilution’ anion is formed, the properties of which can be monitored by time-resolved absorption spectrophotometry.62 In this way electron-transfer equilibria, Ar(1) -+Ar(2) +Ar(1) + Ar(2)-(19) have been studied63 for a variety of donor-acceptor pairs.The data have been examined63 by the Marcus theory of electron transfer with good results in a num- ber of cases. The kinetics of cation-anion associative pairing has also been studied by pulse radiolysis.64 Capture of solvated electrons by benzophenone in solution in tetra- hydrofuran or dioxan in the presence of added alkali-metal ions (as tetraphenyl-boronate salts) was used to give initially free ketyl radical anions which subse- quently equilibrated with the cations present to form a population of ion pairs.Differences between the absorption spectra of the ion pairs and the free ions65 were followed as a function of time (<1 ps) after the free ketyl ions were pro- duced. The rate constant for the forward step in the equilibrium M+ + PhzCO-+[M+Ph,CO-] (20) was found to be close to the diffusion limit. There has been a recent awakening of interest in measuring the relaxation of solvent molecules around charge centres in an attempt to understand more fully the microscopic nature of the liquid state. Although pulse radiolytically produced electrons have most widely been used (see above), a recent study66 showed that ao N.Hirota, in ‘Radical Ions’, ed. E. J. Kaiser and L. Kevan, Wiley, New York, 1963, p. 35. M.Szwarc, in ‘Carbanions, Living Polymers and Electron Transfer Processes’, Wiley- Interscience, New York, 1968. sa S. Arai and L. M. Dorfman, J. Chem. Phys., 1964,41, 2190. 63 J. R. Brandon and L. M. Dorfman, J. Chem. Phys., 1970,53, 3849. a4 D. Beaumond and M. A. J. Rodgers, Trans. Faraday SOC.,1969,65,2973. O6 D. G. Powell and E. Warhurst, Trans. Faraday SOC.,1962, 58, 953. S. Arai, M. Hoshino, and M. Imamura, J. Phys. Chem., 1975, 79, 702. BaxendQle and Rodgers the ketyl radical anions of acetophenone, benzophenone, and some derivatives undergo analogous spectral shifts due to solvation.The time dependence of such shifts was measured and showed two relaxation stages which were interpreted as being due to a rapid formation of a primary solvent shell followed by a slower outer-shell formation. B. Radical Cations in Organic Liquids.-Although less extensively studied than radical anions, the cations formed by charge transfer from solvent positive ions to, e.g., aromatic hydrocarbons have received some attention. For example, studies on the charge-transfer process between solvent cation and aromatic acceptor (e.g. biphenyl or pyrene) in di~hloroethane~~ acetone,g* and cyclo- hexane69 show bimolecular rate constants in excess of those calculated from the Debye theory. The differences are most marked for pyrene in cycl~hexane~~ and constitute evidence of movement of positive holes throughout bulk liquid by a resonance charge migration in addition to molecular diffusion.As with radical anions, so cations can undergo charge exchange with mole- cules of lower ionization potential. Absolute rate constants for biphenyl to pyrene, biphenyl to p-terphenyl, and p-terphenyl to anthracene have been obtained67 and shown to be diffusion-controlled. No evidence for back reactions was obtained. A further example of the importance of charge-exchange studies is provided in recent work70 in which the transfer of positive charge between chlorophyll-a and carotenoid pigments has been the subject of a pulse radiolysis investigation. A primary reaction in photosynthesis is thought to be loss of an electron by a chlorophyll-containing centre on photoexcitation, followed by a neutralization of the chlorophyll cation by charge transfer to a carotenoid auxiliary.It was found70 that in a homogeneous liquid only charge transfer in the reverse direction can occur. Recently the radical cations and anions of several carotenoid pig- ments were generated and characterized using the pulse radiolysis technique. 71 The wavelength maxima were found to be linearly dependent on the number of conjugated double bonds, in general agreement with a theoretical description using Hiickel and PPP techniques. An interesting property of aromatic radical cations is their tendency to add a neutral molecule to build up a sandwich-type dimer species, analogous to excimer formation by excited states of the same molecules, e.g.pyrene + + pyrene + (pyrene), + (21) This phenomenon was first observed72 in glassy matrices subjected to y-radiolysis followed by slight softening. Such studies provided spectroscopic N. E. Shank and L. M. Dorfman, J. Chem. Phys., 1970,52,4441. M. A. J. Rodgers, J.C.S. Faraday I, 1972, 68, 1278. 6s E. Zador, J. M. Warman, and A. Hummel, Chem. Phys. Letters, 1973, 23, 363. 'O I. Lafferty, E. J. Land, and T. G. Truscott, J.C.S. Chem. Comm., 1976, 70. 'l J. Lafferty, A. C. Roach, R. S. Sinclair, T. G. Truscott, and E. 5. Land, J.C.S. Faraday I, 1977, 73, 416. B. Badger, B. Brocklehurst, and R. D. Russell, Chem. Phys. Letters, 1967, 1, 122. 25 1 Contributionsof Pulse Radiolysis to Chemistry characteristics of dimer cations.Kinetic data for the monomer-dimer equili- brium have been obtained from pulse radiolysis experiments73 in which the decay of the monomer cation and formation of dimer cation were monitored using absorption spectrometry with nanosecond time resolution. Thermodynamic data were obtained from temperature studies of these processes74 which led to a binding energy of the pyrene dimer cation AH = -40kJ mol -l with an overall entropy change of AS = 88 J K -1 mol -l, both values very similar to those for the pyrene excimer. For naphthalene and 2,6-dimethylnaphthalene,however, the binding energies obtained 74a were approximately one half of the corresponding excimer values (AH = -25 kJ mol -1).For these same radical ions the entropy change in the dimerization step was close to AS = 8 J K -l mol -l, indicating an overall gain in entropy on dimerization, presumably accounted for by a change in solvent ordering in the neighbourhood of the charge centre. C. Electron Transfer Equilibria and Redox Potentials.-The transfer of a single electron from one molecular species to another in a consecutive reaction sequence is connected to the respiratory metabolism of glucose in mitochondria and the release of stored energy. The species involved in this respiratory chain are flavoproteins, cytochromes, NAD, and oxygen and in order to obtain a complete understanding of the mechanisms involved, quantitative details of the oxidation- reduction behaviour of the various molecular stages must be determined. Conventional polarographic techniques for the measurement of single electron redox data usually are inapplicable to biologically relevant media because of the short lifetimes of the radicals involved, which create irreversibilities. The pulse radiolysis technique has recently been put to effective use for overcoming such problems by allowing a quantity of a singly reduced substrate to be produced rapidly in aqueous phase by reaction with hydrated electrons: e6 + X --f X-(22) Subsequently X -comes to equilibrium with a second electron acceptor (A) pre-sent in lower concentration: X-+A+X+A-(23) The time required for attainment of equilibrium depends on the individual rate constants and the acceptor concentration but is typically complete in 100 ps.Then if the radicals X -and A -are stable for only milliseconds and their optical absorptions are sufficiently well separated for unambigous measurement of equilibrium concentrations, the equilibrium constant for the transfer process can be determined and thence the one-electron redox potential of one of the couples, if the other is known. This technique was used for measuring the potentials of several quinone- 73 (a)M. A. J. Rodgers, Chem. Phys.Letters, 1971, 9, 107; (b) A. Kira, S. Arai, and M. Imamura, J. Chem. Phys., 1971, 54,489. 74 (a) M. A. J. Rodgers, J.C.S. Faraday I, 1972, 68, 1278; (6) S. Arai, A. Kira, and M. Imamura, J. Chem. Phys., 1972,56, 1777.Baxendale and Rodgers semiquinone systems, the 02/02 -couple,75 and nitro-aromatic and nitro- heterocyclic single-electron potentials76 against duroquinone and 9,lO-anthro- quinone 2-sulphonate standards. Parallel measurements of the spin densities on the nitro-groups of the radical anions using in situ radiolysis+.s.r. found a linear correlation between spin density and redox potential in spite of wide structural variations. This technique has similarly been applied to determining the one- electron reduction potential of riboflavine77 as a function of pH. Values ranging from Eh = -0.24 V at pH 5.9 to Eh = -0.46 V at pH 11.9 were found; the pH dependence followed the curve calculated from known pKa values of riboflavine and its semi-reduced form. Recently, Marcus theory has been applied78 to the rate constants of the forward and reverse reactions, resulting in good agreement between experimental and theoretical values.An exhaustive series of nitro- imidazole couples have been studied using the pulse radiolysis method79 in an attempt to demonstrate that the one-electron potential determines the efficiency of such molecules in sensitizing hypoxic mammalian cells (cells having oxygen levels below physiological requirement) to ionizing radiation. Recently, transient absorption spectra and lifetimes (several ps) of carbonium ions, e.g.PhCH2+, Ph2CH+, and Ph3Cf have been determined in halogenocarbon solvents.80 Rate constants for reaction with halide ions and tertiary amines were measured and found to approach the diffusion limit.7 Chemistry of Excited Molecules Although the properties of electronically excited molecules are generally studied using photoexcitation techniques, several situations exist where such methods are less useful than pulse radiolysis. Such investigations have served to supple- ment the photoexcitation studies and have contributed new fundamental know- ledge to all aspects of excited-state chemistry. In this section we have made a selection of topics for brief exposition to highlight the wide-ranging use of the technique. A. Monomer Singlet States.-In certain cases, high-energy excitation will pro- duce fluorescent molecular states not readily produced by the conventional spark sources of fluorometry.For example, the kinetic properties of fluorescent states of cyclohexane and other aliphatic hydrocarbons were monitored by following the decay of electron-pulse-induced emission at 260 nm.81 This fluorescence is thought to arise from a radiative transition from a (T*--(T state populated either by direct excitation or rapid charge recombination. Similarly a fluorescent state of the cyclic ether dioxan was first reporteds2 in a 75 D. Meisel and G. Czapski, J. Phys. Chem., 1975, 79, 1503. 76 D.Meisel and P. Neta, J. Amer. Chem. Soc., 1975, 97,5198. 77 D.Meisel and P. Neta, J. Phys. Chem., 1975, 79, 2459. D. Meisel, Chem. Phys. Letters, 1975, 34, 263. 79 P. Wardman and E. Clarke. J.C.S. Furnduy I, 1976, 72, 1379. *O (a)R.L. Jones and L. M. Dorfman,J. Amer. Chem. SOC.,1974,96,5715;(b)J. R. Sujdak,R. L. Jones, and L. M. Dorfman, ibid., 1976, 98,4875. I. H.Baxendale and J. Mayer, Chem. Phys. Letters, 1972, 17, 458. 83 J. H. Baxendale, D. Beaumond, and M. A. J. Rodgers, Chem. Phys. Letters, 1969, 4, 3. Contributions of Pulse Radiolysis to Chemistry pulse radiolysis study. The spectrum showed a maximum at 280 nm and the time constant for fluorescence was co. 2 ns. B. Excited Dimer States.-The large bulk of knowledge on excimers and exciplexes has been accumulatedB3 from photostimulated fluorescence studies which allow the kinetic properties of the dimeric species to be characterized. The spectroscopic properties of such species as derived from fluorescence measure- ments are restricted to those states involved in radiative process, ID1 -+ 1Do.Kinetic absorption spectrophotometry, using pulse radiolysis, allows the lD1 -, lDnelectronic transitions to be observed, thereby supplying experimental support for the calculations of theoretical spectroscopists. The first excimer absorption spectrum obtained was that of benzene in a pulse radiolysis e~periment.~~ This showed a broad absorption band with Amax = 520 nm which was attributed85 to the Blg -, Elu excimer transition which was calculated to have the appropriate energy at an interplanar separation of 0.33 nm.86 Transient absorption spectra measurements become increasingly important in studies of weakly or non-fluorescent species.The anthracene excimer is especially relevant: the S1 state of anthracene is subject to encounter-controlled, ground- state quenching to yield a photodimer bonded between the 9-and 10-positions: Anth(S,) + Anth(S,) + photodimer (24) Clearly this reaction should proceed via an excimer state which, by analogy with excimers of other aromatic hydrocarbon^,^^ would exist as a weakly bound ‘sandwich’ structure with the two 9-and 10-positions opposite each other. Unlike other excimers, however, no fluorescence from an anthracene species had been observed in fluid medium, presumably because the relaxation to photodimer occurred too rapidly. This was shown to be the case in pulse radiolysis experi- rnents87 on anthracene in benzene in whish anthracene (Sl) was populated by diffusion-controlled energy transfer from benzene (Sl) under conditions where ground-state quenching was marked.A new species absorbing in the near i.r. (A,,, = 1200 nm) was observed which had kinetic properties consistent with anthracene excimer . Other condensed-ring aromatic hydrocarbons have excimer absorption spectra in the near i.r. in agreement with calculations.BB The lifetime of the anthracene excimer in benzene was less than 2 ns,83 as expected if facile u-bond formation followed. C.Triplet States.