摘要:

Ordinary Members PROFESSOR R. J. DONOVAN 1983 PROFESSOR M. C. R. SYMONS 1983 DR G. J. HILLS 1984 PROFESSOR J. M. THOMAS 1983 PROFESSOR A. J. LEADBETTER 1984 DR J. ULSTRUP 1985 DR I . W. M. SMITH 1985 PROFESSOR G. WILLIAMS 1985 PROFESSOR F. L. SWINTON 1983 DR D. A. YOUNG 1984 Honorarj, Secretarj-: DR G. J. HILLS Honorarj- Treasurer : PROFESSOR P. GRAY The President thanked the retiring members of Council, Vice-presidents Professor Sheppard and Professor Wagner, and Ordinary Members Professor King and Professor Purnell, for their services. 5. Reriew of Futurr Acfirifies A programme of future activities of the Division had been tabled and the President drew attention to the forthcoming General Discussions and Symposia. xiOrdinary Members PROFESSOR R. J. DONOVAN 1983 PROFESSOR M. C. R. SYMONS 1983 DR G. J. HILLS 1984 PROFESSOR J. M. THOMAS 1983 PROFESSOR A. J. LEADBETTER 1984 DR J. ULSTRUP 1985 DR I . W. M. SMITH 1985 PROFESSOR G. WILLIAMS 1985 PROFESSOR F. L. SWINTON 1983 DR D. A. YOUNG 1984 Honorarj, Secretarj-: DR G. J. HILLS Honorarj- Treasurer : PROFESSOR P. GRAY The President thanked the retiring members of Council, Vice-presidents Professor Sheppard and Professor Wagner, and Ordinary Members Professor King and Professor Purnell, for their services. 5. Reriew of Futurr Acfirifies A programme of future activities of the Division had been tabled and the President drew attention to the forthcoming General Discussions and Symposia. xi

ISSN:0300-9599    

DOI:10.1039/F198278FX037

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

3 708 REVIEW OF BOOKS is the absence of any reference to possible new and potentially significant applications for polymer latices. Novel applications may well be found in at least two directions, namely, those which exploit the large polymer-aqueous-phase specific surface area of latices, and those which exploit the electrical dissymmetry which is present at the interface between polymer and aqueous phase in the case of electrostatically stabilised latices. No reference is made in this book to the efforts which have so far been made to exploit for medical purposes the adsorptive and binding potentialities of the large area of polymer-aqueous-phase interface in latices. Nor is there any mention of possible catalytic applications of this large interfacial area. So far, catalytic applictions have been confined to those which rely essentially upon enhancement of the counter-ion concentration in regions of the electrical double layer which are near to the polymer surface.However, it is at least possible that the adsorptive capacity of the interface may also be useful in catalytic applications. Some discussion of possibilities such as these would have been welcome. D. C. BLACKLEY Received 14th April, 1982 Shock Waves in Chemistry. Ed. by ASSA LIFSHITZ. (Marcel Dekker, New York, 1981). Pp. ix + 390. Price SFr 182. After a somewhat hesitant start, the use of shock waves to study chemical and physical processes at high temperatures has become an accepted technique and reliable kinetic data can be obtained in this way. Several books have been written, notably by Bradley and by Gaydon and Hurle, which describe not only the underlying principles and the experimental procedures but also give some account of the early results obtained using shock waves to provide high temperatures for short, well defined times in the reactant gases.Inevitably, these books have become rather dated. This new book, edited by Lifshitz, is rather different. It is a collection of self-contained review articles on various aspects of shock waves. The first (by Khandelwal and Skinner) is concerned with hydrocarbon oxidation, and the next (by Tsang) describes the results obtained using the comparative rate technique which he has pioneered. Both these articles include extensive lists of references and represent useful summaries of the present situation.Boyd and Burns have contributed a chapter on dissociation-recombination reactions, while Kiefer describes the laser-schlieren method which he has done so much to develop. There is another chapter by an acknowledged expert, Just, on atomic resonance absorption spectrometry. Under shock-tube conditions it is very seldom that the concentrations of radicals and other species reach a steady state, and so the classical Bodenstein steady-state approximation cannot be used. Instead, it is necessary to integrate the differential equations describing the time-variation of species concentration, and Gardiner, Walker and Wakefield have provided a useful guide to the computational procedures available in this and other aspects of shock-tube work.In addition to these contributions there is another by Bar-Nun on Chemical Aspects of Shock Waves in Planetary Atmospheres which, although interesting in itself, fits rather uneasily with its companions. As is inevitable in a book of this type the standard and style of the chapters varies and there is some overlapping material; none of this, however. represents a serious drawback. What is more difficult to understand is the audience for whom the book is intended. Each chapter is a useful and interesting review which will appeal to a fairly restricted readership, but, in the opinion of this reviewer, the whole volume lacks coherence. The time-honoured phrase ‘should be on the shelves of every library’ probably applies, though the price, over &50 at the current exchange rate, must cause all university librarians to flinch in these days of U.G.C.cuts. There is still room for the definitive up-to-date book to be written on shock waves in chemistry. J. A. BARNARD Received 5th April, 19823 708 REVIEW OF BOOKS is the absence of any reference to possible new and potentially significant applications for polymer latices. Novel applications may well be found in at least two directions, namely, those which exploit the large polymer-aqueous-phase specific surface area of latices, and those which exploit the electrical dissymmetry which is present at the interface between polymer and aqueous phase in the case of electrostatically stabilised latices. No reference is made in this book to the efforts which have so far been made to exploit for medical purposes the adsorptive and binding potentialities of the large area of polymer-aqueous-phase interface in latices.Nor is there any mention of possible catalytic applications of this large interfacial area. So far, catalytic applictions have been confined to those which rely essentially upon enhancement of the counter-ion concentration in regions of the electrical double layer which are near to the polymer surface. However, it is at least possible that the adsorptive capacity of the interface may also be useful in catalytic applications. Some discussion of possibilities such as these would have been welcome. D. C. BLACKLEY Received 14th April, 1982 Shock Waves in Chemistry. Ed. by ASSA LIFSHITZ. (Marcel Dekker, New York, 1981). Pp. ix + 390.Price SFr 182. After a somewhat hesitant start, the use of shock waves to study chemical and physical processes at high temperatures has become an accepted technique and reliable kinetic data can be obtained in this way. Several books have been written, notably by Bradley and by Gaydon and Hurle, which describe not only the underlying principles and the experimental procedures but also give some account of the early results obtained using shock waves to provide high temperatures for short, well defined times in the reactant gases. Inevitably, these books have become rather dated. This new book, edited by Lifshitz, is rather different. It is a collection of self-contained review articles on various aspects of shock waves. The first (by Khandelwal and Skinner) is concerned with hydrocarbon oxidation, and the next (by Tsang) describes the results obtained using the comparative rate technique which he has pioneered.Both these articles include extensive lists of references and represent useful summaries of the present situation. Boyd and Burns have contributed a chapter on dissociation-recombination reactions, while Kiefer describes the laser-schlieren method which he has done so much to develop. There is another chapter by an acknowledged expert, Just, on atomic resonance absorption spectrometry. Under shock-tube conditions it is very seldom that the concentrations of radicals and other species reach a steady state, and so the classical Bodenstein steady-state approximation cannot be used. Instead, it is necessary to integrate the differential equations describing the time-variation of species concentration, and Gardiner, Walker and Wakefield have provided a useful guide to the computational procedures available in this and other aspects of shock-tube work.In addition to these contributions there is another by Bar-Nun on Chemical Aspects of Shock Waves in Planetary Atmospheres which, although interesting in itself, fits rather uneasily with its companions. As is inevitable in a book of this type the standard and style of the chapters varies and there is some overlapping material; none of this, however. represents a serious drawback. What is more difficult to understand is the audience for whom the book is intended. Each chapter is a useful and interesting review which will appeal to a fairly restricted readership, but, in the opinion of this reviewer, the whole volume lacks coherence. The time-honoured phrase ‘should be on the shelves of every library’ probably applies, though the price, over &50 at the current exchange rate, must cause all university librarians to flinch in these days of U.G.C. cuts. There is still room for the definitive up-to-date book to be written on shock waves in chemistry. J. A. BARNARD Received 5th April, 1982