-The measurement of the activity parameters of molecular excited triplet states necessitates the monitoring of TI -+ Tnabsorptions since the 83 J. B. Birks, ‘Photophysics of Aromatic Molecules’, Wiley-Interscience, London, 1970.84 (a)R. Cooper and J. K. Thomas, J. Chem. Phys., 1968, 48, 5097; (b) J. K. Thomas and I. Mani, ibid., 1969, 51, 1834. 86 J. B. Birks, Chem. Phys. Letters, 1968, 1, 625. M. T. Vala, I. H. Hillier, S. A. Rice, and J. Jortner, J. Chem. Phys., 1967, 44, 23. M. A. J. Rodgers, Chem. Phys. Letters, 1972, 12, 612. M. F. M. Post, J. Langelaar, and J. D. W. van Woorst, Chem. Phys. Letters, 1976,42, 133. 254 Baxendale and Rodgers TI SOtransition, being spectroscopically forbidden, usually restricts phospho- rescence measurements to rigid phases where diffusion, and therefore reaction, is obviated. Flash photolysis and pulse radiolysis excitations are both useful in this context. Which technique is better depends upon certain criteria which have to be met : (i) The exciting flash must be able to populate a sufficient concentration of S1 (depends on extinction coefficients of ground state and lamp/laser output spectrum).Further, intersystem crossing must effectively populate sufficient TI states. (ii) If not, a sensitizer having a triplet state of sufficient energy to participate in an energy-transfer reaction with the molecule in question must be found which can satisfy criterion (i). The pulse radiolysis technique largely over-rides these considerations in that energy is deposited initially in the solvent phase, becoming very rapidly localized in a solute via energy-transfer or ion-recombination processes. The utility of the method is best shown by the work characterizing the triplet states of biologically active molecules such as carotenes and visual pigrnent~,~Q porphyrin~,~Oand isoprenoid benzoquinonesgl in addition to a number of simple aromatics.92 Any systematic study of excited-state molecules requires measurement of the quantum efficiencies of the various decay modes open to molecules in an upper level.To measure intersystem crossing, S1 + TI, it is necessary to count the triplets formed, which in an absorption experiment requires knowledge of the T1-Tn extinction coefficients (ET-T).An extremely useful method for determining such data has been developed.93 An energy-transfer technique is used which fundamentally consists of comparing the absorption due to a triplet of known ET-T with that of the unknown one: two experiments are performed, one with the donor molecule alone in solution (benzene or cyclohexane), another with both donor (at the same concentration as before) and acceptor (at about 1 % of the donor concentration).The solvent energy is first intercepted by the donor mole- cule which can transfer all or some to the acceptor, the partition depending on the relative rates of donor decay (measured in the first experiment) and energy trans- fer (measured as the rate of acceptor triplet formation in the second). The optical density due to the acceptor triplet is measured at the end of the transfer process and corrected for the amount of donor triplet lost to natural decay modes using the two measured rates. The ratio of the optical density to the donor triplet in the (a) T.G. Truscott, E. J. Land, and A. Sykes, Photochem. and Pliotobiol., 1973, 17, 43; (6) R. V. Bensasson, E. J. Land, and T. G. Truscott, ibid., p. 53. S.J. Chantrell, C. A. McAuliffe, R. W. Munn, A. C. Pratt, and E. J. Land, J.Luminescence, 1976, 12/13, 887. R. V. Bensasson, C. Chachaty, E. J. Land, and C. Salet, Photochem. and Photobiol., 1972, 16,27.** (a) E. 5. Land, Trans. Furaday Soc., 1969, 65,2815; (b) R. V. Bensasson, E. J. Land, T. Lafferty, R. S. Sinclair, and T. G. Truscott, Chem. Phys. Letters, 1976, 41, 333; (c) E. J. Land, E. McAlpine, R. S. Sinclair, and T. G. Truscott, J.C.S. Furaday I, 1976, 72, 2091. O3 (a) E. J. Land, Proc. Roy. SOC.,1968, A305,457; (b) R.V. Bensasson and E. J. Land, Trans. Faraday SOC.,1971, 67, 1904. 255 Contributionsof Pulse Radiolysis to Chemistry absence of acceptor to the corrected acceptor optical density is equal to ET-T (donor)/ ET-T(accept or), whence E T-T(accept or) can be evaluated. This technique was used initially to measure ET-T for several condensed-ring aromatic ~pecies~3~ and show the solvent dependence93b of both spectral shape and intensity. More recently the method has been utilized in characterizing the spectral features and reactivity parameters for a diverse range of molecular triplet statessg ..92 and it is now widely recognized as a straightforward, conven- ient way of obtaining extinction coefficient data. 8 Surfactants and Micellar Systemsg4 Aqueous solutions of amphiphilic molecules (surfactants) can exist in at least two well-defined states.At low concentration the dispersion is largely one of mono- meric surfactants in water ;at higher concentrations supramolecular aggregates (micelles) form in which the hydrophobic paraffin chains are inwardly disposed with the hydrophilic polar head groups at the surface layer. Micellar shapes can be roughly spherical or cylindrical. Above a critical micelle concentration (c.m.c.) monomers and micelles co-exist in dynamic equilibrium. Since micelle stability results from the same molecular interactions which stabilize bio-membranes and tertiary protein structures, micelles have been extensively studied as model biological entities.In recent years the pulse radiolysis technique has been added to the conventional means of probing such systems. Early experiments using pulse radiolysis concentrated on measuring the reactivity of primary radicals (eag, *OHand *H)with surfactant molecules and micelle-solubilized substratesg5 For common surfactants such as cetyltrimethyl- ammonium bromide (CTAB), sodium dodecyl sulphate (SDS), and Igepal (a polyoxyethylene), bimolecular rate constants with H* atoms range between 108 and log dm3 mol -1 s -1; for OH radicals the range is 109-1010 dm3 mol -1 s -1. These surfactants are most unreactive to ea, as may be anticipated from the lack of reactivity of saturated hydrocarbons. The interaction of hydrated electrons with micelles or micelle-bound substrates is severely modified by the micellar surface charge.For example cetylpyridinium chloride (CPC) reacts efficiently with eiii (rate constant of 7 x lo9dm?mol -1 s -1) below the c.m,c. This rate constant is increased to 5 x 10l2dm3 mol -l s -1 on micelle formation.96 In addition, when CPC was added to CTAB micelles (unreactive to ea;i) at a level of one per micelle, very rapid micelle-micelle hopping by ea: was observed to occur.97 Reactions of primary water radicals with substrates adsorbed at a micelle sur- @O Accounts of pulse radiolysis studies of micellar systems can be found in (a) J. H. Fendler and E. J. Fendler, ‘Catalysis in Micellar and Macromolecular Systems’, Academic Press, New York, 1975; (b) ‘Reaction Kinetics in Micelles’, ed.E. Cordes, Plenum Press, New York, 1973; (c) J. K. Thomas, Accounts Chem. Res., 1977, 10, 133. s5 (a)J. H. Fendler and L. K. Patterson, J. Phys. Chem., 1970, 74, 4608; (b) R. M. Bansal, L. K. Patterson, E. J. Fendler, and J. H. Fendler, Internat. J. Radiat. Phys. Chem., 1971,3, 321 ; (c) J. H. Fendler, G. W. Bogan, E. J. Fendler, G. A. Infante, and P. Jirathana, ref. 26, p. 53. @O M. Grgtzel, J. K. Thomas, and L. K. Patterson, Chem. Phys. Letters, 1974, 29, 393. O7 L. K. Patterson and M. Gratzel, J. Phys. Chem., 1975, 79, 956. Baxendale and Rodgers face or solubilized within its interior have proved instructive. Thus the rate constant for ea, reacting with benzene shows a ten-fold increase in the presence of CTAB micelles on which the benzene is adsorbed.95c Other rate-constant enhancements have been measured for polycyclic aromatic hydrocarbon^,^^ ketones and benzoquinone~,~~ and nucleic acid bases.loO Such enhancements have been variously attributed94u to distortion by the micellar surface charge of the substrate 7-r-system, thereby assisting nucleophilic attack, or to a purely electro- static effect on the diffusion rate.Whatever the real reason, it is clear that reactivities of charged species with lipid-associated substrates at membrance sites cannot be confidently extrapolated from data obtained in dilute aqueous solution. Equilibrium constants for the binding of transition metals to SDS micelles have been evaluated101 using pulse radiolysis to generate eag, which probes the con- centration of metal ion in bulk solution.An interesting recent development has been the observationlo2 of the dismutation of Br2- species produced by the reactions OH + Br--t Bra + OH-(25) Br*+ Br -+ Br,. -(26) where the Br -ions are in the diffuse double layer of CTAB micelles. Two kinetic processes involving Br2 -dismutation were observed, one governed by normal diffusion through the inter-micellar region and the other resulting from diffusion restricted to the micellar surface by electrostatic forces. Such two-dimensional diffusion has obvious relevance to biochemical processes occurring in the vicinity of charged surfaces. There is no doubt that the study of micellar systems by the pulse radiolysis technique is rapidly expanding and future work will concentrate more on using the primary water radicals as probes for learning more about micellar structure and the location of adsorbed or solubilized substrates.9 Organic and Biological Free Radicals This heading covers probably the largest body of information which has resulted from pulse radiolysis studies of aqueous systems and it would be impossible in this article to attempt even a partial coverage of the field. Fortunately a number of comprehensive review articles have appeared recentlyl03 to which reference can be made for detail; our aim will be to attempt an overview of the subject punc- s* (a) S. C. Wallace and J. K. Thomas, Radiat. Res., 1973, 54, 49; (6) J. H. Fendler, H. A. Gillis, and N.V. Klassen, J.C.S. Faraday Z, 1974, 70, 145. sg Ref. la, Ch. 8. looC. L. Greenstock and I. Dunlop, Znternat.J. Radiat. Phys. Chem., 1973,5, 231. lol M. Gratzel and J. K. Thomas, J. Phys. Chem., 1974, 78, 2248. loZ A. J. Frank, M. Gratzel, and J. J. Kozak, J. Amer. Chem. SOC.,1976, 98, 3317. lo3 (a) P. Neta, in ‘Advances in Physical Organic Chemistry’, ed. V. Gold and D. Bethell, Academic Press, London, Vol. 12, 1976; (b) G. E. Adams, in ‘Advances in Radiation Chemistry’, ed. M. Burton and J. L. Magee, Wiley-Interscience, New York, Vol. 3, 1972; (c) M. Simic, in ‘Fast Processes in Radiation Chemistry’, ed. G. E. Adams, E. M. Fielden, and B. D. Michael, Wiley, London, 1975; (d) P. Wardman, in ‘Reports on Progress in Physics’, 1977, in press. Contributions of Pulse Radiolysis to Chemistry tuated with examples to describe the general aspects of free-radical properties which can be obtained from pulse radiolysis experiments.Organic free radicals are generated in pulse radiolysis by reaction of the primary water radicals with organic substrates; according to the selected con- ditions the organic substrates may be oxidized or reduced, e.g. CICHzCOzH+ e& -+ -CH,CO,H + C1-MeNO, + eG -+ MeN0,-H *+ MeCH,OH -+ MekHOH + H, H *+ CBHg-+ CBH7. *OH+ MeCOMe -+ CH,COMe + H,O (31) .OH + CzHl -+-CH,CH,OH (32) *OH+ RSH -+ RS *+ H20 (33) Monitoring of free radicals and their subsequent reactions has been carried out largely by time-resolved optical absorption spectroscopy supplemented by conductivity, polarography, and e.s.r.measurements following pulse radiolysis generation.103a Absorption spectra are especially useful where the primary radical can react at different sites on the substrate, e.g. PhNO, + H*-+ PhN02H (34) or PhNO, + H-+ H&H,NO, (35) The two product radicals in this and other cases have sufficiently different absorption maxima to allow determination of the mode of He reaction. In general, absorption spectra of carbon o-radicals are in the low U.V. region and of low extinction coefficient, which makes their observation difficult. r-Bonded systems usually have absorption maxima in more accessible regions: the more extensive the conjugation, the more red-shifted the absorption becomes. Compilations of free-radical spectra have been published.lo4 A particularly useful feature of optical absorption spectra of many free radicals is a shift in A,,, with pH. Utilization of this fact has made possible the deter- mination of pK values of a multitude of free radicals, e.g. (i) Carboxylic acid dissociation (parent acid value in parenthesis) COZH + C0,-+ H&; PKa = 1.4 (3.7) (36) CHzCOZH + CHZC02-+ HBfq; PKa = 4.5 (4.7) (37) *CH(NHZ)COzH+ *CH(NH,)CO,-+ H:q; pKa = 6.6 (2.2) (38) (ii) Hydroxy-group dissociation CHZCHzOH + CHZCH20-+ HA; PKa = 14.7 (39) CHZ=CHCHOH + CH,=CHdHO-+ H&; PKa = 9.6 (40) Me,COH + Me,CO-+ H8Q; PKa = 12.2 (41) lo4 (a) A. Habersbergerova, I. Janovski, and P. Kourim, Radiat. Res. Rev., 1968, 1, 109; (b)A. Habersbergerova, I.Janovski, and J. Teply, ibid., 1972,4, 123. Baxendale and Rodgers (CFS)eCOH + (CF,),CO-+ HA; PKa = 1.7 (42) PhaCOH + Ph2CO-+ H&; PKa = 9.25 (43) An extremely large number of radicals have been characterized in this way105 and the changes in pKa with degree and nature of substitution have been correlated with empirical Taft and Hammett re1ationships.l03a The time-resolved conductivity technique has been used to good effect where ionic concentrations undergo change as a result of radical formation (e.g. ionic dissociation or hydrogen halide elimination). Recently, interesting chemistry of oxidized dialkyl disulphides has been followed using combined optical and con- duct ivity observations,106viz. *OH + RSSR,/nRs(oH)S (44)LRSSR+ + OH-Formation of RSSR+ results in a decrease in conductivity of the solution since the OH -formed rapidly removes H30 + ions from the solution, i.e.the highly mobile proton is replaced by a less mobile RSSR +-ion. The dimethyl disulphide cation is found to react with reductants such as [Fe(CN)6I4 -and Fez +,the for- mer with a bimolecular rate constant of 1.5 x 1O1O dm3 mol -1 s -1 the latter of 5.2 x lo6 dm3 mol-1 s-l. Whereas the rate constant for reduction by ferro- cyanide was diffusion-controlled for increasingly hindered disulphides, the reduction by ferrous ions became less efficient as the alkyl groups varied in the series Me, Et, Pri, But. Not only *OHradicals but .also pulse-radiolytically pro- duced radicals such as Ag2 +, (Ag(OH)] +, T12 +,HC03, and Br2- -caused oxida- tion of RSSR species.lo7 Electron spin resonance studies of free radicals generated by electron pulses have been particularly useful in characterizing radical structures and yields.1l In addition, time-resolved e.s.r.experiments following pulse radiolysis have demonstrated that non-equilibrium concentrations of spin-states are produced. lo8 Kinetic information is also available from such experiment~.l~~~~l The ultimate decay mode of free radicals is by a bimolecular reaction involving either dismutation or dimerization. By and large, the rate constants governing such processes for organic radicals are in the range of lo9dm3 mol -l s -1 and are largely independent of structure.lO3a Some radicals undergo unimolecular changes prior to the bimolecular decay.Thus hydroxyl radical addition to phenols and hydroquinone, lo lobE. Hayon and M. Simic, Accounts Chem. Res., 1974, 7, 114. looM. Bonifacic, K. Schafer, H. Mockel, and K.