ISSN:0300-9599    

DOI:10.1039/F198278BX039

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

JOURNAL OF THE CHEMICAL SOCIETY FARADAY TRANSACTIONS, PARTS I AND 11 The Journal of The Chemical Society is issued in six sections: Journal of The Chemical Society, Chemical Communications Journal of The Chemical Society, Dalton Transactions Journal of The Chemical Society, Faraday Transactions, I Journal of The Chemical Society, Faraday Transactions, II Journal of The Chemical Society, Perkin Transactions, I Journal of The Chemical Society, Perkin Transactions, I I Thus, five of the sections are directly associated with three of the Divisions of The Royal Society of Chemistry: the sixth is Chemical Communications. This continues to be the medium for the publication of urgent, novel results from all branches of chemistry. Communications should not normally exceed one printed page in length and authors are required to submit three copies of the typescript and two copies of a statement of the reasons and justification for seeking urgent publication of the work.This Section is intended to be essentially a journal for inorganic chemists containing papers on the structure and reactions of inorganic compounds and the application of physical chemistry techniques to, e.g. the study of inorganic and organometallic compounds and problems, including work on the kinetics and mechanisms of inorganic reactions and equilibria, and spectroscopic and crystallographic studies of inorganic compounds. Journal of the Chemical Society, Faraday Transactions, I and II These are, respectively, physical chemistry and chemical physics journals. P A R T I (physical chemistry) includes papers on such topics as radiation chemistry, gas-phase kinetics, electrochemistry (other than preparative), surface and interfacial chemistry, heterogeneous catalysis, physical properties of polymers and their solutions and kinetics of polymerization, etc.P A R T I I (chemical physics) contains theoretical papers, especially those on valence and quantum theory, statistical mechanics, intermolecular forces, relaxation phenom- ena, spectroscopic studies (including i.r., e.s.r., n.m.r., and kinetic spectroscopy, etc.) leading to assignments of quantum states, and fundamental theory, and also studies of impurities in solid systems, etc. Journal of The Chemical Society, Chemical Communications Journal of The Chemical Society, Dalton Transactions Journal of The Chemical Society, Perkin Transactions, I and II These are, respectively, the organic chemistry and the physical organic chemistry sections of the Journal.P A R T I (organic and bio-organic chemistry) is designed to contain papers on all aspects of synthetic, and natural product organic and bio-organic chemistry and to deal with aliphatic, alicyclic, aromatic, carboncyclic and heterocyclic compounds. Papers on organometallic topics are considered for either the Dalton or the Perkin Transact ions. iP A R T I I (physical organic chemistry) is for papers on reaction kinetics and mechanistic studies of organic systems and the use of physico-chemical, spectroscopic, and crystallographic techniques in the solution of organic problems. Notice to Authors ( 1 ) Although authors need not be members of the Royal Society of Chemistry it is hoped that they will be.(2) Authors must indicate the Part of the Journal they wish their paper to appear in. This preference will be respected unless it is obviously erroneous in terms of the scientific content of the paper. (3) Since all papers will be subjected to refereeing, in parallel, by two independent referees, the original typescript (quarto or A4 size) and two good-quality copies should be provided. (4) All papers should be sent to the Director of Publications, The Royal Society of Chemistry, Burlington House, Piccadilly, London W I V OBN. ( 5 ) For details of manuscript preparation, preferred usages, etc. the Instructions to Authors, previously available from the Faraday Society, and now obtainable from The Royal Society of Chemistry, should be consulted. (6) The Society will adopt the following abbreviations for the new journals in all its publications.J. Chem. Soc., Chem. Commun. J. Chem. SOC., Dalton Trans. J. Chem. Soc., Faraday Trans. I J . Chem. Soc., Faraday Trans. 2 J . Chem. Soc., Perkin Trans. I J. Chem. Soc., Perkin Trans. 2 * The author to whom correspondence should be addressed is indicated by an asterisk after his name in the heading of each paper. 11THE FARADAY DIVISION O F THE ROYAL SOCIETY O F CHEMISTRY 1982 Bourke Lectures by Professor E. Clementi Professor E. Clementi (International Business Machines Corporation, U.S.A.) will give the 1982 Bourke Lectures at the following venues: Monday, 1 8 October at Birkbeck College, London Tuesday, 1 9 October at Cambridge University Wednesday, 20 October at Oxford University Thursday, 21 October at Sheffield University Friday, 22 October at St Andrews University Further information may be obtained from: Mrs Y. A.Fish, The Royal Society of Chemistry, Burlington House, London WIV OBN THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY S Y M P O S I U M NO. 1 7 The Hydrophobic Interaction University of Reading, 15-1 6 December 1982 This term refers to interactions between chemically inert residues arising from perturbations in the unique spatial and orientational correlations in liquid water. These effects provide a major contribution to many of the non-covalently bonded structures that form the basis of life processes.Current advances in the statistical mechanics of polar fluids, intermolecular forces, computer simulation, and membrane physics are providing a new basis for the re-examination of various aspects of hydrophobic effects, their origin and their quantitative descri pt ion. Such theoretical treatments will be confronted with recent experimental work on simple model systems which, it i s hoped, will lead to a better understanding of hydrophobic interactions in more complex processes. The following have agreed to contribute to the symposium: A. Ben-Naim, H. J. C. Berendsen, D. L. Beveridge, S. D. Christian, L. Cordone, D. Eagland, D. Eisenberg, R. Lumry, P. J. Rossky, M. C. R. Symons, H. Weingartner, M. D. Zeidler The programme and application form may be obtained from: Mrs Y.A. Fish, The Royal Society of Chemistry Burlington House, London W1V OBNTHE FARADAY DIVISION O F THE ROYAL SOCIETY O F CHEMISTRY GENERAL DISCUSSION NO. 75 Intramolecular Kinetics I University of Warwick, 18-20 April 1983 Organising Committee Professor J. P. Simons (Chairman) Dr M. S. Child Professor R. J. Donovan Dr G. Hancock Experimental and theoretical interest in the time-dependent behaviour of isolated molecules, radicals or ions is strong and increasing. The Discussion will be concerned with the kinetics of processes which occur in isolated species following their preparation in states with non-equilibrium energy distributions (e.g. by photon absorption or collisional activation). Topics covered will include: (a) theoretical and experimental studies of energy redistribution in isolated species; ( b ) observation and theoretical modelling of the competition between intramolecular energy redistribution and radiative decay or radiationless processes (e.g.internal conversion, fragmentation, isomerisation). The preliminary programme may be obtained from : Mrs Y. A. Fish, The Royal Society of Chemistry Burlington House, London W1V OBN Dr D. M. Hirst Professor K. R. Jennings Dr R. Walsh THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION NO. 76 Concentrated Colloidal Dispersions Loughborough University of Technology, 14-1 6 September 1983 The meeting will discuss the experimental investigation and the theoretical description of the properties of concentrated colloidal dispersions, i.e.those systems in which the particle-particle interactions are strong enough to cause significant deviations from ideal behaviour. Both the structural and dynamic features of concentrated systems as determined by scattering, rheological and other techniques will be considered. It is anticipated that a range of dispersion types will be discussed. These will include both 'model' systems and dispersions of importance to industry provided that the data from the measurements can be interpreted. I Further information may be obtained from: Professor R. H. Ottewill, School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS ivTHE FARADAY DIVISION OF THE ROYAL SOCIETY O F CHEMISTRY SYMPOSIUM NO. 18 Molecular and Microstructural Basis of Viscoelasticity and Related Phenomena Robinson College, Cambridge, 8-9 December 1983 Organising Committee Sir Geoffrey Allen (Chairman) Professor Sir Sam Edwards Dr M.La1 Dr R. A. Pethrick Dr P. Richmond Dr D. A. Young (Editor) The past few years have witnessed the development of new concepts which provide a deeper understanding of the relationship between molecular dynamic and microstructural features of systems and their viscoelastic behaviour. This Symposium is designed to bring together original contributions involving theoretical, computational and experimental studies which represent significant advances in this important field of current activity. It is hoped that such contributions, together with the discussion that they will generate, will lead to new insights into the molecular mechanisms underlying the viscoelastic/rheological behaviour of, for example, flexible and rigid rod-like polymer molecules, liquid crystals and composites.In addition to three oral sessions (at which the main papers will be presented and discussed), the Symposium may include a poster session. Such poster papers will not be published in the S ym posi u m volume. Contributions for consideration by the organising committee are invited. Abstracts of ca. 300 words should be sent to: Dr M. Lal, Unilever Research, Port Sunlight Laboratory, Bebington, Wirral L63 3JW not later than 29 October 1982. Full papers for publication in the Symposium volume will be required by 19 August 1983. THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION NO.77 Interfacial Kinetics in Solution University of Hull, 9-11 April 1984 This Discussion will focus attention on reactions involving liquid-gas, liquid-liquid and liquid-solid interfaces (but it will not include electrode kinetics as such). The subject encompasses processes of fundamental, industrial and environmental importance and includes such topics as the rate of dissolution of reactive gases, kinetics at liquid membranes, metal and solvent extraction, Marangoni effects, heterogeneous catalysis and photocatalysis in solution, and the kinetics of dissolution of minerals and drugs. The aim of the meeting is to bring together workers in these diverse fields to highlight the complementary nature of the problems encountered and of the results obtained, and to disseminate ideas concerning new and effective experimental techniques and novel theoretical approaches.Contributions for consideration by the organising committee are invited. Titles should be submitted as soon as possible, and abstracts of about 300 words by 15th April 1983, to: Professor D. H. Everett, Department of Physical Chemistry, School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS VFARADAY DIVISION INFORMAL AND GROUP MEETINGS ~ Polymer Physics Group Measurement Techniques for Polymeric Solids To be held at NPL, Teddington on 1-2 December 1982 Further information from Dr M. J. Richardson, NPL, Teddington, Middlesex lW11 OLW Division - Half - day Symposium Photochemical Reaction Dynamics to include the Tilden Lecture: J. P.Simons To be held at the Scientific Societies Lecture Theatre, London on 7 December 1982 Further information from Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1 V OBN Electrochemistry Group Spectroscopic Studies of Electrode Surfaces To be held at Oxford on 13-14 December 1982 Further information from Professor W. J. Albery, Department of Chemistry, Imperial College, London SW7 2AZ Colloid and Interface Science Group Physical and Biological Aspects of Insoluble Monolayers and Multilayers To be held at the Scientific Societies Lecture Theatre, London on 14 December 1982 Further information from Dr R . Aveyard, Department of Chemistry, The University, Hull HU6 7RX Polymer Physics Group Gels and Gelation To be held in London on 21-22 December 1982 Further information from Dr M.Miles, Food Research institute, Norwich NR4 7UA Division with the Institute of Physics Applications of Electron Microscopy in Chemistry To be held at the Geological Society, London on 10 January 1983 Further information from: Mrs J. Cegielka, Institute of Physics, 47 Belgrave Square, London SWlX 8QX Electrochemistry Group Spring Informal Meeting To be held at the University of Newcastle on 29-30 March 1983 Further information from Dr R . D. Armstrong, Department of Chemistry, University of Newcastle, Newcastle upon Tyne NE1 6RU Theoretical Chemistry Group - Half- da y Spring Meeting To be held at King's College, London on 2 March 1983 Further information from Dr G. G. Balint-Kurti, School of Chemistry, University of Bristol, Bristol BS8 1TS Division - Half- da y Symposium Including the Faraday Lecture: J.S. Rowlinson To be held at Imperial College, London on 10 March 1983 Further information from Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1 V OBN Statistical Mechanics and Thermodynamics Group Liquids and Liquid Mixtures To be held at the University of Hull on 28-29 March 1983 Further information from Dr P. G. Francis, Department of Chemistry, The University, Hull HU6 7RX Division with Macrogroup UK and Polymer Physics Group Annual Chemical Congress: Copolymers To be held at the University of Lancaster on 11-1 3 April 1983 Further information from Dr J. F. Gibson, The Royal Society of Chemistry, Burlington House, London W1 V OBN viSCI Electrochemical Technology Group and Electrochemistry Group Ion Exchange Membranes To be held in Chester on 13-1 5 April 1983 Further information from Society of Chemical Industry, 14 and 15 Belgrave Square, London SW1 8PS Colloid and Interface Science Group Proteins and Colloidal Systems To be held at the University of Leeds on 14-1 5 April 1983 Further information from Dr E.Dickinson, Procter Department of Food Science, University of Leeds, Leeds LS2 9JT Polymer Physics Group, Macrogroup UK and Plastics and Rubber Institute Polyethylenes 1933-1 983 To be held in London on 8-1 0 June 1983 Further information from The Plastics and Rubber Institute, 11 Hobart Place, London SWl W OH2 Industrial Physical Chemistry Group Crystallization Processes in Condensed Phases To be held at Girton College, Cambridge on 5-7 July 1983 Further information from Dr I.D. Robb, Port Sunlight Laboratory, Bebington, Wirral, Merseyside L63 3JW Polymer Physics Group Physical Aspects of Polymer Science To be held at the University of Reading on 14-1 6 September 1983 Further information from Dr D. Bassett, University of Reading, Whiteknights, Reading RG6 2AH viiNOTES I t has always been the policy of the Faraday Transactions that brevity should not be a factor influencing acceptability for publication. In addition however to full papers both sections carry at the end of each issue a section headed “Notes”, which are short self-contained accounts of experimental observations, results, or theory that will not require enlargement into “ f u l l ” papers.The “Notes” section is not used for pre I i m i na r y com m u n ica ti o n s. The layout of a “Note” is the same as that of a paper. Short summaries are required. The procedure for submission, administration, refereeing, editing and publication of “Notes” is the same as for “full” papers. However, “Notes” are published more quickly than papers since their brevity facilitates processing at all stages. The Editors endeavour to meet authors’ wishes as to whether an article is a full paper or a “Note”, but since there is no sharp dividing line between the one and the other, either in terms of length or character of content, the right is retained to transfer overlong “ Notes” to the *‘ full papers” section. As a guide a ” Note” should not exceed 1500 words or word-equivalents.NOMENCLATURE AND SYMBOLISM For many years the Society has actively encouraged the use of standard IUPAC nomenclature and symbolism in its publications as an aid to the accurate and unambiguous communication of chemical information between authors and readers. In order to encourage authors to use IUPAC nomenclature rules when drafting papers, attention is drawn to the following publications in which both rules themselves and guidance on their use are given. Physicochemical Quantities and Units. Manual of Symbols and Terminology for Physicochemical Quantities and Units. (Pure and Appl. Chem., Vol. 51, No. I , 1979, pp. 1-41. Also available as a soft-cover booklet from Pergamon Press, Oxford.) Surface Chemistry. ‘ Definitions, Terminology, and Symbols in Colloid and Surface Chemistry - I .’ (Purr and Appl. Chem.. Vol. 31, No. 4, 1972, pp. 577-638.) Definitions, Terminology, and Symbols in Colloid and Surface Chemistry - 11. Heterogenous Catalysis. ’ (Pure und Appl. Chem., Vol. 46, No. I , 1976, In addition, the terminology and symbols for the following subject areas are available either in the form of soft-cover booklets from Pergamon Press (denoted by *) or have been the subject of articles in Pure and Applied Chemisrry in recent years: activities;* chromatography ; elect roc hemi s t ry ; electron spectroscopy ; equilibria, fluid flow ; ion exchange; liquid-liquid distribution ; molecular force constants; Mossbauer spectra; nuclear chemistry; pH ; polymers; quantum chemistry; radiation;* Raman spectra; reference materials (recommended reference materials for the realization of physico- c hem ica 1 properties : general i n t rod uc t i on, en t ha1 p y , optical rot at i on, surface tension, optical refraction, molecular weight, absorbance and wavelength, pressure-volume- temperature relationships, reflectance, potentiometric ion activities, testing distillation columns); solution chemistry; spectrochemical analysis; surface chemistry; thermo- dynamics, and zeolites. Finally, the rules for the naming of organic and inorganic compounds are dealt with in the following publications from Pergamon Press: ‘Nomenclature of Organic Chemistry, Sections A, B, C, D, E, F, and H’, 1979. ‘Nomenclature of Inorganic Chemistry’, 1971. A complete listing of all IUPAC nomenclature publications appears in the 198 1 Index issues of J . Chem. SOC. pp. 71-90.)