-D. Asmus, J. Phys. Chem., 1975, 79, 1496. lo' M. Bonifacic and K.-D. Asmus, J. Phys. Chem., 1976, 80, 2426. lo8(a) E. C. Avery, J. R. Remko, and B. Smaller, J. Chem. Phys., 1968, 49, 951 ; (b) R.W. Fessenden, ibid., 1973, 58, 2489. logG. Nucifora, B. Smaller, J. R. Remko, and E. C. Avery, Radiat. Res., 1972, 49, 96. ll0 (a) E. L. Land and M. Ebert, Trans. Faraday Soc., 1967, 63, 1181;(b) G. E. Adams and B. Michael, ibid., p. 11 71. 259 Contributions of Pulse Radiolysis to Chemistry OH OH is followed by water elimination, OH 0 &OH H,O + 0 with eventual formation of phenoxy-radicals.Similar H2O elimination has been demonstrated for -OH adducts of anilines,lll toluene,llj pyrroles, and imi- daz01es.l~~Kinetic parameters for many free-radical reactions have been col- lected and tabulated.ll4 An important bimolecular reaction of most organic free radicals is with oxygen. Two modes of reaction are possible: (i) addition *CH,OH + 02+ .O,CH,OH (47) (ii) electron transfer Me,kO-+ 0, + Me,CO + 0; (48) These processes are of particular importance in the biological effects of free radicals in oxygenated ~ystems.~O~c For example, the killing of cells by ionizing radiation has been shown to be enhanced in the presence of 0xygen.~O36 This oxygen enhancement has been shown to involve (in part) free-radical interme- diates in bacterial spores.l15 Experiments using electron pulses quickly following oxygenation of cells in a rapid-mix system aim to characterize the time-scale of the oxygen effect.116 An understanding of this phenomenon and of the molecular mechanisms for cellular sensitivity to free-radical-induced processes has stimu- lated much effort into the study of organic free radicals, both as isolated systems and as components of macromolecules, e.g.proteins and DNA. The involvement of radicals in general,l036 and peroxy-radicals in particular,l03c in biological effects has been reviewed. It would seem that a logical picture has emerged, viz.characterization of the reactivities and spectra of radicals derived from amino-acids and simple peptides by pulse radiolysis coupled to observations ll1 H. Christensen, Internat. J. Radiat. Phys. Chem., 1972, 4,31 1. 11* H. Christensen, K.Sehested, and E. J. Hart, J. Phys. Chem., 1973, 77, 983. 113 A.Samuni and P. Neta, J. Phys. Chem., 1973, 77, 1629. 114 A.J. Swallow, in ‘Progress in Reaction Kinetics’, ed. R. B. Cundall and K. R. Jennings, Pergamon Press, in press. 115 E.L.Powers, in ‘Electron Spin Resonance and Effects of Radiation on Biological Systems’, Nuclear Science Series, Report No. 43, National Academy of Sciences, Washington D.C., 1966. ll6 B. D. Michael, G. E. Adams, H. B. Hewitt, W. B. G. Jones, and M. E. Watts, Radiat. Res., 1973,54, 239.Baxendale and Rodgers of proteins and enzymes. Likewise the radical properties of nucleic acid bases, nucleosides, and nucleotides have been obtained in order to help an understand- ing of the response of DNA to free-radical attack. The simple aliphatic amino-acids lose H. from the a-carbon when attacked by .OH or Ha but are of low reactivity towards ea;.l03b Sulphur-containing amino- acids are very reactive towards both .OH and eLtq. Amino-acids with aromatic residues are very reactive to ea< and moderately so to *OH.103b Transient spectra of the *OH adducts of tyrosine and tryptophan have been characterized; a comprehensive study of the latter117 has shown that *OH adds to the C-2 and C-3 portions of the heterocyclic ions and to a site on the benzenoid nucleus with about equal efficiency. Capture of e,g by the disulphur bridges in cystine is very efficient (k = 1.3 x 101O dm3 mol -1 s -l)Il8 and produces an anionic radical species absorbing strongly in the visible region.Studies119 using enzymes, lysozyme, papain, chyniotrypsin, and ribonuclease show similar spectra on reaction with eaq. It was suggested that eag attacks an enzyme at multiple reactive loci, following which the electron is rapidly passed through the protein intramolecularly to be localized eventually on the disulphide bridge.1036 Free radicals have been successfully used to probe the active site of enzymes. Thus, -OH radicals have been shown to inactivate lysozyme in aqueous solution; hydroxy 1radical scavengers, except thiocyanate ions, protect against inactivation ; hence the (CNS)2.-ion formed by *OH + CNS-+ CNS. + OH-(49) CNS* + CNS-+(CNS)S*-(50) must also deactivate lys~zyme.~~~ It was found that (CNS)2*-reacts with the enzyme with a rate constant of 6.6 x 108 dm3 mol --I s -I. Of the 20 amino-acids which make up lysozyme only tryptophan was found to react with (CNS)r -at a significant rate. Subsequently tryptophan-1 08 was confirmed as being responsible for the activity.120 This use of free radicals generated by pulse radiolysis sug- gests that such species may be more widely used as specific probes for identifying reaction centres. The free bases of nucleic acids, nucleosides, and nucleotides are all very reactive towards *OHradicals and, with a few exceptions, hydrated electrons.103* Much effort has been expended in identifying the loci of attack and the nature of radi- -cals formed.121 The reaction of oxygen with the product radical (including DNA-) has been shown to be extremely efficient122 ( > 109 dm3 mol-1 s -1). The intense 117 R.G. Armstrong and A. 5. Swallow, Radiat. Res., 1969, 40, 563. 118 R. Braams, Radiat. Res., 1966, 27, 319. lle (a)5. V. Davies, M. Ebert, and R. J. Shalek, Infernat.J. Radiaf.Biol., 1968,14, 19; (b)G. E. Adams, R. B. Cundall, and R. L. Willson, in ‘Chemical Reactivity and Biological Role of Functional Groups in Enzymes’, ed. R. M. S. Smellie, Academic Press, New York, 1970. laoJ. E. Aldrich, R. B. Cundall, G. E.Adams, and R. L. Willson, Nature, 1969, 221, 1049. J. F. Ward, in ‘Advances in Radiation Biology’, ed. Lett and Adler, Academic Press, New York, Vol. 5, 1975. laaR. L. Willson, Internat. J. Radiat. Biol., 1970, 17, 349. Contributions of Pulse Radiolysis to Chemistry interest in the oxygen enhancement of radiation damage in living systems has led to searches for 02-mimics, i.e. substances which also enhance radiation sensi- tivity. Many are now known, some interesting ones being stable nitroxyl free radicals, e.g. triacetoneamine-N-oxyl (TAN). Pulse radiolysis studies of the reactivity of TAN with deoxyribonucleotides and DNA have been performed.123 Recently, pulse radiolysis techniques have been used to investigate the redox properties of many radiation sensitizers (see above).The reactivity of the radicals (CSN)Y -, Bra. -,ecq, and C02. -with biological macromolecules has been used to probe macromolecule-su bstrate binding. Thus eag produced by pulse radiolysis in the presence of protein (c.g.serum albumin) undergoes addition and the rate of loss of LG, can be followed absorptiometrically. When a drug molecule (e.g. penicillin) is bound to the protein, this reactivity is modified and measurements of the rate constants can lead to evaluation of the concentrations of free and bound drug and hence to equilibrium constants and, via a temperature dependence, to binding energies.’ 24 Similar experiments have been performed for observing the binding of detergents and proteins’ and drugs by DNA and dye-DNA intercalation.126 10 Gas-phase Chemistry Early studies on gases were limited because of technical difficulties which derive from the fact that owing to the lower densities compared with liquids the energy absorption from a radiation pulse, and therefore the concentration of species produced, is much lower. However, the use of multiple reflection cells and the development of fast sensitive detection equipment has now made it possible to make observations comparable with those in liquid systems.Since an authorita- tive and detailed review of the pulse radiolysis of gases has recently been pub- lished1Z7 it will suffice for present purposes to outline the scope of these studies. The work has covered the reactions of atoms, free radicals, electrons, ions, and excited states, using measurements of optical absorption and emission and, in the case of electrons, microwave conductivity.Among the atom reactions those of oxygen have received most attention. In particular the formation of 03 in oxygen by the three-body reaction 0 + 0, + M ’0, + M (51) has been analysed in great detail. It proceeds by two steps: 0 + 0, + 03* (52) 133 R. L. Willson and P. T. Emmerson, in ‘Radiation Protection and Sensitization’, ed. Moroson, Quintiliani, Taylor, and Francis, London, 1970. lZ4 (a) G. 0. Phillips, D. M. Power, C. Robinson, and J. V. Davies, Biochirn. Biophys. Acta, 1970, 215,491 ;(b)G. 0.Phillips, D. M. Power, and J. T. Richards, Israel J. Chem., 1973, 11,517; (c)J.S. Moore, G. 0.Phillips, D. M. Power, and J. V. Davies, J. Chem. SOC.(A), 1970, 1 115. le5 C. V. Eadsforth, D. M. Power, E. W. Thomas, and J. V. Davies, Internat. J. Radiat. Biol., 1976, 30, 449. lZ6 (a) D. W. Whillans, Biochim. Biophys. Acta, 1975, 414, 193; (6) C. L. Greenstock and G. W. Ruddock, ibid., 1975, 383, 464. 127 M. C. Sauer, ‘Advances in Radiation Chemistry’, Vol. 5, Wiley, New York, 1976, p. 97, which contains detailed references to all the work quoted in this section. 262 Baxendale and Rodgers 03*+ M + 0,+ M (53) Rate constants for the deactivation step have been obtained for a variety of M, and at low pressures absorptions due to 03* in various states of vibrational excitation have been observed. The reactions of 0 (obtained from Ar + C02 or Ar + NzO mixtures) with benzene and a range of substituted benzenes have been followed and the absorption characteristics of the adducts obtained.Reactions of hydrogen atoms (from Ha or Ar + H2) with a range of aromatics and with 02, CO, NO, and C2H4 have been measured using either the optical absorption of the products or of H itself with Lyman cc-radiation as monitoring light. Using the identifiable absorptions of the radicals themselves, reactions of NH2 (from NH3), SH (from HzS), CH (from CH4), and CN (from C2N2) with a variety of inorganic and organic molecules have been studied and in the latter case the effect on reaction rates of having CN in various vibrational states has been examined. Both absorption and emission have been used to study the excited states of Ar, Kr, and Xe.In argon the four 1s excited states have been observed in absorption and their decay characteristics obtained. Also, their appearance as a result of emission from the corresponding 2p states has been established. 1.r. absorptions, which are assigned to excited dimers formed from the 1s states, are present in all three irradiated gases and have been used to measure the excimer lifetimes. In nitrogen the behaviour of vibrationally and electronically excited states has been investigated. The lifetimes of Nz(A3Zn +), Xz(B3ng), and Nz(a’lZu-) in various states of vibronic excitation have been obtained using the absorption of the excited species. The kinetics of the emission from Nz(C3nu)have been shown to be consistent with its production by the neutralization reaction e- +, N4+. Other studies of emission from electronically excited states of species such as 02+, CO2+, CH, and CO as well as N2 have given rates of collisional deactivation by various inert gases. Excited states of aromatics can be produced by energy trans- fer from, e.g.,excited argon, or by reaction of the aromatic cation with electrons or with the corresponding anion formed by electron capture. Using emission from the excited singlet to monitor the reactions with anthracene, rate constants of 2 x 1015 and 2 x 10’4 dm3 mol -1 s -1 have been obtained. Electron capture by a variety of organic halides is found to occur with rate constants which vary from about lo* (EtBr) to 1014 (cc14) dm3 mol s -I. De-tailed examination of the cc14 reaction has shown effects of gas pressure which arise from the time required for the electrons to be thermalized; for example, this requires milliseconds in argon at 1 Torr but nanoseconds in hydrocarbons, NzO, NH3, etc. at the same pressure.

ISSN:0306-0012    

DOI:10.1039/CS9780700235

出版商:RSC    

年代:1978

数据来源: RSC

摘要:

The Chemistry of Dental Cements By A. D. Wilson LABORATORY OF THE GOVERNMENT CHEMIST, DEPARTMENT OF INDUSTRY, CORNWALL HOUSE, STAMFORD STREET, LONDON SE1 1 Introduction The ancient art of dentistry and the need for dental materials arose from man’s attempt to combat and iepair the effect of dental diseases, which are largely associated with the development of civi1ization.l Exodontics, the extraction of teeth, and prosthetics were known to the civilization of Antiquity; examples of Greek exodontic instruments and Etruscan gold bridgework have been found.2 Much of dentistry, especially modern dentistry, is concerned with the restora- tion of the function and appearance of teeth following the lesions caused by caries. Restoration takes the form of the complete replacement of teeth, pros- thetics; or conservation of the natural dentition, conservative dentistry.The latter includes the art of the restoration of eroded teeth using a filling material or a cemented inlay or crown. The subject of dental materials forms an essential part of restorative dentistry. It is not merely a specialized branch of materials science, itself a complex amal- gam of the chemical and physical sciences and technologies, but combines also the elements of cosmetic art, biological science, and clinical practice. It remains an art as well as a science and the requirements of one may impinge on those of the other. Thus basic chemistry cannot always be dissociated from biological and aesthetic requirements and this adds to the complexities of research.The development of restorative dentistry has been related to the availability of suit-able materials. The search for these has continued throughout the history of dentistry and has been largely dependent on general advances in science and technology. The requirements for dental restorative materials are demanding. Since they are prepared by the clinician in the surgery they must have good manipulative properties and adequate working time allied to the ability to set and harden rapidly once placed in position. They should, ideally, adhere to tooth material. Moreover, dental restorative materials are required to function in the sensitive but hostile environment of the mouth. Thus they should be bland towards living tissues, yet be able to withstand the aggressive action of acids generated from sugars by micro-organisms.No one material meets all these requirements G. Toveradet, ‘A Survey of the Literature of Dental Caries’, Nat. Head Sci. Res. Council, Washington, Publication 225, 1952; P. 3. Breklus and W. D. Armstrong, J. Amer. Dent. Assoc. 1936, 23,1459. B. W. Weinberger, ‘History of Dentistry’, Mosby, St. Louis, U.S.A., 1948, vol. 1. The Chemistry of Dental Cements and all have limitations of performance and application. For this reason a wide variety of dental restorative materials have been formulated. 2 Dental Cements One of the most important classes of materials used in dentistry is that of the dental cements. These materials, which are prepared directly in the surgery, are used for many purposes and depending on their formulation they may be used to attach prefabricated crowns and inlays to teeth, to line cavities for the protection of dental pulp against chemical and thermal insult, and even for the direct filling of cavities.All dental cements are based on the hardening reaction between a powdered solid and a viscous hydrogen-bonded liquid. The plastic pastes formed by mixing these components set rapidly to hard salt-like gels.3 The liquids, which may be water-based or organic, act as acids or proton donors. The cement powders are essentially amphoteric or slightly basic substances which act as proton acceptors in the cement-forming reaction. They may be simple oxides, such as zinc oxide, or complex aluminosilicate glasses containing appreciable amounts of fluoride. The essential cation in these powders-zinc in the zinc oxide cements and alu- minium in the aluminosilicate cements-are cations with a small ionic radius and high ionic potential and are known in glass science to have a network- forming capacity.Beryllium and magnesium, occasionally found in minor amounts in both powder and liquid, also have a network-forming capacity. This capacity decreases with increasing basicity of the cation, and is greatest for cations of high charge and small ionic radius. The most versatile powder for cement formation is zinc oxide which when treated with a wide range of organic and inorganic proton-donating liquids yields a whole family of cement^.^ Their development is closely bound up with dentistry.Historically, the first was invented by Sorel in 1855.5 In this cement zinc oxide powder was combined with an aqueous solution of zinc chloride to form a cementitious mass. Its use in dentistry was recommended by Feichtinger in 18586 but it did not prove to be a success (another Sorel cement based on magnesium oxide and solutions of magnesium chloride is still used in the building industry).7 Subsequent development of zinc oxide cements in the 1880’sfollowed two courses. In one, the Sorel’s liquid was replaced by aqueous solutions of phosphoric acid, giving rise to the zinc phosphate cement;8 and in the other, A. D. Wilson in ‘Scientific Aspects of Dental Materials’, ed.J. A. von Fraunhofer, Butter- worths, London, 1975, Ch. 4. A. D. Wilson in ‘Scientific Aspects of Dental Materials’, ed. J. A. von Fraunhofer, Butter- worths, London, 1975, Ch. 5. E. Sorel, Compt. rend., 1855, 41, 784. J. W. Mellor, ‘A Comprehensive Treatise on Inorganic and Theoretical Chemistry’, Longman, London, 1929, Vol. IV, p. 546. E. Sorel, Compt. rend., 1867, 65, 102; E. S. Newman, J. Res. Nat. Bur. Standards, 1955, 54, 347. * C. S. Rostaing, G.P.N. 6045/1878; G.P.N. 11 253/1880; C. N. Peirce, Dent. Cosmos., 1879, 21, 696. Wilson various organic liquids were substituted, eugenol proving the most su~cessful.~ These traditional cements were fully developed by the end of the 19th century : thereafter the subject stagnated until recent years when a number of research studies have been conducted.Liquid chelating agents, other than eugenol, have been reported,loJl 2-ethoxybenzoic acid being the most notable.ll A practical cement based on aqueous solutions of poly(acry1ic acid) has also been developed as a practical material.12 Not all dental cements are based on zinc oxide. The traditional dental silicate cement utilized instead an aluminosilicate glass powder.13J4 This cement and the zinc phosphate cement are the only dentally useful ones found in a large family of cements based on reactions between oxides and phosphoric acid solution.15 The dental silicate cement remained unique until recently when another cement based on aluminosilicate glasses was discovered.This cement, which utilizes aqueous solutions of poly(a1kenoic acids), has been termed by Wilson and Kent13J6 a glass-ionomer or ASPA cement; ASPA is an acronym of Alumino- Silicate PolyAcrylic Acid. Undoubtedly the most important recent development in dental cement technology has been the emergence of a new class of cement, the ionic polymer cement system where anionic polyelectrolytes are employed as cement forming liquids. The ionic polymer cements are at present represented by the zinc polycar- boxylate cement of Smith4J2 (which uses a zinc oxide powder) and the glass- ionomer cement of Wilson and Kent.13J6 This field of dental science is one where further developments are to be expected. The range of dental cements is summarized in Table 1.Despite their diverse nature certain features are common to all. The cements themselves fall within Wygant’s definition of reaction cement,17 a term applied to cements other than those which set by hydraulic action. Dental cements may be more closely defined as arid-base reaction cements. The cement-forming liquids are acidic, viscous hydrogen-bonded liquids capable of donating protons, exemplified by eugenol and phosphoric acid. The powders are amphoteric oxides, either zinc oxide or aluminosilicate glass, which act as proton acceptors. The cement-forming reaction is essentially an acid-base interaction between these two components, the gel-salt formed, in the liquid phase, acting as a binding matrix. General equations for the cement-forming reaction may be written as follows: J. S.King, Denf. Cosmos, 1872, 14, 193; L. C. Chisholm, Dent. Register, 1873, 27, 517; J. F. Flagg, Dent. Cosmos, 1875, 17, 465; D. D. Smith, ibid., 1878, 20, 521 ; J. Wessler, Deutsch. Mschr. Znhnheilk, 1894, 12, 478. T. H. Nielsen, Actri. Odonf. Scand., 1963, 21, 159. l1 G. M. Brauer, E. E. White, and M. G. Mashonas, J. Dent. Res., 1958, 37, 547. l2 D. C. Smith, Brit. Denf. J. 1968, 125, 381; B.P.N. 1 139430/1969. l3 A. D. Wilson in ‘Scientific Aspects of Dental Materials’ ed. J. A. von Fraunhofer, Butter- worths, London, 1975, Ch. 6. T. Fletcher, B.P.N. 3028/1878; Brit.J. Dent. Sci., 1879,22,74; P. Steenbock, G.P.N 174557, B.P.N. 15 176/1903; B.P.N. 15 181/1904; C. G. Voelker, Dent.Surnm., 1916, 36, 177. l5 W. D. Kingery, J. Amer. Ceram. SOC., 1950, 33, 239, 242. l6 A. D. Wilson and B. E. Kent, J. Appl. Chem. Biotech., 1971, 21, 313; Brit. Dent. J., 1972, 132,133; B.P.N. 1 316 129/1973. l7 J. F. Wygant in ‘Ceramic Fabrication Processes’ ed. E. D. Kingery, M.I.T.Press, Cam-bridge, Mass., 1958, Ch. 18. 267 Table 1 Classification of dental cements Cement Oxide powder Liquid * Cementing* Active (proton acceptor) (proton donor) (gel-salt ) Filler Zinc Phosphate Zinc oxide and Phosphoric acid, Zinc and magnesium Zinc oxide and hmagnesium oxide aqueous solution phosphate magnesium oxide -Zinc Carboxylate Zinc oxide and Poly(acry1ic acid) Zinc and magnesium Zinc oxide and 9magnesium oxide pol yacrylate magnesium oxide 8Silicate Complex fluorine- Phosphoric acid, Aluminium phosphate Complex h containing aqueous solution and calcium fluoride fluoroalumino-aluminosilicate glass silicate glass core sheathed by a siliceous hydrogel Glass Ionomer Complex fluorine- Poly(a1kenoic acid) Calcium and aluminium Complex containing aqueous solution polyalkenates fluoroalumino-aluminosilicate glass silicate glass core sheathed by a siliceous hydrogel Zinc oxide eugenol Zinc oxide Eugenol Zinc eugenolate Zinc oxide Zinc oxide eugenoll Zinc oxide Eugenol/2-ethoxy-Zinc eugenolate and Zinc oxide 2ethoxybenzoic acid benzoic acid 2-et hoxybenzoate *Principal phase Wilson MO + H2A = MA + HZO proton proton salt-gel acceptors donors matrix MOxSi02 + H,A = MA + xSi0, + HzO (2) proton proton sal t-gel acceptors donors matrix where M represents the cement-forming cation and A the cement-forming anion; for convenience of representation both M and A are taken to be bivaient. The cement-forming reaction is one where hydrogen bridges in the liquid phase are progressively replaced by more rigid metal ion bridges, a process which causes the liquid to gel and the gel to harden.This generalized account of cement-formation is in accord with the views of Wygantl' who has emphasized that maintenance of some continuity of structure is essential for cement- formation. This criterion can only be satisfied by amorphous systems since these possess structural flexibility and, indeed, the matrices of all dental cements are now known to be essentially amorphous.There is other experimental confirma- tion of Wygant's view. 1.r. spectroscopic studies18 on zinc oxide eugenol cements have shown that there is a structural affinity between the associated eugenol dimer in the liquid and the bisligand zinc eugenolate chelate in the matrix of the set cement. Electrical conductivity and permittivity studies on the dental silicate cementlg have indicated that setting and hardening processes proceed with significant discontinuities. The metal ions in dental cements A13+ and Zn2+, derived mainly from the powder but in some cases also from the liquids, are small ions of high ionic potential having some glass-forming ability. The same may be said of the phosphate group in dental silicate cements.Both phosphoric acid and poly- (acrylic acid) solutions used in dental cements will form glasses on heating. These observations are not coincidences but reflect the fact that the same structural considerations apply to cement gels as well as to glass structures. The properties of dental cements are shown in Table 2. All set very rapidly and with certain exceptions are strong materials. Indeed the dental silicate cement is the strongest inorganic cement known and its strength is an order of magnitude greater than that of Portland cement. 3 Phosphate-bonded Cements Cements based on the hardening reaction between a metal oxide or silicate and a concentrated solution of phosphoric acid can be formulated to set within a few minutes and to develop strength rapidly.This property makes them suitable for dentistry. There are two main types: the dental zinc phosphate cement employed for luting crowns and inlays, and the dental silicate cement used for the aesthetic filling of front teeth. Until quite recently they remained the materials of choice for these applications but the advent of the composite resins and iono- mer cements has greatly reduced their usage. A. D. Wilson and R. J. Mesley, J. Dent. Res., 1972, 51, 1581. l9 A. D. Wilson and B. E. Kent, J. Derlt. Res., 1968, 47, 463. t44 0 5Table 2 The properties of typical examples of the various dental cementsa %Phosphate bonded Ionomer Non-aqueous b Zinc Silicate Zinc Glass-ionomer Simple Reinforced Reinforced 2 phosphate polycarboxy-ZOE ZOE ZOEIEBA 5 late Powder/liquid 4.2 4.0 3.6 3.5 2.6 2.4 6.2 proportion (g/ml-l) Setting time, 37"C/min 4.25 3.75 3.25 4.0 3.75 3.75 6.0 Compressive strength, 128 226 85 175 13 39 91 24 h/MN m-2 Tensile strength, 8 13 12 12 1.6 3.5 8 24 h/MN m-2 Modulus of elasticity, 13 18 6.2 9.0 24 h/GN m-2 Opacity, C .70 1.o 0.50 1.o 0.69 1.o 1.o 1.o Adhesive to Enamel/MN m-2 0 0 4.0 0 0 0 Adhesive to Dentine/MN m-2 0 0 3.O 0 0 0 (a) B.E. Kent, B. G. Lewis, and A. D. Wilson, Brit. Dent. J., 1973, 135, 322. Wilson Phosphoric acid solutions used in phosphate cements have concentrations ranging from 45 to 60 % w/w as H3P04(Tables 3 and 4). The structures of these solutions is believed to be one where the phosphate groups are hydrogen bonded Table 3 Chemical composition oj some zinc phosphate cementsaSbl % w/w Powder 89.4 90.3 88.9 3.2 9.6 8.0 6.8 - 1.5 0.6 - 1.6 - 100.0 99.9 100.0 Liquid Total phosphate 57.1 54.7 61.4 (as H3P04) A1 2.4 2.5 3.1 Zn 5.6 4.7 - H2O to 100.0 to 100.0 to 100.0 (a) B.Axelsson, Odont. Revy, 1965, 16, 126; (b) A. D. Wilson, G. Abel, and B. G. Lewis, Brit. Dent. J., 1974, 137, 313. Table 4 The chemical composition of some dental silicate cementsa/ % w/w Powder Si02 41.6 38.8 31.5 A1203 28.2 29.1 27.2 CaO 8.8 7.7 9.0 NazO 7.7 8.2 11.2 F 13.3 13.8 22.0 Pzo5 3.3 3.O 5.3 ZnO 0.3 2.9 H20 2.2 1.6 3.1 (by difference) Less 0 for F -5.6 -5.8 -9.3 99.8 99.3 100.0 Liquid Total phosphate 48.8 49.3 50.7 (as H3P04) A1 1.6 1.9 1.5 Zn 6.1 4.2 8.7 Water to 100.0 to 100.0 to 100.0 (a)A.D. Wilson, B. E. Kent, D. Clinton, and R. P. Miller, J. Muterials Sci., 1972, 7, 220. 