ISSN:0300-9599    

DOI:10.1039/F198278FP073

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J . Chem. Soc., Faraday Trans. I , 1982, 78,2813-2887 Viscoelastic Properties of Concentrated Latices Part 1 .-Methods of Examination BY RICHARD BUSCALL,? JAMES W. GOODWIN,* MICHAEL W. HAWKINS AND RONALD H. OTTEWILL School of Chemistry, University of Bristol, Bristol BS8 1TS Received 10th August, 1981 Concentrated polystyrene latices form strongly interacting systems, and under appropriate conditions of particle radii, volume fraction and ionic strength the repulsive forces can be strong enough to give rise to measurable viscoelastic behaviour. In this paper an experimental investigation of the viscoelastic behaviour of concentrated latices is described. The methods used included creep compliance and shear-wave propagation. The former was used to determine the low stress viscosities and the latter to determine the shear modulus.The particle radii were varied from 26 to 98 nm and the volume fraction range was 0.2-0.4. In recent years polymer latices have attained considerable importance as model systems for the investigation of various colloidal phenomena, and in particular for fundamental studies on interaction forces between colloidal particles. The simple geometry of the particles, i.e. spherical, makes a direct comparison possible with theoretical calculations. In very dilute dispersions the particles can move freely in the dispersion medium and undergo Brownian motion. However, with an increase in the number concentration of the particles so that the surface-surface separation between the particles is of the order of the distance over which the repulsive forces act, then the range of motion of the particles becomes restricted and structural features can be observed in the systems. For instance, over certain ranges of volume fraction and particle size, iridescence can be observed on irradiation with visible light.1-4 The structure factor of such systems can be observed with visible light for dilute systems5 and with neutron beams with concentrated systems.6 Typically, the structure factors observed suggest that the particles exist in an arrangement with a persistent high degree of ordering.The rheological behaviour of systems of this type is viscoelastic,'> with the elastic component becoming progressively more dominant as the strength of the interaction is increased. The latter increase can be achieved by various approaches, but two direct methods are to increase volume fraction at constant electrolyte concentration or to reduce the electrolyte concentration of the system.The rigidity of these dispersions can often be similar to that found in gels. It is of considerable importance to be able to relate the bulk rheological properties of a concentrated colloidal dispersion to interparticle forces and thence to the effect of variables such as particle size, electrolyte concentration, the diffuse double-layer potential of the particles and to the nature of the medium and the particle, i.e. relative permittivity and Hamaker constant. By using concentrated, well-characterised polystyrene latices under defined conditions we have obtained reasonable explanations, in terms of current theories of particle-particle interactions, for the viscoelastic behaviour of concentrated dispersions as measured by creep compliance, shear-wave propagation and the application of a continuous shearing stress. t Present address: ICI Corporate Laboratory, Runcorn, Cheshire.28732874 CONCENTRATED LATICES EXPERIMENTAL MATERIALS Polystyrene latices L50, L60 and L70 were prepared by the polymerisation of styrene at 60 OC using potassium persulphate as the initiator by the method described previouslyQ but with addition of sodium dodecylsulphate as the emulsifier. Latices L80 and L200 were grown from L50 in the absence of emulsifier. The potassium persulphate was AnalaR grade and was recrystallised from cold water before use.Styrene was distilled under nitrogen at reduced pressure in order to remove the inhibitor and any polymerised material. Excess emulsifier, unreacted monomer and unwanted electrolyte were removed from the latices by dialysing ten times against a five-fold volume ratio of 0.01 mol dmW3 sodium chloride solution; fifteen times against once-distilled water and finally five times against twice-distilled water. The dialysis tubing was boiled in several quantities of distilled water before use. The volume fraction of the latices after purification was in the region of 0.2. More concentrated sols were prepared by ultrafiltration using dialysis tubing as a filter membrane. The required electrolyte level was obtained by adjusting the pH to between 7 and 7.5 with sodium hydroxide and then dialysing the latices against the appropriate sodium chloride solution until no change in the conductance of the dialysate was observed in 24 h.Latex concentrations were determined gravimetrically by drying duplicate samples to constant weight at 80 OC; the volume fraction was calculated by taking the density of polystyrene to be 1.054 kg dm-3.10 Particle-size measurements were made from electron micrographs with the aid of a Zeiss TGZ 3 particle-size analyser. The electron microscope was calibrated using diffraction-grating replicas of known spacing. The latices had the number-average diameters and coefficients of variation given in table 1. TABLE 1 .-SUMMARY OF LATEX PREPARATIONS preparation [K,S,O,I [NaCl] variation latex temperature/K /mol dmP3 /mol dm-3 diameter/nm (%I L50 333 0.0129 0.0526 52.6 13.1 L60 333 0.0129 0.0526 62.2 L70 333 0.01 29 0.0526 68.6 14.6 L80 333 0.0028 0 78.4 11.6 L200 333 0.0028 0 196.6 5.6 - APPARATUS SHEAR-MODULUS MEASUREMENTS The apparatus was based on that used by van Olphenll for the study of montmorillonite gels and was the same as that described by Goodwin and Khidher.* It comprised two parallel Perspex discs, 2.5 cm diameter, mounted in a vapour-tight Perspex cylinder in a way that allowed the disc separation to be varied.Each disc was connected to a lithium chloride crystal transducer, marked as A and B in fig. 1, via a thin Perspex shaft. The crystals were of the type used in inexpensive audio cartridges (e.g. ACOS GP 33 or BSR 5H). A pulse generator supplied a transient pulse to the lower crystal which caused a small rotation of the lower disc in its own plane, typically just less than lop4 rad.Simultaneously, both channels of a twin-channel oscilloscope were triggered, the output of the upper crystal was displayed on one channel and a lo3 Hz square-wave calibration trace on the other. The traces were recorded photographically. The receiver-crystal response resulted in an oscilloscope trace which consisted of a linear portion followed by a damped sine wave. The initiation of the pulse corresponded to the start of the trace and the arrival of the pulse at the upper disc was indicated by the appearance of the sine wave. The length of the trace to the first peak of the sine wave was measured and the distance was converted to the propagation time, At, using the lo3 Hz calibration. In practice AZ was measured for a series of gap settings, normally between 3 and 25 mm, and the propagation velocity was obtained from a plot of the data with a precision of better than 1 %.R.BUSCALL, J. w . GOODWIN, M. w. HAWKINS AND R. H. OTTEWILL 2875 D FIG. I .-Schematic illustration of the shear-wave propagation cell : A and B represent the receiving and driving crystal transducer assemblies, C the Perspex discs, D the dispersion and E the silicone rubber seals. The frequency of the wave was determined by the effective force-constant of‘the lower disc assembly. The latter had contributions from the crystal mountings, the seals and the sample. However, the compliance of the sample was always much larger than that of the measuring system, and so the frequency of the shear wave varied only slightly between different samples and was in the range 185-225 Hz.ESTIMATION OF SHEAR MODULUS The response of a linear viscoelastic fluid to an oscillatory shearing strain can be described in terms of a complex modulus, G*(o). The latter has two components which are defined by the equation G*(w) = G’(o) - ioq’(o) where G’(w) and ~’(o) are the dynamic rigidity and the dynamic viscosity, respectively; both are functions of o, the angular frequency of deformation. Under steady-flow conditions (w = 0) G’ vanishes and purely viscous behaviour is observed. The steady-state viscosity is thus defined by (1) ~(0) = limq’. (2) O - r O Under non-steady-shear conditions (w > 0) both dissipation and storage occur in varying degrees, and at high frequencies G’ approaches a limiting value given by Go = limG.w-+m2876 CONCENTRATED LATICES The components of the dynamic modulus are related to the velocity of the wave, u, and the density of the samples, p, by u”( 1 - r2) (1 +r2)2 (3) G‘= and (4) with r = A/2 .xo, where A = 2nu/o is the wavelength of the wave and x, is the critical damping length. In general both u and r are required in order to calculate G’. In practice however, u was the easier parameter to measure with the present apparatus, although estimations of r could be made. In most instances the damping was small, and when this was the case precise estimates of r were not required, since for r < 0.1 G‘ z u2p ( 5 ) corresponding to critical damping distances of ca.10 mm. In the majority of the measurements little or no change in signal level was observed as the disc separation was varied over the range 3-25 mm, and so eqn ( 5 ) was used to evaluate G’. In the case of a few of the more dilute systems, marked attenuation was observed, and in these instances rather precise estimates of r were required. Since these were not directly available with any great accuracy the following scheme was employed. Eqn (4) was rearranged to give r as a function of q’ so that r could be estimated from independently determined values of q’. The latter were derived from creep-data (see below). CREEP-COMPLIANCE MEASUREMENTS Creep-compliance measurements were made using a Deer PDR 81 rheometer fitted with double concentric-cylinder platens machined from Perspex.These were ca. 50 mm in length and 50 mm in diameter with gaps close to 1.5 mm. The precise widths of the inner and outer gaps were scaled to give the same mean shear rate in each gap. Within each gap the shear-rate varied by ca. 5% across the gap, but this variation was unimportant as the objective was to make measurements in the low-stress Newtonian region. A constant torque was applied to the upper cylinder and its resulting displacement measured as a function of time, when for thinner samples the steady-state rate of displacement could also be read directly from a tachometer. The torque was adjustable in the range 0-1 kg m, and this corresponded to a shear-stress range of 0-2.4 N m-2. Measurements were made at 25 f 0.1 O C .The torque settings were standardised by balancing the rotor with small weights. The end-correction was determined using a 60% w/w aqueous sucrose solution. The calibration was subsequently checked by measuring the viscosity of 20 and 40% w/w sucrose solutions. The results obtained were in good agreement with literature values.12 Viscosity data obtained for a number of different shear-thinning fluids were found to agree well with data obtained using an R 16 Weissenberg rheogoniometer fitted with combined concentric-cylinder and cone- and-plate platens; no end correction was necessary for these. The creep compliance resulting from the application of a constant shearing stress c is defined as J(t) = e(t)/o (6) where e(t) is the strain measured after a time t has elapsed.For a linear viscoelastic fluid J(t) has the general form13 where 1 /Go and t/q(O) are the instantaneous elastic compliance and viscous compliance, respectively, and the terms in the summation represent contributions to the retarded elastic compliance associated with a spectrum of characteristic times Aj of the material. It will be seen below that the creep behaviour of ordered latices can be described employing a single retarded term, i.e. j = 1.R. BUSCALL, J. W. GOODWIN, M. W. HAWKINS AND R. H. OTTEWILL 2877 VISCOSITY MEASUREMENTS In some instances creep measurements were made at stresses large enough to cause non-linear behaviour, and in these cases steady-state viscosities were determined over a range of stresses. Similar measurements were also made as a function of shear rate using an R16 Weissenberg rheogoniometer fitted with 10 cm diameter combined cone-and-plate and concentric-cylinder platens of the type described by Mooney and Ewart.14 RESULTS EFFECT OF LATEX CONCENTRATIONS .Creep-compliance and shear-wave velocity measurements were made using Latex L70 (a = 34.3 nm); the latex had been dialysed against a ten-fold volume excess of 5.0 x lop4 mol dm-3 sodium chloride solution at pH 7. CREEP MEASUREMENTS Linear behaviour was observed provided the shear stress was kept sufficiently small. Creep curves for three volume fractions, 4 = 0.138, 0.141 and 0.177, are reproduced in fig. 2 and 3. At the lower concentration the viscous compliance was large enough 0.002 - I z N -g 0.001 h ---.v b UJ 0 I- - I z E N --. h --. W b I I I 1 1 1 1 1 1 1 0 120 240 360 time/s 3 .1 (bJ 2 - 1 - 0 - 1 0 10 20 30 LO time/s FIG. 2.-Creep curves for Latex L70 dialysed against 5 x loA4 mol dm-3 sodium chloride; (a) 4 = 0.177; (b) 4 = 0.138; t, application of stress; 1, removal of applied stress.2878 0.04- 0.03 - I z 0.02- "E ---. h --- Y) v e 0.01 CONCENTRATED LATICES I # - - 0 Lf 0 10 20 30 4 ( time/s FIG. 3 . 4 u r v e of creep compliance (c/o) against time for Latex L70 at a volume fraction of 0.141. Sodium chloride concentration = 5 x mol dm-3. (-) Experimental data; 0, points calculated from eqn (8) using Go = 190 N m-3 and q(0) = 730 N s m-2. 1, application of stress; 1, removal of applied stress. I 0.005 a/N m-z FIG. 4.-Instantaneous strain plotted against applied stress for 4 = 0.177; Latex L70.Sodium chloride concentration = 5 x mol dm-3. to obscure any elastic effects. Newtonian behaviour was observed provided o < 0.3 N m-2, and the limiting viscosity ~ ( 0 ) had a value of 2.9 N s m-2. In contrast at the higher concentration (4 = 0.177) the time-dependent complicance was too small to be easily measurable, and essentially only the instantaneous elastic compliance was observed. The instantaneous strain is plotted against shear stress in fig. 4; the plot is linear over the measured range and the slope corresponds to a modulus Go of 590 N m-2. ~ ( 0 ) was estimated to be ca. lo6 N s m-2. Over a rather narrow concen- tration range, roughly 0.14 < 4 < 0.16, viscous and elastic effects were resolvable, as can be seen from fig.3. In this example, where 4 = 0.141, linear behaviour was observed provided the shear stress was < 1.2 N m-2. A good fit to the experimental creep curves was obtained using eqn (7) with a single retarded term. In the cases where the retarded elastic compliance was sufficiently well-defined to be evaluated precisely, its equilibrium value was found to be equal to the value of the instantaneous compliance, and the characteristic time R equal to the ratio v(0)/Go. Thus the total compliance was given byR . BUSCALL, J. w . GOODWIN, M. w . HAWKINS AND R. H. OTTEWILL 2879 TABLE 2.-zERO-SHEAR-RATE RELATIVE VISCOSITY AS A FUNCTION OF VOLUME FRACTION FOR LATEX L70 0.140 5.0 0.138 3.49 0.130 2.2 1 0.125 1.43 0.1 10 1.25 0.100 0.97 0.090 0.77 The accuracy of the fit can be assessed from fig.3. In this case 1 was 4.04 s. Eqn (8) corresponds to the simplest possible case of a Burgers body.ll The mechanical analogue of the latter is sketched in fig. 5. Low shear-stress viscosity measurements were made at concentrations between 4 = 0.08, where the latex was quite fluid and just shear-thinning, and 4 = 0.15, above which the limiting viscosity was too large to measure. A typical flow curve is plotted in fig. 6; in this case 4 was 0.131. Shear thinning was observed when the stress was > 0.27 N m-2. Similar behaviour occurred at other concentrations, except that at GO FIG. 5.-Mechanical analogue of eqn (8). higher concentrations the maximum stress for Newtonian flow tended to increase; for lower concentrations it seldom fell below 0.25 N m-2.The limiting viscosity q(0) is plotted as a function of volume fraction in fig. 7. Above 4 z 0.12, q(0) increases very steeply, illustrating the rather abrupt change from fluid to gel-like behaviour which occurs as the latex concentration is increased. As can be seen the steep part of the curve is difficult to define precisely as a result of the sensitivity of q(0) to small concentration changes. Nevertheless, the overall effect of concentration is represented reasonably well by the curve.2880 CONCENTRATED LATICES N ‘E 0.100 - m z \ 0.075- 0.050 0.05 0.1 0.2 0.5 1.0 2.0 FIG. 6.-Logarithmic plot of viscosity against shear stress, for Latex L70 at a volume fraction of 0.141 a/N m-’ Sodium chloride concentration = 5 x mol dm-3.6 FIG. 7.-Logarithm of the zero-shear-rate viscosity of Latex L70 as a function of the volume fraction. Sodium chloride concentration = 5 x mol dmP3. SHEAR-MODULUS RESULTS In creep the latices behave as simple Burgers’ bodies with a characteristic time L = q(0)/Go. Their dynamic behaviour can be deduced from this model (fig. 5) by solving the differential equation representing the deformation of the stress by an oscillatory strain. Alternatively, it can be obtained by direct transformation of eqn (8), since G*(co) is the Fourier transform of 1/J(t).15 By either route it can be shown that the model has two relaxation times, namely [3/2 d(5/4)]A. The components of G*(o) are in turn related to the limiting parameters Go and ~ ( 0 ) byR . BUSCALL, J.w. GOODWIN, M. w. HAWKINS AND R. H. OTTEWILL 2881 The form of G’ and q’ as a function of cuA is shown in fig. 8. G’ approaches the limiting value Go when wil > 10. This condition allows Go to be determined from transverse-wave propagation data in the manner described below. h s c o . c 0.01 I I 10.01 0 .l 1 10 wh FIG. 8.-Schematic diagram of q’/q(O) and G / G , as a function of wl to represent the form of eqn (90) and (9b). w is the applied angular frequency and 1 the characteristic time. Wave-velocity measurements were made in the concentration range 0.12 < 6 < 0.31. The data are plotted in fig. 9. For 4 < 0.12 the attenuation was so severe that wave-propagation could not be detected, thus indicating that the driving frequency cu was probably < l/A.In the range 0.12 < 4 < 0.14 attenuation was rather pronounced ; nevertheless it was possible to determine the velocity accurately. For 4 > 0.14 little or no attenuation was observed, and x,, the damping length, was estimated to be ca. 20 mm, increasing to the order of 100mm with increasing Concentration. Thus for 4 > 0.14 the damping parameter r in eqn (3) was small and G’ was given by eqn (5) with insignificant error. At a concentration of 4 = 0.141 the creep measurement gave a value for il of 4.04; this corresponds to a value for COA of 4700, and so at this concentration G’ = Go. Go is plotted as a function of volume fraction in fig. 9. It was clear from the wave damping observed for volume fractions between 0.12 and 0.14 that G’(w) # Go # u2p in this region.However, it was not entirely clear whether the disappearance of a detectable shear wave when 4 < 0.12 was a result of a relaxation time which was less than w-l, or whether the elastic property was lost at this concentration. Relaxation was nevertheless thought to be the more probable explanation, since Go was expected on theoretical grounds to be a smooth function of concentration (cf. part 216). Confirmation of this idea was obtained in two ways. If the first values for Go, extrapolated using a fit to the data for 4 > 0.14, were employed, together with measured values of q(O), in order to predict values of G’ and u from eqn (4) and (9), then the predicted values were found to approach zero at the same volume fraction as did the measured wave velocities (4 < 0.125).Secondly, eqn (8) was used together with ~ ( 0 ) and extrapolated values for Go in order to estimate2882 CONCENTRATED LATICES 3 J c 0.1 0.2 0.3 f$ chloride. Closed symbols Go; open symbols G’. FIG. 9.