271 The Chemistry of Dental Cements to the water liquid lattices.20 Experimental evidence21-23 shows that there are other species present in solution in addition to H3P04 and HzPOd-, namely the hydrogen bonded dimer H8P208 and the triple ion H5P208-. N.m.r. studies21 suggest that more highly polymerized forms are also present such as (H3P04)3 and (H3P04)d. Phosphoric acid solutions used for cement formation contain aluminium or zinc or both (Tables 3 and 4). Whereas zinc is present as a simple salt in solu- tion24~25 aluminium forms a variety of complexes with phosphoric acid.21@,26927 At low A1 :P ratios multi-ligand complexes are present corresponding to the formulae Al(H3PO&, where n = 2,3, or 4, (the state of protonation is unknown).As the A1 :P ratio is increased other complexes are formed i.e. AI,HzP042+, Al,(HzP04)2+, and AI,H3P02+. There is some evidence for the existence of dinuclear aluminium complexes at higher A1 :P ratios and the formation of such species may play an important role in cement-forming reactions. A. Dental Zinc Phosphate Cement.-The zinc phosphate cement has long been used in dentistry for cementing crowns and inlays and is still widely used4 owing to a combination of good manipulative properties and strength.The cement appears to have originated in 1878-18808 and was prepared as a paste from strongly ignited zinc oxide and phosphoric acid in concentrated aqueous solution. In this simple form the cement proved clinically unsatisfactory. The cement was only accepted in dentistry when, as reported by Fleck in 1902,28 improved forms became available. Cement formulations have remained essen- tially unchanged since the early 20th century. Typical modern compositions are given in Table 3.29 The chief problem with the zinc phosphate cement has always lain in the excessive vigour of the cement-forming reaction. Indeed, when plain zinc oxide powder and a simple aqueous solution of phosphoric acid are mixed to- gether they react very rapidly and with the generation of much heat to form a crystalline mass which has little value as a cement.The reaction may be mod- erated in two ways. Firstly, by incorporation of zinc and aluminium salts in the liquid, and secondly by deactivating the zinc oxide powder, which is highly dispersed and so very reactive, by sintering. Sintering has the effect of consolidat-ing the powder to a denser form of reduced surface area and reactivity. This 2o J. R. van Wazer, ‘Phosphorus and its Compounds’. Interscience Publishers Inc. New York, 1958, Vol. 1, pp. 486-491. *l J. W. Akitt, N. N. Greenwood, and G. D. Lester, J. Chem. SOC.(A), 1971, 2450. 22 M. Selvaratnam and M. Spiro, Trans. Faraday SOC.,1965, 61, 360. 23 K. L. Elmore, J. D. Hatfield, R.L. Dunn. and A. D. Jones, J. Phys. Chem., 1965, 69, 3520. 24 A. D. Wilson and R. J. Mesley, J. Dent. Res., 1968, 47, 644. 25 J. A. R. Genge, A. Holroyd, J. E. Salmon, and J. G. I,. Wall, Chem. and Ind., 1955, 357. z6 R. F. Jameson and J. E. Salmon, J. Chem. SOC.,1954, 4013. 27 A. Holroyd and J. E. Salmon, J. Chem. SOC.,1956, 269; J. E Salmon and J. G. L. Wall, J. Chem. SOC.,1958, 1128; N. Bjerrum and C. R. Dahm, Zeir. Phys. Chem. Bodenstein Festband, 1931, 627. c8 H. Fleck, Dent. Items, 1902, 24, 906. B. Axelsson, Odont Revy, 1965, 16, 126; A. D. Wilson, G. Abel, and B. G. Lewis. Brit. Dent. J., 1974, 137, 313. Wilson process can be enhanced by the incorporation of other oxides in the sinter and magnesium oxide, sometimes supplemented by silica, is used for this purpose in preparing zinc phosphate cement powders.Zhuravlev et aZ.,3O who studied the chemistry of the process, found that magnesium oxide reacts with zinc oxide to form a solid solution of zinc oxide in magnesium oxide and the powder cakes. Similarly, silica reacts with zinc oxide to form the mineral willemite, a variety of zinc orthosilicate. Although the cement has been in existence for nearly 100 years, little is known about the cement-forming reaction and microstructure. The reaction may be presumed to be one of acid-base between zinc oxide and phosphoric acid with formation of zinc phosphate salt matrix. The general features are probably similar to those found in the dental silicate cement-forming reaction, which has been studied in detail (see Section 3B).There is always an excess of zinc oxide over phosphoric acid in zinc phosphate cement mixes and the set cement consists of zinc oxide particles embedded in a zinc phosphate matrix. At one time the matrix was thought to consist entirely of hopeite-crystalline zinc ortho- phosphate tetrahydrate, Zn3(P04)2, 4H20.31 This view is now known to be an oversimplified one. Only when a plain zinc oxide and phosphoric solution are mixed together are hopeite crystallites formed in the vigorous reaction, and then as a non-cementitious mass. In practical dental ~emenfs,3~*~~ where the reaction is attenuated by incorporation of aluminium in the liquid and magnesium oxide in the powder, a glassy matrix of amorphous zinc orthophosphate is produced.However, this matrix is not completely stable and crystallites of hopeite develop . at the surface in the presence of free water. There is a period of inhibition which is related to the speed of the reaction. Surface crystallization is an unfavourable process for it destroys any bond that may have been formed between the cement and the tooth and consequently this type of dental cement is a non-adhesive one. The phosphoric acid concentration of the cement liquid is an important parameter that determines many cement properties. An increase in the phos- phoric acid concentration ratio retards the setting reaction.33 No explanation has been advanced for this observation; possibly it is related either to a deficiency of water in the system (to aid transport of ions and hydrate reaction products) or to the fact that the H3P04 concentration decreases as the phosphoric acid concentration increases owing to dimerization (see p.272 and ref. 22). The compressive strength of this cement and its resistance to aqueous attack increases with phosphoric acid concentration. Inspection of the phase diagram% for ZnO-Pz05-HzO shows that when excess ZnO is present and the phosphoric acid concentration is less than 73.1 % w/w (as H3P04) then the zinc phosphate species formed is Zn3(P04)2,4Hz0. If this species is related to that in the cement, 30 V. F. Zhuravlev, S. L. Volfson, and B. I. Sheveleva, J. Appl. Chem. (U.S.S.R.),1950,23,121. 31 A. Dobrowsky, Chem.Tech. (Berlin), 1942, 15, 159. 32 G. E. Servais and L. Cartz, J. Dent. Res., 1971,50, 613; L. Cartz, G. Servais, and F. Rossi, J. Dent. Res., 1972, 51, 1668; S. Crisp, H. J. Prosser, I. K. O’Neill, and A. D. Wilson, J. Dent. Res., 1978, in the press. 33 H. K. Worner and A. R. Docking, Austral. J. Dent., 1958, 3, 215. 34 J. E. Salmon and H. Terry, J. Chem. SOC.,1950, 2813. The Chemistry of Dental Cements then its volume fraction in the matrix will increase as the phosphoric acid content of the liquid increases, with a limit at 73.1 % w/w H3P04. The properties of the cement may be briefly described. It has good manipulative properties and sets rapidly to a cement which can attain a compressive strength of 100 N mm-2 in 24 h. However, it suffers from a number of disadvantages, it is brittle, weak in tension, soluble in acids, opaque, and irritant to living tissues.Moreover, it is not adhesive either to enamel or dentine. B. The Dental Silicate Cement.-Until quite recently the dental silicate cement was the most important of all the dental cements. However, alternative and superior materials are now available for the aesthetic restoration of front teeth and its use has declined considerably. The early history of the cement is obscure. It may have originated as early as 1878, but successful versions were not developed until 1903-190514935 and the first modern type did not appear until 1908.36 Apart from the recently introduced glass-ionomer cement, the dental silicate cement is the only major dental cement not based on zinc oxide powder.The powder is, in fact, a type of calcium aluminosilicate glass which is ion-leachable in acid solution. All current glasses are prepared using a fluoride flux, and are, in consequence opal glasses containing phase-separated droplets of fluorite. The liquid component resembles that of the zinc phosphate cement and is a concentrated aqueous solution of phosphoric acid containing metal salts. The chemical compositions of some typical examples are given in Table 4.37 When these two components are mixed together a rapidly setting cement is formed which develops a strength of ca. 250 MN m-2 in 24 h. The set cement has the valuable quality of translucency which is not possessed by zinc oxide cements.This property is of cosmetic importance, making the cement suitable for the aesthetic restoration of front teeth. Despite its prominence in aesthetic restorative dentistry for more than 70 years the correct chemistry of its setting and structure remained unknown until quite recently. The accepted view was that setting resulted from the formation of silica gel, i.e. a setting mechanism similar to that found in the silicate cements formed by mixing sodium silicate and acid solutions. However, this is not so and the true mechanism of set was established by Wilson and his co-workers37 using several techniques : wet-chemical methods,38 i.r. spectros~opy,~~ electron-probe micro-analysis (EPMA),37g40 electron micro~copy,~~ electrical conduc- tivity,lg and pH measurement^.^^ The following account is based on this work.Before describing the cement-forming reaction it is appropriate to discuss the nature of the unusual glasses used for these cements. Their role is to supply metal ions to the liquid (at an appropriate rate) and this they are able to do be-36 M. Morgenstern, Ost-ung. Vjschr Zahnheilk, 1905, 21, 514. 36 F. Schoenbeck, U.S.P.,N. 897 160/1908. 37 A. D. Wilson, B. E. Kent, D. Clinton, and R. P. Miller, J. Materials Sci.,1972, 7, 220. 38 A. D. Wilson and B. E. Kent, J. Dent. Res., 1970, 49, 7, 21. 39 A. D. Wilson and R. J. Mesley, J. Dent. Res., 1968, 47, 644. 40 B. E. Kent, K. E. Fletcher, and A. D. Wilson, J. Dent. Res., 1970, 49, 86. A. D. Wilson and B.E. Kent, J. Dent. Res., 1969, 48, 212. Wilson cause they have the unusual property, for a silicate glass, of being acid decompos- ible. This is because they are aluminosilicates characterized by a high aluminium content (the A1 :Si ratio approaches unity). A simple silicate, such as silica, is a macromolecule based on a three-dimensional network of [Si04] tetrahedra linked by Si-0-Si bridges and is impervious to acid attack. The behaviour of aluminosilicates is different. In these materials AP + can isomorphically replace Si4+ ions in the network (up to a maximum of 1 :1) which thus consists of [A1041 and [SiO4] tetrahedra linked by Al-0-Si bridges. Replacement of Si4+ by AP+ increases the negative charge on the network, which has to be balanced by network dwelling cations such as Ca2+ and Na+.This glass structiire is susceptible to acid attack, since positively charged hydrogen ions can penetrate and disrupt the negatively charged network by attacking the AP+ sites and severing the Al-0-Si links. AP+, Ca2+, and Na+ ions are liberated and ortho- silicic acid, which polymerizes to silica gel, is formed. The cement-forming reaction, which occurs in a number of overlapping stages, is essentially an acid-base interaction between the glass powder and the acidic liquid. In the fist stage of the reaction H3P04 ionizes to HzP04- (detectable by i.r. spectroscopy39) and the hydrogen ions released attack the glass powder. AP+, Ca2+, Na+, and F-are liberated from the glass leaving behind an ion-depleted layer of silica gel at the surface of the glass particles (as indicated by i.r.spectroscopy39 and EPMA37t40). This decomposition is a rapid one. The liberated ions-AP+, Ca2+, Na+, and F-ions-migrate, possibly as fluoride complexes, into the aqueous phase of the cement paste where they accumulate together with H2PO4-, Zn2+, and Al3+ ions already present in the liquid. As the pH of the system increases the ionic species precipitate as salts. The solubiliza- tion and precipitation of these ionic species is illustrated by Figure 1. The principal reaction is thz formation of an insoluble aluminium phosphate salt, the gel matrix. Associated side reactions are the precipitation of calcium fluoride and formation of soluble sodium dihydrogen phosphate.When about 50 % of the total phosphate has precipitated the cement sets sharply. A schematic representa- tion of this reaction is depicted in Figure 2. Precipitation of ions continues long after the cement has set and strength and hardness also continue to increase37 (Figure 3). The hardening process is largely controlled by increases in pH. Initially there is only a small increase in the apparent pH of the system from an initial figure of 0.8 to 1.7 at set, despite the extent of the interaction between the powder and the liquid because of the buffering action of phosphoric acid solution in the region of its pK, (2.1). The subsequent considerable increase in hardness over the following 24 h is accom- panied by a much larger increase in apparent pH (from 1.7 to 5.0-5.5) which completes the precipitation process despite little further interaction between powder and liquid.In this pH region there is no buffering reaction. Electrical conductivity measurements18 indicate that slow diffusion reactions continue for several months and that the ionic character of the cement slowly diminishes; these changes are accompanied by small increases in cement ~trength.~2The possibility of slow hydration reactions cannot bt ruled out. G. C. Pdenbarger, I. C. Schoonover, and W. Souder, J. Amer. Dent. Assoc. 1938, 25, 32. The Chemistry of Dental Cements H2PO;------\,A1 PO, Ca F, NaH2P04 liquidlmatrix region glass particle Figure 2 Schematic representation of the cement-forming reaction of a dental silicate cement Wilson 4 u 1 I A P"2 z0 0 0 Figure 3 Variation of soluble phosphate, pH, electrical conductivity, and hardness, with the age of the dental silicate cement (Reproduced by permission from J.Mater. Sci.,1972, 7, 220) The microstructure of the set cement has been established using electron and optical micro~copy3~ and electron probe microanalysis (EPMA) 37s40 the latter technique giving the spatial location of the elements and identifying regions where the various reactions occur. The set cement has a composite structure with acid-attacked glass particles embedded in an amorphous and particulate matrix. EPMA st~dies37~~0 show that the matrix contains aluminium, calcium, sodium, phosphorus, and fluorine, but not silicon, and that aluminium tends to be associated with phosphorus and calcium with fluorine (Figure 4).The matrix appears to consist principally of amorphous aluminium orthophosphate (a conclusion confirmed by i.r. spectroscopic data39) containing isolated areas of crystallites : fluorite, CaF2, and augelite, A12(OH)3P04.37 However, the pre- sence of' aluminium fluorophosphates cannot be discounted since n.m.r. studies2I on phosphoric acid solutions containing aluminium and fluoride ions have shown that fluoro-alumino-phosphate complexes containing direct Al-F links, are formed as well as alumino-phosphate complexes. The properties of dental silicate cements, like those of the zinc phosphate cement, are sensitive to the phosphoric acid concentration of the liquid.43 The setting time of cements increases with the acid strength of the liquid: slightly, in the range 50-60 % w/w Hap04 and very sharply above 68 % w/w H3P04 (Figure 5).This phenomenon is paralleled by an observed sharp increase 48 A. D.Wilson, B. E. Kent, R. F. Batchelor, B. G. Scott, and B. G. Lewis, J. Dent. Res., 1970, 49, 307. The Chemistry of Dental Cements core of core of glass particle glass particle VI c C3 t c e In c aJ--EAl -<, -\ aJ r\ \1 . . . . . . . . ... . . . C Na *'**... -m cC 0 P L 20 40 3 1 20 40 61 wn Pm Figure 4 Distribution of elements across e boundary of a glass particle (Reproduced by permissionfrom J.Materials Sci., 1972, 7, 220) I5 c.-E Q) m 5 I I I 1 I x) 40 50 60 70 liquid composition, 010 w/w H,PO, Figure 5 Eflect of phosphoric acid concentration in the liquid on the setting time and compressive strength of'dental silicate cements: P,plain phosphoric acid; PAZ, phosphoricacid solutions containing aluminium and zinc Wilson in the viscosity and reduction of water vapour pressure of such solutions. Moreover, cements prepared with liquids containing more than 60 % w/w H3P04 absorb water, These observations indicate that retardation of the setting reaction in this situation would be due to a deficiency of water which is required to transport ions and hydrate reaction products (e.g.aluminium phosphates and silica gel).The compressive strength of this cement, like that of the zinc phosphate cement, is sensitive to the phosphoric acid concentration of the liquid. However, the form of the strength/acid concentration curve is different, with a maximum strength corresponding to an optimum acid concentration (Figure 5). This con- centration lies between 49 and 55 % w/w (total phosphate calculated as H3P04), the exact concentration depending on whether metal ions are present or not. The form of this curve requires some explanation. Decrease of cement strength with decreasing acid concentration, i.e., increasing water content, observed at acid concentration below the optimum, is explained by the weakening effect of water present in the matrix in excess of that required to solvate the aluminium phosphate and silica hydrates.Decrease of strength with increasing acid con- centration above the optimum can be explained by reference to the A1209- P205-HzO phase diagram.26 The phase diagram indicates that when excess A1203 is added to phosphoric acid solutions in the concentration range 0-65.3 % w/w (as H3P04) the stable aluminium phosphate hydrate formed is AIP04, 3.5H20. Above 65.3 7; w/w H3P04 another hydrate, AIP04, 2H20 is formed. If the former species is superior to the latter as a cementing hydrate, the effect of phosphoric acid concentration is explained. The optimum phosphoric acid concentration would tend to be less than 65.3 % w/w (as H3P04), since water is required for silica gel formation.4 Ionic Polymer Cements The ionic polymer cements,u*45 which include the most recent dental cements, are based on acid-base reactions between aqueous solutions of poly(a1kenoic acids) (40-50 % w/w) and certain acid-decomposi ble metal o~ides,12*~6947 aluminosilicate glasses,16.48 and minerals.49 In the setting reaction liberated cations become bound to polyanions in the aqueous phase to form a hard polysalt gel. These materials are of interest as they are intermediate between a filled organic polymer and an inorganic cement. They are novel in that the gel- matrix contains both covalent C-C and ionic bonds and so can properly be termed ionic polymer (or ionomer) cementsu Many of these cements are weak, hydrolytically unstable and of little practical utility.However, hydrolytically stable cements are formed from zinc and copper 44 A. D. Wilson and S. Crisp, Brit. Polymer J., 1975, 7, 279. 46 A. D. Wilson and S.Crisp in ‘Ionic Polymers’, ed. L. Holliday, Applied Science Publishers, London, 1975, Ch. 4. 40 K. A. Hodd and A. L. Reader, Brit. Polymer J., 1976, 8, 131. 47 S.Crisp, H. J. Prosser, and A. D. Wilson, J. Materials Sci.,1976, 11, 36. 48 A. D. Wilson, Brit. Pol-vmer J., 1974, 6, 165; D. A. Wilson and S. Crisp, ‘Organolithic Macromolecular Materials’, Applied Sciences Publishers, London, 1977, Ch. 4. 49 S. Crisp, A. D. Wilson, J. Elliot, and P. Hornsby, J. Appl. Sci. Biofech.,1977, 27, 369 279 The Chemistry of Dental Cements o~ides,~2~4~~4~ and the minerals special aluminosilicate (ASPA) glas~es,~~,~~ willemite and muscovite.4Q Of these cements those of zinc oxide, the so-called zinc polycarboxylate cements12 and of calcium aluminosilicate glasses, the ASPA (or glass-ionomer) cement^,^^^^^ are of importance as the find practical applications for dentistry.These cements are notable for being bland towards living tissues and adhesive towards enamel, dentine, and base metals. Thus, stainless steel orthodontic buttons can be directly attached to teeth. Traditional dental cements are not adhesive and often irritant towards living tissues. Ionic polymer cements have certain features in common with the alginate impression materials, which are formed by an aqueous ionic reaction in solution between alginic acid and metal salts.A. Ion Binding and Gelation.-Ionic polymer cements stiffen and set as the result of the neutralization of an aqueous solution of a poly(a1kenoic acid) and the formation of an insoluble gel of multivalent metal polysalts. There are under- lying molecular phenomena associated with these physical changes. During neutralization there are changes in the molecular configuration of the polymer chains, which extend as they acquire negative charges, causing the viscosity of solutions to increase.50 Chemical gelation is also associated with the phenomenon of ion-binding. Zon-binding.This is a phenomenon chiefly to be found in polyelectrolyte solutions. Polymer chains containing a high density of charged functional groups exert a considerable attraction on counter-ions which tend to remain in the proximity of these chains.These interactions can give rise to configurational and solvation changes. Generally, as in the case of ionic polymer cements, the chains are polyanions and the counter-ions are cations. There are a variety of binding situations which are determined by the nature of the cation and the polyanion, i.e. size, configuration, charge, charge distribution, polarizability, etc. A distinc-tion may be made between non-specific long range electrostatic attractions, ‘ionic atmosphere binding’, and specific short range attractions, ‘site ion- binding’.51.52 Specific ion binding may be identified by a number of techniques which have been reviewed by Strau~s.5~ The strength of ion-binding depends on the acid strengthp3 molecular con- figuration,u and the number and distribution of‘ ionized carboxy acid groups on the polymer chain.Ion-binding is most marked in polyelectrolyte solutions, where the co-operative effect of electrical charges strung along the polymer chain exerts a considerable influence over the counter-ion. Multivalent cations are more 6o A. Katchalsky and H. Eisenberg, J. Polymer Sci., 1951, 6, 145. I1 H. Morawetz, Fortschr. Hockpolym. Forsch., 1958, 1, 1; M. L. Miller, ‘The Structure of ** Polymers’, Reinhold, New York, 1966, Ch. 12. U. P. Strauss, in ‘Polyelectrolytes’, ed. E. Selegny, M. Mandei, and U. P. Strauss, Reidel Publishing Co., Dordrecht-Holland/Boston, U.S.A., 1974, p.79. 63 V. Crescenzi, A. de Cherico, and A. Ripamonti, Ricercia Sci., 1959,29, 1424; V. Crescenzi, V. De Rosa, and D. Maldarella, ibid., 1960, 30, 1680. N. Muto, T. Komatsu, and T. Nakagawa, Bull. Chem. SOC.Japan, 1973,46,2711; N. Muto, ibid., 1974, 47, 1 122. Wilson strongly bound than univalent cations55 and chelation greatly enhances bond strength.56 There are several different solvation states associated with the formation of ion-pairs, the ions may be in direct contact or separated by one or more layers of water of solvation. According to Ikegami,57 whereas unionized poly(acry1ic acid) has no water of solvation, there are two hydi-ation regions associated with a fully ionized polyacrylate chain: an intrinsic sphere of primary hydration surrounding each carboxylate group and a cylindrical sheath of secondary water produced by the co-operative action of the carboxylate groups along the chain. These hydration states are affected by the nature of the cation.Multi- and bi-valent cations can partly or wholly displace both primary and secondary water, the extent of this disruption depends on the nature of the cation and the degree of ionization of the polyacid chain. By contrast, Li+, Na+, and K+ ions only penetrate the secondary hydration region with the minimum disruption of solvated water. Gelation. Ion-binding and desolvation of salt bridges result in the gelation of polyelectrolytes by precipitation. Examples are the precipitation of naturally occurring polyacids, alginic and pectic acids, by calcium ions,5* and the gelation of aqueous solutions of poly(acry1ic acid) by multivalent cations.55.59 Wall and Drenan59 have attributed this type of gelation to the formation of salt-like cross- links but recognised that there were factors other than coulombic interactions, and indeed configuration changes and desolvation of ions play a role.Ikegami and Imai55 consider that precipitation of a polyelectrolyte does not occur if the ion-pair formed remains hydrated-excepting a salting-out effect at high cation concentrations. Precipitation at low concentration of cations can only occur if a hydrophobic salt bridge is formed and thus formation of this type of ion-binding is accompanied by desolvation, which can be observed by volume changes. Univalent ions, e.g.Na+, are only weakly bound and do not precipitate poly- (acrylic acid) solutions except at high concentrations. More strongly bound bivalent ions, such as Ca2+, even when present in low concentration, will pre- cipitate poly(acry1ic acid) that has been ionized to the extent of 25 %. However, the highly hydrated Mg2+ ions although strongly bound will not precipitate poly(acry1ic acid) until ionization is appreciable (60 % or more) and water is dis- placed from the salt bridge. Such observations indicate that the formation of ‘contact ion-pairs’, rather than ‘solvent-separated’ ion-pairs, is necessary for gelation. Although numerous investigations have been made of ion-binding in dilute solution, extrapolation of these results to the situation pertaining to cement gel must be treated with caution.Studies of ion-binding in concentrated solutions 66 A. Ikegami and N. Imai, J. Polymer Sci., 1962, 56, 133. F. T. Wall and S. J. Gill, J. Phys. Chem., 1954, 58, 1128; A. M. Kotliar and H. Morawetz, J. Amer. Chem. SOC.,1955, 77, 3692; H. P. Gregor, L. B. Luttinger, and E. M. Loebl, J. Phys. Chem., 1955, 59, 34. 61 A. Ikegami, J. Polymer Sci., 1964, A2,907; A. Ikegami, Biopolymers, 1968,6,431. I. Michaeli, J. Polymer Sci., 1960, 48, 291 ;D. A. Rees, Chem. and Ind., 1972, 630; G. R. Seeley and R. L. Hart, Macromolecules, 1974, 7, 706. tis F. T. Wall and J. W. Drenan, J. Polymer Sci., 1951, 7, 83. 28 1 The Chemistry of Dental Cements and in gels present some difficulties, and are rare.The only technique that has been used in this connection is i.r. spectroscopy. Leyte et aL60 have used this method to study ion-binding between alkali and alkaline earth metal ions and poly(a1kenoic acid) and have concluded that the binding is purely ionic and nonspecific in character. Crisp et have applied the attenuated total ~1.~~9~~ reflectance technique to study ionomer cement gels formed by the reaction of metal oxides and silicates with poly(acry1ic acid) solutions. The frequency shifts of the symmetric and asymmetric stretching bands of the carboxylate group were used to determine the type of bonding. The binding of Na+, MgZ+, Ca2+, and perhaps Zn2+ ions to poly(acry1ic acid) were found to be purely ionic (Table 5).The evidence for covalent character in the Zn2+ ... -0OC linkage is Table 5 COz-stretching frequencieslcm-l of some metal polyacrylatesa Metal Asymmetric Symmetric Structure stretch stretch Na+ 1540 1406 Ionic Mgz+ 1535 1403 Ionic Ca2+ 1533 1404 Ionic Znz+ 1540 1398 Ionic ~13+ 1600, 1530 1390 Ionic, structure (1) cu2+ 1605, 1545 1405 Ionic, structure (1) or (2) H+ 1690 1435 (a) S. Crisp, H. J. Prosser, and A. D. Wilson, J. Materials Sci., 1976, 11, 36. inconclusive. The Al3+ and Cu2+ apparently form both ionic and complex forms. Structure (1) may be assigned to the A13+ complex and either structure (1) or structure (2) to the Cu2+ complex. IM B.Poly(a1kenoic acid) solutions.