-Shear modulus plotted against volume fraction for Latex L70 in 5 x mol dm-3 sodium G’ from the experimental wave-velocity data. These values for G’, which are plotted as open circles in fig. 9, agreed well with those obtained by the first method. These tests confirm that relaxation (i.e. gross damping) was responsible for the loss of wave-propagation below 4 x 0.12. A different technique would be required in order to measure Go in this region, since increasing the measuring frequency only increases the ratio of damping length to wavelength but does not increase its absolute value.As shown above the viscoelastic behaviour can be represented by a simple model with two relaxation times with values of [3/2 d(5/4)]q(0)/Go. The characteristic time R is plotted as a function of volume fraction in fig. 10. The variation is similar to that of q(0) since the limiting modulus is relatively much less sensitive to volume fraction. R has a lower value of the order of lo-* s and increases rapidly to values of the order of seconds. While it was not possible to estimate 3, at volume fractions much higher than 0.14, it is suspected that it continues to increase and becomes very large at higher concentrations, since more concentrated samples (4 > 0.2) were capable of supporting their own weight for long periods without observable flow.It is clear that the relaxation processes responsible for stress relaxation and viscous flow are very sensitive to particle interactions. EFFECT OF PARTICLE SIZE It has been seen that the visGoelastic properties of ordered latices are very dependent upon volume fraction. An increase in volume fraction naturally leads to a decrease in mean particle separation. The latter can also be modified by changing the particle size while keeping the volume fraction constant. Thus the viscoelastic properties are also expected to depend upon particle size.R. BUSCALL, J. W. GOODWIN, M. W. HAWKINS AND R. H. OTTEWILL 2883 0.10 0.12 0 .I4 0.16 4) FIG. lO.-characteristic time plotted against volume fraction for Latex L70 in 5 x lo-* rnol dm-3 sodium chloride.Further wave-propagation measurements were made using latices L50, L70, LSO and L200 dialysed against mol dm-3 sodium chloride solution. The experiments were confined to volume fractions high enough to produce little or no wave attenuation. The data for each particle size are plotted as a function of volume fraction in fig. 1 I . For a given volume fraction the effect of an increase in particle size is a decrease in modulus. This is intuitively what would be expected and in part 2 it will be shown that it can be modelled quantitatively. EFFECT OF ELECTROLYTE CONCENTRATION The range of double-layer interactions is reduced by the addition of electrolyte and so the shear modulus would be expected to decrease with increasing salt concentration.Shear-modulus measurements were made over a range of sodium chloride concen- trations using Latex A60 at a constant volume fraction of 0.232 0.005. At low sodium chloride concentrations (+ mol dm-3) wave propagation was observed with negligible attenuation whereas at high salt concentrations (> mol dm-3) no rigidity was detected. Broadly speaking the effect of increasing the electrolyte level was similar to that obtained by decreasing the latex concentration at constant salt concentration, in both cases a point was reached where the latices would not support the propagation of shear waves, either because of a decrease in modulus, or because of an increase in relaxation rate, or both. Detailed measurements were confined to salt concentrations between and 2 x loe3 mol dm-3 as in this range the critical2884 CONCENTRATED LATICES 0.2 0.3 0.4 @ containing particles of different radii; 0, 26; a, 34; 0, 39; M, 98 nm.FIG. 1 1.-Go plotted as a function of volume fraction for latices in mol dmp3 sodium chloride 1500 N E u 5 1000 500 1 o - ~ 1 0-2 [NaCl]/mol FIG. 12.-Go plotted against sodium chloride concentration for Latex L70 at a volume fraction of 0.232. damping length was at least 20 mm, implying A + 1. The data are plotted in fig. 12. Over the measured range little effect of salt was observed until a salt concentration of mol dm-3 was reached, above which Go decreased. This behaviour will be discussed in detail in part 2. NON-NEWTONIAN FLOW BEHAVIOUR The investigation described above was largely concerned with the linear rheology of concernation latices, i.e.the limiting behaviour at small stresses and strains. Nevertheless in steady flow the latices were shear-thinning when a critical stress,R. BUSCALL, J . W. GOODWIN, M. W. HAWKINS AND R. H. OTTEWILL 2885 typically 1 N my2, was exceeded. A typical shear-stress against shear-rate curve is plotted in fig. 13. The differential viscosity (aa/a~) appears to approach a constant value at high shear rates to give a limiting viscosity. Bingham yield values extrapolated from this and similar flow curves were found to depend on the square of the volume fraction (fig. 14); this type of concentration dependence is often observed for colloidal systems.17 0 250 500 750 1000 +/S -1 FIG. 13.--Shear stress plotted as a function of shear rate for Latex L50 in 2.67 x lo-* mol dm-3 sodium chloride at a volume fraction of 0.13 1 .iji 0 0.02 0.04 0.06 dJz FIG. 14.-Bingham yield stress, gB, plotted against q52 for Latex L50 in 2.67 x mol dm-3 sodium chloride. In addition there was a linear relationship between the Bingham yield stress and the shear modulus over the range examined. This is shown in fig. 15. The slope of the line gives a proportionality between the two which is of the order of 0.025. However, it is not completely clear at present why this numerical value is obtained.2886 CONCENTRATED LATICES 30 'i' 20 E z m t3 \ 10 0 ? G,/103 N m-* FIG. 15.-Bingham yield stress, cB, against limiting shear modulus, Go, for Latex L50 in 2.67 x lo-* mol dm-3 sodium chloride. CONCLUSIONS The viscoelastic properties of concentrated latices have been shown to depend upon particle size, volume fraction and electrolyte concentration.For example, it was found that increasing the particle concentration could cause a change from a rather fluid state, with no detectable elastic properties, to a gel-like state with a high zero-shear-rate viscosity and an appreciable modulus. Sufficiently concentrated samples were capable of supporting their own weight in an inverted container for many months. The effect of increasing particles size was to increase the particle concentrations at which significant viscoelastic behaviour was first observed. Thus it can be inferred from these data that latices with diameters $ 200 nm would only be viscoelastic at concentrations approaching a close-packed limit.Particular attention was paid to measurement of the limiting (Hookean) shear modulus, which for sufficiently concentrated latices could be determined from a single wave-velocity measurement at ca. 200 Hz. The limiting modulus increased exponentially with increasing particle size. The effect of electrolyte was more complex as the limiting modulus was more or less constant at low salt concentrations (< mol dm-3) but decreased at higher concentrations. These trends will be described in more detail in part 2. These systems show elastic properties because a small deformation of the particle array increases the extent of interaction between neighbouring particles. Consequently factors that increase the strength of the electrostatic interaction between particles are found to increase the limiting modulus.In part 2 the mechanism of the elastic response will be considered in more detail, and it will be shown that the experimental results are consistent with a simple theoretical model which relates the limiting modulus to the interparticle potential. A large part of this work was carried out while R. B. held a Fellowship supported by ICI Ltd under their Joint Research Scheme. M.W.H. acknowledges financial support from the S.R.C. in the form of an advanced course studentship. We are grateful to Jill Files and Peter Rogers for their help in preparing some of the latices. We also thank Dr Th. F. Tadros for many helpful discussions.R. BUSCALL, J. w . GOODWIN, M. w. HAWKINS AND R. H. OTTEWILL 2887 1 W. Luck, M. Klier and H. Wesslau, Naturwissenschafi'en, 1963, 50, 485. P. A. Hiltner and I. M. Krieger, J. Phys. Chem., 1969, 73, 2386. S. Hachisu, Y. Kobayashi and A. Kose, J. Colloid Interface Sci., 1973, 42, 343. J. W. Goodwin, R. H. Ottewill and A. Parentich, J. Phys. Chem., 1980, 84, 1580. J. C. Brown, P. N. Pusey, J. W. Goodwin and R. H. Ottewill, J. Phys. A, 1975, 8, 664. R. H. Ottewill, Prog. Colloid Polym. Sci., 1980, 67, 71. J. W. Goodwin and A. M. Khidher, Colloid and Interface Science, ed. M. Kerker (Academic Press, New York, 1974), vol. IV, p. 529. J. W. Goodwin, J. Hearn, C. C. Ho and R. H. Ottewill, Colloid Polym. Sci., 1974, 252, 464. H. van Olphen, Ciays Clay Miner., 1956, 4, 204. ' J. W. Goodwin and R. W. Smith, Discuss. Faraday Soc., 1974, 57, 126. lo J. B. Bateman, E. J. Weneck and D. C. Eshler, J. Colloid Interface Sci., 1959, 14, 308. l2 International Critical Tables, ed. E. W. Washburn (McGraw-Hill, New York, 1929), vol. V, p. 23. l 3 J. D. Ferry, Viscoelastic Properties of Polymers (Wiley, New York, 1961). l4 M. Mooney and R. H. Ewart, Physics, 1934, 5, 350. B. Gross, Mathematical Structure of the Theories of Viscoelasticity (Hermann, Paris, 1953). l6 R. Buscall, J. W. Goodwin, M. Hawkins and R. H. Ottewill, J. Chem. SOC., Faraday Trans. I , 1982, 78, 2899. J. W. Goodwin, in Colloid Science, ed. D. H. Everett (Specialist Periodical Report, The Chemical Society, London, 1976), vol. 11, p. 242. (PAPER 1 / 1274)