-Both types of ionic polymer cement used in dentistry, the zinc polycarboxylate cement and the glass-ionomer cement, use similar liquids-concentrated aqueous solutions of poly(a1kenoic acids). These solutions are prepared directly by aqueous polymerization of alkenoic acids using ammonium persulphate as the initiator and propan-2-01 as a chain transfer agent.45 The first liquids used in ionomer cements were 40-50 % w/w solutions of poly(acry1ic acid), molecular weight range (Mw)20 000-50 00016,42962 (Table 6). J. C. Leyte, L. D. Zuiderweg, and H. J. Vledder, Spectrochirn. Acta, 1967, 23a, 1397. S. Crisp, M. A. Pringuer, D. Wardleworth, and A. D. Wilson, J. Dent. Res., 1974, 53, 1414. B. W. Bertenshaw and E.C. Combe, J. Dent., 1972173, 1, 13; 1976, 4, 87. Wilson A number of alternative poly(a1kenoic acid)s have been synthesized and include the homopolymers of maleic and methacrylic acid and copolymers of acrylic acid with other alkenoic acids, particularly itaconic acid.63-65 Table 6 Composition of some glass-ionomer cementsa Powder composition/ % w/w Fusion Oxide glasses Fluorine-containing glasses mixture A B C D E F Si02 35.9 30.8 21.9 35.0 32.2 29.0 A1203 30.6 26.1 37.2 29.7 27.3 16.6 CaO 33.5 43.1 40.9 26.2 3.O -CaF2 ---9.1 37.5 34.3 ----5.ONasAlFa -----5.3AlF3 -----9.9AIPO4 -Liquid composition 1. 50.0% w/w poly(acry1ic acid), flw= 23 OOO 2. 47.5% w/w poly(acry1ic acid), flw = 23 OOO and 5.0% w/w tartaric acid. 3.47.5% w/w poly(acrylic/itaconic acids), mw= 23 OOO and 5.0% w/w tartaric acid. (a) A. D. Wilson and B. E. Kent, B.P. 1 316 129/1973; (b) A. D. Wilson, Brit. Polymer J., 1974, 6, 165; (c) A. D. Wilson and S. Crisp, Brit. Polymer J., 1975, 7, 279; (d)A. D. Wilson and S. Crisp, ‘Organolithic Macromolecular Materials’, Applied Science Publishers, London, 1977, Ch. 4. The 2 :1 acrylic acid-itaconic acid copolymer is superior to poly(acry1ic acid) in that its viscosity in 50 % w/w aqueous solution is much less than that of poly- (acrylic acid) and, moreover, does not increase with time and thus cause gelation. At first sight it appears surprising that the copolymer which has a higher COOH :total C ratio than poly(acry1ic acid), and thus more propensity to form hydrogen bonds, should yield aqueous solutions which are less viscous and more stable.Three reasons for this behaviour may be advanced. Firstly, poly(itaconic acid) is known to form seven-membered rings with intramolecular hydrogen bonds66 so reducing the propensity for intermolecular hydrogen bonds to be formed. Secondly, a random copolymer will not have the stereo-regularity of a homopolymer and so have less tendency to form a regular cross-linked array. Thirdly, some pendant groupings on the copolymer chains are more bulky than those present in poly(acry1ic acid), thus increasing steric effects. S. Crisp, B. G. Lewis, and A. D. Wilson, J. Dent. Res., 1976, 55, 299. A. Jurecic, B.P. 1 304 987/1973; S. Crisp, A.J. Ferner, B. G. Lewis, and A. D. Wilson, J. Dent., 1975, 3, 125; ESPE, B.P., 1382881, 1382882/1975; J. A. Barton, jun., G. M. Brauer, J. M. Antonucci, and M. J. Raney, J. Dent. Res., 1975,54, 310. S. Crisp, B. G. Lewis, and A. D. Wilson, J. Dent. Res., 1975,54, 1173. B. E. Tate, Adv. Polymer Sci., 1967,5, 214. The Chemistry of Dental Cements C.Ion-LeachablePowders.-The zinc oxide powder used in zinc polycarboxylate cements is specially prepared by heating it either alone or with magnesium oxide (up to 10 % w/w) to 1100-1300°C for several hours.45 This process is required to deactivate the powder and retard the setting reaction. These metal oxide powder~~~l~~ are generally similar to those used in zinc oxide cements (see Section 3A).The formulation of the glasses used in the glass-ionomer cement is a far-more complex topic. Like the dental silicate cement, the glass-ionomer cement employs ion-leachable glasses as the powder component. These glasses are unusual ; they are essentially calcium aluminosilicates characterized by a high A1 :Si ratio, higher even than that found in the dental silicate cements, and are decomposed to silica gel by mineral acids. There are two basic glass formulations used in this type of cement; these are (a)Si02-Al203-CaO and (b)Si02-AI203-CaFz. Also compositions intermediate between these two main types may be used. In practice more complex formula- tions are used which incorporate auxiliary fluxes in the fusion mixture, e.g. Na3AlF6, AlP04, Ca,(PO&, and NazC03.A number of advantages accrue from the use of a fluoride flux in glass preparation: (i) temperature of fusion is lowered to 1OOO-1200°C; (ii) release of fluoride during the cement-forming reaction improves work- ability, and these glasses, unlike simple oxide glasses, can form workable pastes with plain poly(a1kenoic acid) solution, (iii) fluoride appears to enhance the strength of cements (see Table 7), and (iv) in dental applications topical release of fluoride from cement confers a cariostatic property on adjacent dental enamel. Table 7 Eflect of fluoride in a glass on the strength of glass-ionomera Glass composition (atomic ratios) A C Si 2.0 2.0 A1 2.0 2.0 Ca 2.0 2.0 0 9.0 8.8 F 0 0.4 Powder/liquid ratio/g ml -l 2.5 2.5 Compressive strength 74 125 24 h/MN (a) A.D. Wilson, Brit. Polymer J., 1974, 6, 165. These glasses vary in appearance from clear to opal. The microstructure of one of the opal glasses (Glass F, Table 6) has been studied in detail, 65 and found ‘7 B. W. Bertenshaw and E. C. Combe, J. Dent., 1972/73, 1, 65. T. I. Barry, R. P. Miller, and A. D. Wilson, ‘XI Conference on the Silicate Industry’,Budapest, 1973, p. 881. Wilson to be complex. There is a main calcium aluminosilicate matrix interspersed with phase-separated droplets. The droplets themselves are of complex morphology with an inner core of fluorite sheathed by a layer of calcium aluminosilicate (of higher calcium content than that of the surrounding matrix). The glasses are ion-leachable and for the same reasons as those advanced for the acid-decomposible nature of those used for dental silicate cements (Section 3B p.274). The reactivity of these glasses, towards acids, varies and in general depends on the A1203 :Si02 ratio. Obviously the higher the A1 :Si ratio the higher will be the negative charge on the aluminosilicate network and hence the greater will be its susceptibility to acid attack. Since poly(acry1ic acid) is weaker than phosphoric acid, the A1203 :Si02 ratio in glasses intended for glass-ionomer cements is greater than that in those for dental silicate cements. D. Setting Reactions in Ionomer Cements.-Experimental investigations on the setting reaction of zinc polycarboxylate cements are limited; however, an i.r.spectroscopic study confirms that zinc oxide interacts with poly(acry1ic acid) solution to form zinc polyacrylate salt.47 More extensive studies have been made on the glass-ionomer cements. The setting reaction between an ion-Ieachable glass (Glass F, Table 6) and a poly- (acrylic acid) solution has been followed using chemicalsg and i.r. spectroscopic methods.61 The cement-forming reaction takes place in a number of overlapping stages: the acid-attack on the glass when ion-leaching occurs, the initial precipita- tion and gelation, and long-term reactions-diffusion and hydration processes. Firstly, on mixing the glass powder and poly(acry1ic acid) solution an immediate reaction occurs.Changes in the i.r. spectra (Table 8) are consistent with the loss of hydrogen ions from the poly(acry1ic acid) solution (COOH -+ COO-) and the decomposition of the aluminosilicate glass network to silica gel. (EMPA68 indicates that this attack occurs in the surface regions of the glass particles). AP+, Ca2+, Naf, F-, and Pod3- ions are released from the glass and as the reaction continues they accumulate in solution (Figure 6) together with ionizing poly(acry1ic acid) chains : concurrently, the pH of the aqueous phase increases. When these processes reach a certain point, the second stage of the cement- forming reaction is initiated as cations and anions begin to precipitate and gela- tion occurs (Figure 6), in this example after 10 min at 23°C.The precipitation of an insoluble salt gel can be seen to result from the binding of multivalent cations to ionized poly(acry1ic acid) chains (see Section 4A) with formation of desolvated ionic cross-links (salt bridges). As the reaction proceeds, more cations become bound, the number of cross-links increases and the salt gel hardens. Although both calcium and aluminium are major constituents of the glass, i.r. spectroscopics1 and chemical resultse9 (Table 8, Figure 6) show that gelation or intial set results from the formation of calcium polyacrylate alone. According to i.r. spectroscopic data aluminium polyacrylate is not formed for about one hour, i.e. after set, although the cement used was prepared at a low powder/ liquid ratio (1.5 g ml-l) and consequently set more slowly than that used in the S.Crisp and A. D. Wilson, J. Dent. Res., 1974, 53, 1408, 1420. N00 03 2 n % tY2 %Table 8 Some changes occurring in the i.r. spectra (ATR) of a setting glass-ionomer cementa Species BandJcm Mode Components Fresh Set Hardened fPowder Liquid Pastel5 min Cement Cement/24 h COOH 1 700 C=O, stretch -Strong Strong Attenuated Weak 2 COO-(Al) 1600 COO-, asym. ---Appears Strong stretch COO-(Ca) 1540 COO-,asym. --Appears Enhanced Strong stretch Silica gel 1050 Si-0, stretch --Appears Enhanced StrongGlass 940 Strong --(a) S. Crisp, M. A. Pringuer, D. Wardleworth, and A. D. Wilson, J. Dent. Res., 1974, 53, 1414. Wilson I 1 I I I IIIII I I I 1 I l,,l I I 1 11 IIII I 10 100 1mo time, min Figure6 The extraction and precipitation of ions in a setting glass-ionomer cement shown as soluble ionltime curves (Reproduced by permission from J.Dent. Res., 1974, 53, 1420) chemical studies (3 g ml-1). At this stage of the reaction the cement is flexible and elastomeric. Subsequently, as the cement hardens to a rigid mass, aluminium, as well as calcium polyacrylate is formed (Table 8), and in the fully set cement both salts are present in equal amounts. Several reasons have been advanced to explain the delayed formation of aluminium polyacrylate. Differences in the morphology between Ca2f and AP+ ions in the glass surface may result in differential leaching. The aluminium ion, which is fully hydrated, is less mobile than the calcium ion.There is an increased entropy factor necessary for the formation of aluminium polyacrylate arising from the more demanding steric requirements for the binding of aluminium ions to polyacrylate chains. The rheological changes accompanying these chemical processes are of clinical importance and can be affected by the chemical composition of the cement. Dental cements should be easy to mix, remain workable while being manipulated into position, and then set rapidly. Cements which steadily increase in stiffness are not desirable. Ideally, the paste should remain fluid and plastic right up to the moment of set, i.e. the set should be sharp, and thus combine maximum working time with minimum setting time. In this connection the presence of complexing agents in glass-ionomer cements plays a significant role.The fluoride ion is important in controlling rheological parameters and setting characteristics and Crisp and WilsonM have shown that the fluoride ion aids the extraction of cations from the glass powder. The release of fluoride ions also improves workability; the cement paste prepared from the 1150°C glass is The Chemistry of Dental Cements workable while that from the 1300°Cglass is not. Simple oxide glasses are also known to yield unworkable pastes with simple poly(acry1ic acid) solutions. These observations suggest that the formation of fluoride complexes plays an important role in the cement-forming reaction and aluminium is known to form a whole series of complexes ALF2+,AIF2+, etc.The effect of complex formation will be to aid the extraction and transport of cations and to prevent their pre- mature binding to polyanion chains. Other complexing agents notably tartaric acid, can play a similar role, when incorporated in the Such an addition improves ease of manipulation, prolongs working time and sharpens the set of the cement paste. Indeed, powders of simple oxide glasses not containing fluoride, which yield intractable and slowly setting pastes when combined with plain polyacid solutions, give workable cements when tartaric acid is added to the liquid. It has been suggested that the formation of metal-tartaric acid complexes aids the extraction of cations from the glass and, also, temporarily prevents them binding to the polyanion chains, thus preventing premature gelation and sharpening the set.In efTect this amounts to the acid-base reaction taking place, initially, between the glass and tartaric acid and not between the glass and the polyacid. A further suggestion has been made that tartaric acid bridges pairs of metal ions as shown in structure (3). 0 H II I H 0 This molecular unit may be more effective than a simple metal ion in bridging polyanion chains by reason of it having some flexibility. E. Cement Structure.-Little is known about the structure of the zinc poly- carboxylate cement. The binding of zinc ions to polyacrylate chains is, as indi- cated above, almost purely ionic.*’ Since nearly all the water contained in the cement is ‘evaporable’ and not tightly bound,71 the structure of the cement may be similar to that of the fused zinc polyacrylate cement of Niel~en.~2 The structure of this cement was studied using X-ray diffraction and i.r.transmission spectroscopy, and measuring various mechanical and rheological properties.72 It was concluded that pendant half-salt, in-chain di-salt, and a cross-chain di-salt structure were present in the material (Figure 7). The microstructure of the glass-ionomer cement has been established lo A. D. Wilson, S. Crisp, and A. J. Ferner, J. Dent. Res., 1976, 55,489; S. Crisp and A. D. Wilson, J. Dent. Res., 1976, 55, 1023. 71 J. M. Paddon and A. D. Wilson, J. Dent.Res., 1977, 56, Special issue A, A175.’