ISSN:0300-9599    

DOI:10.1039/F19827802873

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I , 1982, 78, 2889-2899 Viscoelastic Properties of Concentrated Latices Part 2.-Theoretical Analysis BY RICHARD BUSCALL,? JAMES W. GOODWIN,* MICHAEL W. HAWKINS AND RONALD H . OTTEWILL School of Chemistry, University of Bristol, Bristol BS8 1TS Received 10th August, 1981 Shear-modulus data for polystyrene latices at various volume fractions and sodium chloride concentrations are compared with a simple model based on the theory of interaction between electrical double layers. Stress relaxation and low shear-rate viscosity are also discussed in the context of particle interaction forces. In the previous paper1 (part 1) it was shown that concentrated polystyrene latices containing particles in the size range 30-100 nm can form viscoelastic systems. The elastic properties occur because of the strong repulsive forces acting between the particles.The forces are electrostatic in origin and are particularly pronounced at low ionic strengths and high volume fractions. The experimental techniques used in this investigation were the measurement of the creep compliance and the measurement of shear-wave velocity . In these measurements the application of a small mechanical force causes a small deformation of the system and a consequent distortion of the interparticle arrangement. Hence an increase in the potential energy of the particles occurs by alteration of the positions of neighbouring particles with respect to a central particle. Thus the basic problem becomes that of relating the particle-particle pair potential to the rheological moduli.In the present paper the experimental data reported in part 1 are analysed theoretically in the light of this model. THEORY PAIR POTENTIAL FOR INTERACTING PARTICLES The potential energy of interaction between a pair of particles in a dilute electrolyte is composed of two main contributions, the electrostatic repulsion, VR, due to overlap of the electrical double layers, and the van der Waals attraction, VA, arising from electromagnetic fluctuations. As pointed out by Derjaguin and Landau2 and Verwey and O~erbeek,~ these two contributions can be added to give the total energy of interaction, VT, as follows : VT = vR+ VA. (1) For spherical particles of radius a, with a centre-to-centre separation distance R and a diffuse layer potential V/d, (with iyd < 50 mV) then4 4 7 1 ~ ~ ~ a2 y~i R exp [ - K(R - 2a)l V H = t Present address : ICI Corporate Laboratory, Runcorn, Cheshire.28892890 CONCENTRATED LATICES where K is the Debye-Hiickel reciprocal double-layer thickness parameter as defined by K = (2rE:i3 ~ *y where I is the ionic strength of the electrolyte solution, E, is the permittivity of free space, E is the permittivity of the medium and the other symbols have their usual meanings. Eqn (2) is applicable for the condition that the particles interact at constant diffuse double-layer potential and Ica < 3 . Under the above conditions eqn ( 2 ) is a good approximation. The potential energy of attraction between two spheres in a liquid medium is given (3) where x = (R-2a)/2a and A is the composite Hamaker constant for polystyrene particles in water.In a concentrated colloidal sol each particle interacts with more than one other particle at a time, and as a first approximation the total potential energy of interaction might be taken as the sum of pairwise interactions; the potential energy of any one particle might thus be written as x2+2x+ x2+2x 1 1 VA = - 2 ( F T z + x 2 + 2 x + l +2 In by5 1 1 n i-1 P.E. = + (VT)i (4) where n is the number of nearest neighbours. The pairwise additivity approximation has formed the basis of attempts to account for the structure and properties of concentrated colloids using the techniques of liquid-state theory.6 The assumption given by eqn (4) is likely to be a reasonable approximation for the condition that (R-2a) > 2 / ~ .The Monte Carlo approach6 and the use of a cell model' are alternative approaches to the problem but are more complicated. The aim of this investigation was to compare shear-modulus data obtained experimentally with theoretical estimates made using a pairwise additivity approach. STRUCTURE IN CONCENTRATED DISPERSIONS It has been established by various authors using optical diffraction and other techniques that under certain conditions of volume fraction, particle size and electrolyte concentration and type, latex particles can form an ordered array with the particles sited on a time-average basis at points on a At low volume fractions clear evidence has been obtained from scattering experiments that the form of the radial distribution function is that of a liquid-like array.ll In an ordered array the particle separation is related to the volume fraction by where q5 is the volume fraction and brn is a constant which is characteristic of the type of array [e.g.0.74 for hexagonal or face-centred cubic (f.c.c.) arrays, 0.68 for body -cen t red cubic (b . c .c .)I. SHEAR MODULUS FOR A N ORDERED ARRAY: LATTICE MODEL In a concentrated dispersion it is clear that there is short-range order of the particles which can be modelled by an f.c.c. lattice. For the determination of the shear modulus, a small shear strain is applied to such a system, which initiates a shear wave of frequency 200 Hz. At the latter frequency the time period of the wave is smallR. BUSCALL, J . w. GOODWIN, M. w. HAWKINS AND R.H. OTTEWILL 2891 compared with the relaxation time of the particles in the array. Similarly, the relaxation time of the electrical double layer is also small compared with the relaxation time of the lattice. Under these conditions the equations used to calculate the restoring force of the lattice can be those corresponding to the equilibrium condition of the electrical double layer. On this basis it is possible to formulate a model for the shear modulus.12 FIG. 1.-Diagram of the coordinate system for evaluating the effect of shear on pairwise particle interaction. Consider a pair of particles, initially separated by a distance R, one at 0, the other at A (fig. 1). The effect of a small shearing strain, ds/dz, is to displace the second particle a distance S from A (R,8,@) to A’ (Rl,81,Ql).The concomitant change in particle separation R is related to S by ( 6 ) 2R 6R - (6R)I = 2 R6 sin Q, - S2 which for small strains can be linearised to give ds dz 6R = R-cos8 sin8 sin@. For small strains the force acting along R is given by a 2 v, F = 6 R w... and the restoring force, i.e. the component of F in the x-direction, is (7) a2 V* F, = 6R - sin 8, sin @, aR2 (9)2892 CONCENTRATED LATICES whence substituting for 6R from eqn (7) we obtain for small strains - - R 3 2 sin28 cos 0 sin2@. aR2 dz The contribution of F, to the shear stress is FJA,,, where A,, is the projected area of the particle-pair in the xy-plane. The volume occupied by the two particles is zR3/34,, so that 71 R2 = 34m Cos e (1 1) and the contribution of F, to the shear stress becomes 34m a2VT ds (J1r--- cos 28 sin 28 sin W.ZR aR2 dz Thus averaging the pairwise contribution to the shear stress over all orientations giving an average shear stress 34m a2VT ds 7[ 4z2R aR2 d z l I cos 28 sin 28 sin 2@ d<D do. - 01=------ Since each particle has n nearest neighbours, the total shear stress is given by n i-1 0 = C 6, = nd, or Finally we obtain for the shear modulus the expression The subscript zero is used to indicate that, as relaxation was not considered, this is the high-frequency limit of the dynamic rigidity. The superscript th is used to indicate a theoretical value of the modulus calculated using this model. For face-centred cubic and hexagonal structures a reduces to 0.833. If a body-centred cubic array is used in the model a = 0.510.The question of the type of structure adopted by ordered latices is not yet fully resolved, as there is light-scattering and electron-microscopic evidence to favour both f.c.c. and b.c.c. packing.l0? 1 3 9 l4 For the present purposes we have assumed an f.c.c. structure. In practice the choice makes less difference to the predictions than the difference in values would imply. Since R also depends upon the type of packing, i.e. R = 2a(0.74/4)$ (f.c.c.) R = 2a(0.68/4)4 (b.c.c.). As an illustration of the effect predicted, moduli are plotted as a function of volume fraction in fig. 2 for particles with a radius of 25 nm in mol dm+ 1 : 1 electrolyte. The upper curve was calculated assuming f.c.c. packing, the lower assuming b.c.c.; the two curves differ only by ca.20%. In practice the difference would probably be even smaller as the effect of ignoring next-nearest-neighbour interaction is probably less valid for b.c.c. than for f.c.c. arrays. At the low electrolyte concentrations and volume fractions used in this work theR. BUSCALL, J. W. GOODWIN, M. W. HAWKINS AND R. H. OTTEWILL 2893 0 I 0 0.1 0.2 0.3 6 FIG. 2.-Gih plotted against volume fraction as calculated assuming a = 25 nm, K = lo8 m-' and I V/d I = 50 mV: (- . -) assuming a face-centred cubic structure, (-) assuming a body-centred cubic structure. interactions are dominated by electrostatic repulsion. Thus taking VT = VR the theoretical shear modulus Gkh can be obtained. Differentiating eqn (2) twice and combining the result with eqn (16) gives ( K'R~+;~ICR+~) exp [ - IC(R - 2a)] Gih = 4 n a ~ ~ , a2Wd for ica < 3.For ica > 10, the equation for VR can be takenI5 as thus giving VR = 2 n ~ ~ ~ a l y i l n { l +exp[-ic(R-2a)]) IC' exp [ - IC (R - 2a)l ({ 1 + exp [ - K(R - 2a)3>' 2 n a ~ ~ , v i a R Ghh = (17) RESULTS AND DISCUSSION EFFECT OF VOLUME-FRACTION SHEAR MODULUS Measurements of shear modulus were made on Latex L70 over the volume fraction range 0.146 < 4 -c 0.303. Below 4 = 0.146, the shear modulus could not be measured with the apparatus available. All the samples used were dialysed against the same large volume (20 dm3) of 5 x The results given in table 1 show that the shear modulus increased smoothly from 280 N rn-' at 4 = 0.146 to 3450 N m-' at 4 = 0.303, an increase of approximately an order of magnitude.mol dm-3 sodium chloride solution adjusted to pH 7.2894 CONCENTRATED LATICES TABLE ~.-SHEAR-MODULUS RESULTS AS A FUNCTION OF VOLUME FRACTION FOR LATEX L70 (a = 34.3 nm) IN 5 x mol dm-3 NaCl 4 G,/N rnp2 4 G,/N m-2 0.146 280 0.227 1380 0.152 370 0.241 1590 0.163 450 0.267 2060 0.174 570 0.283 2590 0.187 690 0.295 2990 0.203 1040 0.303 3450 2 4 6 (CP/$$/lO4 N in-' V-' FIG. 3.-Plot of the experimental limiting shear modulus, Go, against Gihh/~S for various volume fractions of Latex L70 in 5 x loe4 mol dm-3 sodium chloride solution. Gih/y; values calculated with KU = 2.48, a = 0.833 and I$,,, = 0.74. In order to utilise either eqn (17) or eqn (19), an assumption is needed about the packing of the particles in order to calculate R.If the latter is assumed to be face-centred cnbic, then all parameters are known except Vd. Dialysis was chosen for the preparation of samples in order to ensure that the bulk electrolyte solution reference state remained the same for all dispersions as q4 was varied. In fig. 3 the measured value of the shear modulus, Go, is plotted against the quantity Gkh/t&, as calculated from eqn (17) using rca = 2.48, a = 0.833 and 4m = 0.74. A linear plot is obtained up to Go = 3 x los N m-2, which supports the model used. The slope gives a value of I ry, I = 50 mV. This is a physically realistic value and is of a similar magnitude to values of zeta potential obtained from electrophoresis measurements16 carried out on polystyrene latices prepared in a similar fashion to those used in the present work.The linear plot also indicates that the use of pairwise additivity is a good approximation for the present case where d, < 0.30. In other cases depending on size, multibody interactions will have to be taken into account.R. BUSCALL, J. w . GOODWIN, M. w . HAWKINS AND R. H. OTTEWILL 2895 RELAXATION PROCESSES The possible relaxation processes which occur in this system include stress relaxation owing to movement of water molecules, relaxation of the ion atmosphere surrounding the particles and the stress relaxation owing to the diffusional motion of the particles. The first two of these processes are considerably more rapid than the last, so that the rate-controlling step is the particle motion. On this basis the relaxation time would be expected to be proportional to the frictional coefficient of an individual particle in the ordered array.Thus if the motion of a single particle is considered within the field created by its neighbours, then its relaxation time must be proportional to f/g, where f is the frictional coefficient and g is the restoring force constant for the field generated in the same manner as that used to calculate Ghh. The particle relaxation time, AD, is proportional to As shown previously the dynamic behaviour of the latex could be explained using a single characteristic time for the material given by f ~ / [ o . 5 6 ( a 2 v,/~R~)I. (20) = V(O)/GO (21) where ~ ( 0 ) is the zero-shear-stress viscosity of the dispersion. Hence equating this with the particle relaxation time given by the pr~portionalityl~ we obtain f x 0.673q(O)/R.(22) An approximate value of the frictional coefficient can thus be estimated from the experimental data. At a high dilution f should have the Stokes law limiting value of Jb = 6na47, which for Latex L70 in 5 x mol dmP3 sodium chloride solution at 20 "C is 5.8 x 10-lo N s m-l. However, in the dilute limit the value off is larger than this calculated value since an isolated particle is retarded by its own double layer.18 The latter effect, however, is small for particles of radius 34.3 nm. Because eqn (22) is based on the assumption of an interacting system it is not possible to obtain the dilute limit from it.f, however, does appear to approach this limiting value at low concentrations.For example, with 4 = 0.10, A and f were estimated to be 1.3 x s and 8.05 x N s m-l, respectively, giving a value forf/fo of 1.39. The values obtained for other volume fractions are plotted in fig. 4. Also shown in the figure is VR plotted as a function of 4 as calculated from eqn (17). The frictional coefficient increases rapidly in the region where VR is predicted to attain a value large compared with the thermal energy of the particles, thus indicating that the movement of the particles is do'minated by electrostatic interaction between particles rather than by hydrodynamic interaction with the solvent. The sensitivity of the zero-shear-rate viscosity ~ ( 0 ) to volume fraction, and hence the related parameters A andf, is very marked at low volume fractions.' Although ~ ( 0 ) and Go both increase rapidly with volume fraction, the former is more sensitive to small changes in volume fraction and is thus more sensitive to particle interactions.EFFECT OF SODIUM CHLORIDE CONCENTRATION The results shown in fig. 3 cover a range of volume fractions from 0.15 to 0.30 at a constant salt concentration of 5 x mol dm-3 The slope of the linear plot obtained gave a value of I tyd I of 50 mV. A similar set of results were obtained using mol dm-3 sodium chloride solution over a volume fraction range from 0.18 to 0.29 (fig. 5). The results shown in these two figures showed that the derived value of I wd I was not dependent on the volume fraction of latex used.2896 CONCENTRATED LATICES FIG. Q 4.-Variation of the frictional coefficient ratio,f/fo, and V,/RT against volume fraction for Latex L70 in 5 x mol dm-3 sodium chloride solution: (-)f/foo, (---) VT/RT. FIG.5.-Plot of the experimental limiting shear modulus, Go, against G>/w% for various volume fractions of Latex L70 in mol dm-3 sodium chloride solution. Gkh/wi values calculated with ica = 3.57, a = 0.833 and &, = 0.74.R. BUSCALL, J . W. GOODWIN, M. W. HAWKINS AND R. H. OTTEWILL 2897 30 0 1-0 2.0 [NaCl] mol dm-3 FIG. 6.-Plot of the potential I 'y, I for Latex L70 against sodium chloride concentration in the dialysate: 0, single-point determinations; 0, values obtained from variation of Go with volume fraction. 3 PI 'E 2 z m 2 . 0 0 1 0 l i d t ' FIG. 7.-Limiting shear modulus, Go, plotted against volume fraction for particles of different sizes: 0, 26.3; 0, 34.3; A, 39.2; A, 98.3 nm.Curves calculated for various values of Iydl: (-) 50, (---) 55, (- . -) 50, (- - - - - -) 89 mV. The effect of varying the salt concentration over the range lOP4-2 x mol dm-3 on the value of Go was then examined at a' constant volume fraction of latex of 0.232. The experimental data were used in conjunction with eqn (1 7) and (1 9) to find values of yd required to produce a fit. These values are shown plotted as a function of sodium 94 FAR 12898 CONCENTRATED LATICES chloride concentrations in fig. 6. The values obtained in the salt concentration range 4 x 10-4-1 .8 x mol dm-3 appear to be reasonable. EFFECT OF PARTICLE SIZE Go was measured as a function of volume fraction for four latices. The data from these experiments are given as points in fig.7. Optimum values of V/d required to obtain fits using eqn (17) and (19) are also shown in the figure. Different and constant values of I V/d I were required for each latex; these values varied from 50 to 90 mV, which is in the range typically found in electrokinetic experiments on latices. It is apparent from fig. 7 that Go depends markedly on the number-average value of the particle radius. The agreement between the experimental data and the fitted results is excellent over the range investigated. APPENDIX RELATIONSHIP BETWEEN BULK AND SHEAR MODULUS FOR A N INTERACTING ARRAY Another method used for the investigation of interparticle forces in colloidal sols is the compression cell or pressure-filtration technique.lg In the compression experiment a small volume of sol, typically 10crn3, is confined between a deformable rubber membrane and a Millipore filter and is subjected to a constant pressure p .The effect of the applied pressure is to expel part of the dispersion medium through the filter. By making measurements at several pressures it is possible to construct a pressure-volume-fraction curve for the sol. One parameter that can be derived from such a curve is the quantity K = 4 (dP/d4) which dimensionally has the form of a bulk modulus. It represents the resistance to compression of the array of interacting particles. The relation between K and Go can be developed as follows. At constant temperature the bulk modulus can be written in the form where U is the internal energy of the array of particles and u the molar volume, which can be taken as 47rN a3/34, whence 43 d2U K = - - P dd2 with P = 47ca3/3 and N Avogadro's number.Hence, assuming pairwise additivity U = NnVT/2 and or, since R = 2 a &,/& then for a face-centred cubic array Comparing this equation with eqn (6) it is found that Go/K = 9 4 3 2 &,. For face-centred or hexagonally close-packed arrays Go/K = 1.3 17, and for a body-centred cubic array G,/K = 1.477, corresponding to values for Poisson's ratio for the interacting particulate array, as distinct from the latex as a whole, of 0.172 and 0.142, respectively.R. BUSCALL, J . W. GOODWIN, M. W. HAWKINS AND R. H. OTTEWILL 2899 Valhes of G,/Kclose to unity have been found experimentally for a number of different types of colloidal dispersion^.^'? 2o A large part of this work was carried out while R.B.held a Fellowship supported by ICI Ltd, under their Joint Research Scheme. M. W. H. acknowledges financial support from S.R.C. in the form of an advanced course studentship. We are grateful to Jill Files and Peter Rogers for their help in preparing some of the latices. We also thank Dr Th. F. Tadros for many helpful discussions. R. Buscall, J. W. Goodwin, M. W. Hawkins and R. H. Ottewill, J. Chem. Soc., Faraday Trans. I , 1982,78, 2887. E. J. W. Verwey and J. Th. G. Overbeek, Theory of the Stability of Lyophobic Colloids (Elsevier, Amsterdam, 1948). S . Levine, Discuss. Faraday SOC., 1954, 18, 202. H. C. Hamaker, Physica, 1937, 4, 1058. W. van Megen and I. Snook, Faraday Discuss. Chem. Soc., 1978, 65, 92. P. A. Hiltner and 1. M. Krieger, J . Phys. Chem., 1969, 73, 2386. S . Hachisu, Y. Kobayashi and A. Kose, J . Colloid Interface Sci., 1973, 42, 342. lo J. W. Goodwin, R. H. Ottewill and A. Parentich, J. Phys. Chem., 1980, 84, 1580. J. C. Brown, P. N. Pusey, J. W. Goodwin and R. H. Ottewill, J. Phys. A , 1975, 8, 664. l2 J. W. Goodwin and A. M. Khidher, Colloid and Interface Science, ed. M. Kerker (Academic Press, New York, 1976), vol. IV, p. 529. l 3 R. H. Ottewill, Prog. Colloid Polym. Sci., 1980, 67, 71. l4 R. Williams and R. S . Crandall, Phys. Lett., 1974, &A, 224. l5 H. R. Kruyt, Colloid Science (Elsevier, Amsterdam, 1952), vol. 1. l6 R. H. Ottewill and J. N. Shaw, J. Electroanal. Chem., 1972, 37, 133. l7 R. Buscall, J. W. Goodwin and R. H. Ottewill, to be published. l9 L. Barclay, A. Harrington and R. H. Ottewill, Kolloid Z . Z . Polym., 1972, 250, 655. 2o I. C. Callaghan and R. H. Ottewill, Faraday Discuss. Chem. Soc., 1975, 57, 110. 2 B. Derjaguin and L. Landau, Acta Physicochim. URSS, 1941, 14, 633. ' R. J. R. Cairns, W. van Megen and R. H. Ottewill, J. Colloid Interface Sci., 1981, 79, 51 1. F. Booth, J. Chem. Phys., 1954, 22, 1956. (PAPER 1 / 1275) 94-2