* L. E. Nielsen, Polymer Preprints, 1968, 9, 596; J. E. Fields and L. E. Nielsen, J. Appl.Polymer Sci., 1968, 12, 1041. Wilson jH2\ /fH2\\CH //CH2 \ /HZ CH CH CH co co co 0 0 I O\ /O ZnOH ZnOH Zn Pendant half -salt In-chaindi-salt / \ \ /CH-COOH/“ \ Cross-chain di -salt Figure 7 Proposed molecular structure for zinc polyacrylate salts (see Figure 8 on page 293).68 Although chemically different the structure re- sembles that of the dental silicate cement: the matrix is particulate and the glass particles are degraded in the surface regions to silica gel. The composition of the matrix is quite different and composed of calcium and aluminium poly- acrylates and unlike that of the dental silicate cement is adhesive to tooth struc- ture and base metals.73 As indicated previously, the binding of Ca2f ions to polyacrylate chains is purely ionic whereas that of A13f ions involve some complex formation.How- ever, the exact molecular structure remains the subject of speculation; possibly there are structures present similar to those found in Nielsen’s salt. In addition, since aluminium and calcium will be six-co-ordinate then there will be ligands, in addition to the polymer-COO- groups, such as HzO, OH-, and F-. Studies on the hydration states of these cements indicate that about half the water is ‘non-evaporable’ and tightly bound which suggests that ligand water is present in these cements. Structures of the type illustrated in Figure 9 have been postulated.4* This cement is adhesive towards base metals, tooth enamel (apatite), and ’* P.Hotz, J. W. McLean, I. Sced, and A. D. Wilson, Brit. Dent. J., 1977, 142, 41. The Chemistry of Dental Cements I I CH2 H20 H20 CH2 CH2 H20 10-\/ I C -0--\2+720CH-C-O-CS-O-C-CH Ca --A-CH-CH2 H20 CH2 CH2 H20 I /\H20 I /\H20 I I I CH2 I0I III I CH-C\ H20CH2 A-H20 CH2 I I C\I 2+/H20 CH2 Ca -/ \I ,o 1 "2O CH -C' H20/\ ii I ,H20 H20 CHz 16 I I CH2I Figure 9 Some postulated molecular structures present in the glass-ionomer cement dentine (c0llagen).~3 It does not adhere to chemically unreactive surfaces such as porcelain, gold, and platinum. Adhesion to oxide surfaces is believed to be by hydrogen and metal ion bridges formed between cement carboxylate groups and oxygen anions.5 Non-aqueous Zinc Oxide Dental Cements Zinc oxide can be treated with a large number of proton donating organic liquids to form cements, some of which are of practical use. These organic liquids include eugeno19 and the 2-methoxy phenols,'4 the S-diketones,lo other chelating agents,lO and certain monocarboxylic acids,75 which include 2-ethoxybenzoic acid.ll Cements based on eugenol are the oldest and remain the most important in this class. They were developed in the years 1872-1875 when various com- binations of phenolic compounds with zinc oxide were being explored for cement formation.9 This cement has retained its popularity in dentistry as a temporary filling material and cavity base lining, to the present day.Although weak and 74 G. M. Brauer, H. Argentar, and G. Durany, J. Res. Nat. Bur. Stand. Sect. A, 1964, 68, 619. 76 E. W. Skinner, E. J. Molnar, and G. Suarez, J. Dent. Res., 1964, 43, 915; E. J. Molnar, U.S.P.N.3 028 24711962, 290 Wilson hydrolytically unstable it has ideal working properties, is easy to use and is tolerant towards living tissues. As with many dental cements its structure and setting reaction have only recently been elucidated. The two components of the zinc oxide cement merit some discussion. Eugenol is a dimeric liquid, where the molecules are connected in pairs by an inter- molecular phenolic hydrogen bridge. A second hydrogen bond (intramolecular) forms a five-membered ring (see Figure 10).CH2 =CH CH2 CH2 = CH CH2 \ \ CH3-O, ,o Zn H.CH3 \ Matrix Region0 / CH$H =CH2 CH2CH = CH2 Matrix Region Water eluted Figure 10 Hydrolysis of zinc eugenolate to eugenol and zinc hydroxide showing the molecu- lar structures of zinc eugenolate and eugenol (Reproducedby permission from J. Dent. Res., 1973, 52, 253) The hydrogen bonds are dissimilar and are to be distinguished in the i.r. spectrum of eugenol (which exhibits H-0 stretching frequencies at 3460 and 3520 cm-l 18$?6) (Table 9); guaiacol has-a similar structure (Table 10). Although Gerner et a1.76 consider the intramolecular bond in eugenol to be weaker than in the intermolecular bond, this conclusion is doubtful as the Badger-Bauer'? formulae used, strictly only applies to linear hydrogen bonds.78 In fact, dipole measurements79 have shown that the intramolecular hydrogen bond in guaiacol is much stronger than would be expected from such calculations. The zinc oxide powder is in a reactive form and is prepared either by oxidation of zinc metal or low temperature ignition of zinc salts.Increasing the ignition temperature deactivates the zinc oxide.80 The paste, prepared by mixing the two components, sets to a friable cement consisting of zinc oxide particles 'I6 M. M. Gerner, B. A. Zadorozhnyi, L. V. Ryabina, V. N. Batovskii, andV. I. Sharchilev, Russ. J. Phys. Chem., 1966, 40, 122 (translated from Zhur. $2. Khim., 1966, 40, 231). 77 R. M. Badger and S.H. Bauer, J. Chem. Phys., 1937, 5, 839. 'IB L. P. Kuhn and R. A. Wires, J. Amer. Chem. SOC.,1964, 86, 2161. 7e J. H. Richards and S. Walker, Trans. Farad. SOC.,1961, 57, 399. 6o D. C. Smith, Brit. Dent. J., 1958, 105, 313. 291 The Chemistry of Dental Cements Table 9 I.r. spectral diferences between eugenol and a zinc oxide eugenol cement*a Eugenol Deuterio-eugenol Cement Assignment 3520, 3460 2600, 2560 Absent O-H or 0-D stretching 1370 1333 1319 Ring stretching coupled in eugenol with O-H or 0-D in-plane deformation 1269 1269 1266, 1287 C-0 stretching, aromatic O-cH3 (split in cement) 1209 963 Absent O-H or O-D in-plane deformation 794 Reduced Absent Probably an i.r. inactive mode activated by coupling with O-H out-of-phase deformation 48-50 < 400 Absent 0-H out-of-plane deformation (broad background absorption) *Absorption bands given as cm-l (a) A.D. Wilson and R. J. Mesley, J. Dent. Res., 1972, 51, 1581. Table 10 I.r. spectral diferences between guaiacol and a zinc oxide guaiacol cement*a Guaiacol Deuterio-guaiacol Cement Assignment 3490, 3440 2590, 2550 Absent O-H or 0-D stretching 1357 1332 1319 Ring stretching, coupled in guaiacol with O-H or 0-D in-plane deformation 1208 951 Absent O-H or O-D in-plane deformation 650-500 Not detected Absent 0-H out-of-plane deformation (broad background absorption) *Absorption bands given as cm-l (a) A. D. Wilson and R. J. Mesley, J.Denr. Res., 1972, 51, 1581. embedded in a salt-like matrix.81 The reaction is accelerated in the presence of moisture80*s2or by the addition of zinc ~alts.8~~~~ The setting reaction between zinc oxide and eugenol is accompanied by changes in i.r. spectra as phenolic hydrogen is progressively replaced by Zn. Bands associated with phenolic OH groups diminish and ultimately disappear; similar observations have been made for the reactions between zinc oxide and deuterio- A. D. Wilson, D. J. Clinton, and R. P. Miller, J. Dent. Res., 1973, 52, 253. 8a R. F. Batchelor and A. D. Wilson, J. Dent. Res., 1969, 49, 883. W. Harvey and N. J. Petch, Brit. Dent. J., 1946, 80, 1. Figure 8 The microstructure of the gloss-ionomer cement (Crown Copyright) The Chemistry of Dental Cements eugenol, guaiacol and deuterio-guaiacolls (Tables 9 and 10).These are the only spectral changes that occur in these cement-forming reactions and indicate that phenolic salts are formed. Copeland et in 1955 reported that the cement matrix of the zinc oxide-eugenol cement has the empirical molecular formulae Zn(CloHllO2)2 and its X-ray diffraction pattern corresponds closely to that of zinc eugenolate salt. They proposed a chelate structure for zinc eugenolate, arguing that only phenolic bodies with methoxy groups in the ortho position were capable of cement formation with zinc oxide. The structure is seen as electrically neutral with two eugenolate ligands enclosing a central zinc ion (Figure 10). Wilson and Mesleyls have pointed out that the i.r. spectra of eugenol and zinc eugenolate are remarkably similar; the only spectral differences are associated with the loss of OH groups, which is in accordance with the similarity between the dimeric structure of eugenol and the bisligand chelate structure of zinc eugenolate.However, these workers in commenting on the work of Copeland et al., have pointed out that although the three principal X-ray diffraction lines (at 17.0, 8.8, and 7.0 A) are present in both the matrix material and zinc eugenolate salt, there were numerous minor differences. Indeed, the value of X-ray diffraction data may be doubtful for El-Tahawi and Craigs6 have shown that the zinc eugeno- late matrix material is mainly amorphous.Gerner et al.76consider that the co-ordinate Zn-OCH3 link is weak, but this conclusion is not definitive since Wilson and Mesleyls consider that it is based on incorrect assignments. Although water is generated in the cement forming reaction, thermal analysis shows it to be absent in the set cement,18 an observation which has led to the speculation that H2O plays a structural role. Wilson and Mes!ey18 have argued that the central zinc ion may be six-co-ordinate, with the two eugenolate ligands in a square planar configuration with H2O molecules occupying the two remaining diametrically opposed sites. It was further argued that since only one H20 molecule is available for each zinc ion, then these HzO molecules are shared and serve to bridge individual chelate molecules (Figure 1 1).Permittivity studies86 support this view as they provide evidence for the initial generation of free water and its subsequent removal during the course of the reaction. Although the cement-forming processes may be represented as a simple acid- base reaction: 2CloHl100H + ZnO = (Cl,Hl102)2Zn + H20 (3) the exact mechanism has yet to be established. The addition of water to the powder80 or the liquid,87 or an increase in humidity of the surrounding atmo- sphere,82 serve to accelerate the reaction. The reaction is also accelerated by zinc acetate.80~83 The observations suggest that the reaction is an ionic one between simple or complex zinc ions and eugenolate ions, with the formation 84 H. I.Copeland, G. M. Brauer, W. T. Sweeney, and A. F. Forziati, J. Res. Nat. Bur. Stand. Sect. A., 1955, 55, 133. M. Braden and R. L. Clarke, J. Denr. Res., 1974, 53, 1263. 86 H. M. El-Tahawi and R. G. Craig, J. Dent. Res., 1971, 50, 430. 81 R.Viellefosse, H. Vayson de Pradenne, and J. P. Zumbrunn, Rev. Odont-Stromat, 1958, 5, 488. 294 Wilson H?O I Figure 11Proposed H20-bridged structure for the zinc oxide eugenol cement matrix of zinc eugenolate. Zinc ions can be provided by addition of a zinc salt, e.g. zinc acetate which, therefore, acts as an accelerator. In the absence of zinc salts, zinc ions can be generated by the action of water on zinc oxide to convert it to a hydrate, which can then provide zinc in ionic form, as Zn OH+ in Zn(H2O)s OH+.In the absence of' water the reaction is inhibited. However, once the reaction is initiated it becomes autocatalytic, since the reaction generates water. The rate of set depends on the nature of the zinc oxide used80 and the temperature of ignition used in preparing the zinc sa!t is also important. Zinc oxide becomes progressively deactivated as this temperature is increased and when ignited to above lO0O"C the cement-forming reaction is totally inhibited. The microstructure of zinc eugenolate cements may be briefly described. Studies using electron microscopy81 have shown that the set cement consists of zinc oxide particles covered by outgrowths of zinc eugenolate which form a loosely connected matrix. The zinc eugenolate matrix develops in the region of the paste originally occupied by the eugenol liquid. At one time the zinc eugeno- late matrix was considered to be crystalline. However, recent work indicates that the matrix is mainly amorphous, although small amounts of crystalline material are formed in cements accelerated by zinc acetate.85 The friable nature of this cement has been attributed to its voided structure, poorly inter-connected matrix, The Chemistry of Dental Cements and the inherent weakness of zinc eugenolate domains where the mononuclear ligands are joined by water bridges only.This cement can only be used as a temporary filling material because the matrix is hydrolytically unstable.81-88 The CHsO-co-ordinate link of the zinc eugenolate chelate is weak and the chelate hydrolyses into eugenol and zinc hydroxide.The zinc eugenolate is washed away from the zinc oxide particles and from the matrix. Some residual coherence is imparted to the hydrolysed cement by plate-like growths of ;zinc hydroxide which are relics of zinc eugenolate domains (Figure 10). The hydro- lysis of zinc eugenolate may be regarded as a reversal of the original cement- forming reaction. Other phenolic substances have cement-forming capability provided certain structural conditions are met. While simple phenol does not form a cement certain methoxyphenols, other than eugenol, do.74 These are all 2-methoxy- phenols: 2-methoxyphenol (guaiacol), 4-propenyl-2-mcthoxyphenol (‘iso-eugenol’ an isomer of eugenol) and 4-n-propylmethoxyphenol .The more acidic 4-propenyl-2-methoxyphenoi reacts somewhat faster than eugenol since the propenyl group withdraws electrons from the aromatic ring. The 3-methoxy- phenols, for geometric reasons, cannot form chelate rings with zinc ions and certain 2-methoxyphenols which have substituents adjacent to the methoxy and hydroxy groups cannot form cements, either because of the steric effects of these groups, which interfere with the chelation reaction. Examples include two isomers of eugenol, 3-allyl- and 5-allyl-2-methoxyphenols,and 5-n-propyl-2-methoxy- phenol. Other liquid chelating agents in addition to the phenols are capable of cement formation.10 These include salicylaldehyde, 7-n-propyl-8-hydroxyquinolineand, more particularly, a whole range of /3-diketones for which diketones, the sub- stituent groups adjacent to the keto groups are important. Thus acetylacetone, which has methyl groups, readily forms a cement with zinc oxide whereas others, which contain bulky alkyl groups, do not. A. D. Wilson and R.F.Batchelor, J. Dent. Res., 1970, 49, 593.

ISSN:0306-0012    

DOI:10.1039/CS9780700265

出版商:RSC    

年代:1978

数据来源: RSC