ISSN:0300-9599    

DOI:10.1039/F19827802889

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1982,78, 2901-2910 Ion Binding to Polyelectrolytes as Described by the Poisson-Boltzmann Equation Comparison with 23Na Nuclear Magnetic Resonance Relaxation Experiments BY GUDMUNDUR GUNNARSSON AND HANS GUSTAVSSON*t Division of Physical Chemistry 2 and 1, Chemical Centre, P.O.B. 740, S-220 07 Lund 7, Sweden Received 17th August, 1981 Experimental data on 23Na n.m.r. transverse relaxation rates in aqueous solutions of the sodium salts of poly(acry1ic acid) and chondroitin-4-sulphate at different polymer concentrations are presented, as well as experimental data on 23Na n.m.r. longitudinal and transverse relaxation rates for poly(acry1ic acid) at various degrees of neutralization. The data are compared with a theory based on the Poisson-Boltmann equation.The theory is also compared with literature data on 23Na n.m.r. linewidths in Na+-DNA solutions as a function of the concentration of salt containing monovalent or divalent counter-ions. The comparison between the theory and experiments reveals that the Poisson-Boltmann theory in all cases accounts satisfactorily for the experimental results. The search for an appropriate general description of the interactions between simple ions and polyions is of great interest to both chemists and biophysicists. One attempt in this direction is the theory of Manning, which accounts reasonably well for many experimental observations.' A disadvantage of Manning's theory is that it assumes that the salt concentration is much higher than the concentration of ionic groups on the polyelectrolyte.The theory of Manning is furthermore limited to linear polyelec- trolytes. An alternative theory, not suffering from these limitations, is the Poisson- Boltzmann (P.B.) equation, first used by Guoy2 and C h a ~ m a n . ~ In the P.B. equation one can allow for the dimensions and geometry of the polyelectrolyte and the concentration of the polyelectrolyte can be taken into account through the cell m0de1.~~~ The main results of the theory of Manning can be derived from the P.B. equation in the limit of a line charge at infinite dil~tion.~?' The binding of counter-ions to polyelectrolytes can be probed by many experimental methods. One such method is n.m.r. where, in favourable cases, the various parameters are sensitive to short-range counter-ion polyelectrolyte interactions.8 In this paper we present 23Na n.m.r.relaxation rates for aqueous solutions of poly(acry1ic acid) (PA) and chondroitin-4-sulphate (CS). The experimental results at various degrees of neutralisation and polyelectrolyte concentrations are compared with theoretical results from the P.B. theory. Comparisons are also made between the theory and literature values of Na-DNA interactions as measured by 23Na n.m.r. at different concentrations of monovalent and divalent counter-i~ns.~ EXPERIMENTAL The poly(acry1ic acid) was used as obtained from the B.D.H. Ltd, and pure (protein free) chondroitin-4-sulphate was a kind gift from Prof. L. A. Fransson, Lund. t Present address: ASEA AB, Department KYJU, S-721 83 Vaster&, Sweden.290 12902 ION BINDING TO POLYELECTROLYTES The degree of neutralisation, a, is defined as the stoichiometric ratio of added NaOH (or HCl) to polyelectrolyte ionizable groups. The n.m.r. measurements were performed as described elsewherelo on a homebuilt Fourier- transform spectrometer with a wide-bore Oxford Instruments magnet operating at 67.4 MHz and a modified Varian XL-100 spectrometer operating at 26.4 MHz. The excess transverse relaxation rates, R2, ex, were obtained from high-resolution absorption spectra according to (1) where A v ~ , and Av;, ref are the linewidths at half height for the sample and the reference, respectively. Longitudinal relaxation rates, R,, were obtained with the conventional inversion- recovery technique. The reference was an aqueous 0.2 mol kg-' NaCl solution with the experimentally observed relaxation values R l , ref = 16.8 s-l and Av4, ref = 5.9 Hz.4, ex = ~ A v : , s-Av+, ref) N.M.R. THEORY AND ION BINDING In the systems under consideration here the usual fast-exchange conditionll for sodium relaxation is fulfilledlo and the observed relaxation rate, Ri, obsd, is therefore given by (2) where only the two states of bound and free counter-ions are considered. pf and P b are the fractions of free and bound ions, respectively. The. intrinsic relaxation rates, Ri, for quadrupolar relaxation of nuclei with spin quantum number I = 312 are12 Ri, obsd = PfRi, f+PbRi, b where the quadrupolar coupling constant, x , is a measure of the intensity of the interaction, and Z, is the correlation time determined by the dynamics of the interaction.The form of the function,f,, is determined by whether the equation is for the longitudinal (i = 1) or the transverse relaxation rate (i = 2).1°7 l1 The excess relaxation rate, Ri, ex, is given by (4) Ri, ex = Ri, obsd- Ri, f = Pb(&, b - Ri, F). Often Ri, b is independent of parameters such as polyion and salt concentration and in such cases changes in Ri,ex are proportional to changes in Pb. Determination of Ri, ex does not, however, allow for a determination of P b since Ri, b is generally unknown.1° ION BINDING T O POLYELECTROLYTES AS DESCRIBED BY The polyelectrolytes in this study are modelled as infinite cylinders of radius Yb, with a smeared-out surface charge density z. To take the concentration of the polyelectrolyte into account we apply the cell and assign to each polyelectrolyte an electroneutral cylinder of radius Y, containing the counter-ions, added salt and water.The water is modelled as a dielectric continuum of dielectric constant E,. The monomer concentration of the polyelectrolyte, cp, is thus TH E P 0 I S S 0 N- B 0 LTZ M A N N EQ U A TI 0 N cp = 1/(n G bmin) ( 5 ) a = bmi,/b (6) where bmin is the distance between charges in the fully charged polyelectrolyte. The degree of neutralisation, a, is where b is the average distance between charges.G. GUNNARSSON AND H. GUSTAVSSON 2903 A description of the ion distribution around the polyelectrolyte is obtained from a solution of the P.B. equation. In spite of the approximations inherent in the P.B.equation it has been shown to give a good description of the ion distribution around polyele~trolytes.~~-~~ In cylindrical symmetry the P.B. equation is where zi is the valence of ion i and cio the concentration of ion i at r,. Here we have assumed that ty(rc) = 0, which can be done since the absolute value of the electrostatic potential is arbitrary. From the electroneutrality of the cell we have Eqn ( 5 ) can be solved analytically for an infinite cylindrical polyelectrolyte with counter-ions only, but without added ele~trolyte.~~ To obtain a solution with added salt one has to resort to numerical methods. The numerical procedure used here is described in ref. (16). It is not a simple matter to relate the ion distribution, which is obtained from the P,.B. equation, to degrees of ion binding found in experiments. The definition of the degree of ion binding depends on the experimental technique used to probe the ion binding.16 For spectroscopic measurements such as 23Na n.m.r.it may be assumed that only ions close to polyelectrolyte charged groups have spectroscopic properties different from those of the bulk s01ution.l~ One can thus define bound ions as those which are within a distance A from the polyelectrolyte surface, where A is typically of the order of 1-5 A. An alternative definition is to consider ions within a given volume, &,, from each monomer unit as bound. A and Vb are related by A = ( vb/n6+r$)t-rb. (9) These definitions are ambiguous, but they can be expected to reproduce the trends in ion association seen using spectroscopic methods when parameters like polyelec- trolyte and salt concentration are varied.We shall therefore use the following definition of the degree of ion ety r,+A PSI, = 6 C,, exp ( -fi) 2nr dr. rb The fraction of bound sodium ions as obtained from the P.B. therefore CI, PbPB = Psp a- CNa where cNa is the total Na concentration. association, PSp : l8 (10) equation, pEp, is (11) RESULTS AND DISCUSSION As examples of polyions with variable charge density one may choose polyelectrolytes where the ionizable group is weakly acidic, as in PA for example. Here it is necessary to consider H+ binding to the polyelectrolyte carbocylates as ‘site binding’; put in a different way, the protons must bind much more strongly to the ionizable groups on the polyelectrolyte than the counter-ions we wish to study, in order to avoid competition effects.Thus with ‘site binding’ of H+ the charge density can be considered to vary linearly with the degree of neutralisation.2904 ION BINDING TO POLYELECTROLYTES I 1 I I I 0.2 0.4 0.6 0.8 1.0 [Y FIG. 1.-23Na excess transverse relaxation rates, R2, ex, at 67.4 MHz, plotted as a function of the stoichiometric degree of neutralization, a. The circles were obtained from measurements in which 0.095 mol dm-3 aqueous poly(acry1ic acid) was titrated with 1 .O mol dm-3 NaOH and the filled points were obtained from measurements in which the solution obtained in the above-mentioned experiment was titrated with 1 mol dm-3 HC1. The theoretical curves were obtained from eqn (4) and (1 1) by using &, b-R2, = 165.3 s-'.vb = 200 A3, Rb = 3 A, bmin = 2.5 A. E = 78.3, t = 25 O C . kf, = 250000. 60 40 20 r n --- ?is d 0 I I I I I 0.2 0.4 0.6 0.8 1.0 a FIG. 2.-23Na excess longitudinal relaxation rates, R l , at 67.4 MHz, plotted as a function of the stoichiornetric degree of neutralisation, a. The circles were obtained from experiments as described in fig. 1. The theoretical curve from P.B. theory was obtained by letting the theoretical and experimental curves coincide at a = 1 . Parameters are as in fig. 1. The points in fig. 1 show the results on 23Na transverse relaxation rate measurements of the titration of PA with NaOH and of back-titration of the resulting solution with HCl. Also shown is a theoretical curve obtained from P.B. theory [eqn (4) and (1 l)].In calculatin Pb from the P.B. equation we have assumed that ions within a volume to experimentally observed relaxation rates we assume that R i , b in eqn (4) is independent of a and multiply the theoretical values by a constant to get the observed relaxation rate at a = 1. In fig. 2 we show the results from longitudinal relaxation rate of vb = 200 x from each monomer unit are bound. To relate the theoreticalp, valuesG. GUNNARSSON AND H. GUSTAVSSON 2905 measurements for the titration of PA with NaOH. Also shown is a theoretical curve obtained from P.B. theory in a way similar to that described above. The experimental ratio R 1 , e x / R z , e x increases slightly at low a, indicating an increase in z,. It is reasonable to assume that the increase in z, has the same origin here as in the case of poly(methacry1ic acid) (PM), extensively discussed elsewhere.lO Thus the implication is that a slight coiling of PA occurs at low charge density, although to a much lesser extent than in PM which has strongly hydrophobic methyl groups.Depending on the degree of coiling, there is a certain risk that neither the model of an extended cylinder (for the P.B. solutions) nor the line-charge model (Manning) applies. Although the small increase in z, with decreasing o( for PA implies thatp, decreases faster than R l , ex and R2, ex at low a it certainly cannot account for the total excess relaxation in fig. 1 and 2 for low a. Therefore the experiments clearly indicate that there is some extent of ion binding to the polymer at low a and that there is no true discontinuity.There is, however, still an unexplained excess of relaxation at low a. This may probably be explained using an improved n.m.r. theorylg which is presently being developed. The results of the P.B. theory are in reasonable agreement with the present observations, but are at variance with the conclusions of Manning,20 who states that there is no binding of counter-ions to the polyion for a < 0.35. 23Na+ chemical-shift (6) measurements10 have no unambiguous interpretation, and since it is well known that sodium ions interacting with carboxylic groups (-COOH) have a negative shift, whereas Na+-COO- interactions give a positive shift of about the same magnitude, it is not expected that 6 should be a reliable measure of Na+ binding to polyacids with carboxylic groups.This is especially true in cases where -COO-- and -COOH coexist (i.e. at low and intermediate a in PA for instance). 70-0 0 O I m x . d 0 A 1 I 0.00 0.05 0.10 015 0.20 c,/mol dm-3 FIG. 3.-23Na excess transverse relaxation rates, R2, ex and Rl, ex ( x ) at 64.7 MHz plotted as a function of the concentration of monomer units in poly(acry1ic acid), cD, for polymers of three different molecular weights (A, 2000; 0, 5000; 0, 250000). The points are from experiments and the curve is a theoretical one obtained from eqn (4) and (1 1). Parameters are as in fig. 1 (a = 1). In fig. 3 are presented results from 23Na transverse relaxation-rate measurements on the dilution of PA having three different molecular weights at a = 1.Also shown are Rl, ex measurements for the highest molecular weight PA and the results of P.B. theory (obtained as in fig. 1). The theoretical curve quite well parallels the experimental2906 ION BINDING TO POLYELECTROLYTES R,, ex points. This is also true for the R2, ex values for the two lowest PA molecular weights, where relaxation is approximately monoexponential (i.e. R, z R2). The slowest Na+ motion which affects the relaxation processes in this system is probably the exchange of sodium ions between bound and free states in the interpolyion solution for very long PA chains (i.e. with very slow reorientation).1° The slow motion is expected to affect the components of transverse relaxation differently.lO9 l1 The expectation, in a slightly simplified theory, is that and that TABLE 1 .-FRACTIONS OF SODIUM BOUND TO POLY(ACRYLIC ACID) OBTAINED FROM BIEXPONENTIAL ANALYSIS OF 23Na+ SPECTRA M , (PA) = 250000, a = 1 ; R2, ref = 18.6 S-'; CNa+ = C PA- P B expt.C,,+/mmol dm-3 T2,/ms T2,/ms TJms z,/ns (x = 220 kHz) P.B. 147 8.7 11.0 11.8 1.2 0.45 0.45 58 8.3 11.3 12.8 1.4 0.41 0.43 29 6.9 11.9 13.1 2.2 0.42 0.42 19 5.9 12.2 13.3 2.9 0.38 0.405 10 4.1 13.4 14.0 4.6 0.37 0.40 7.5 4.3 14.2 - 4.7 0.39 0.395 5 2.5 14.8 7.0 0.39 0.39 - Here TZl is much more changed by a slow motion (long z,) than is TZ2 or T,. The linear approximation used for R2, ex (z 0.4/T2, + 0.6/T2,) in fig. 3 for the highest molecular weight PA may, for long correlation times, give very erroneous results.1° Using a biexponential analysis the two transverse components may be separated, and a measure of the correlation time obtained.T2, and 7'22 values at a few concentrations are given in table 1, together with the correlation times obtained. With the assumption of a constant quadrupole coupling constant 01 = 220 kHz)l0 the fraction of bound sodium ions have been calculated. As can be seen in table 1, the values agree closely with those obtained with the Poisson-Boltzmann equation. A more exact theory (where the effect on relaxation times from all relevant counter-ion motion timescales are treated separately),lg demanding a slightly more complicated analysis procedure, may prove to give different results in some details. (This is especially true for slow motions, where terms proportional to p b can be important for excess relaxation.The theory may also be able to explain the excess in R2 and R, at low values of a in fig. 1 and 2.) The trend in pB as presented in table 1 is expected to be little changed. Thus, the increase in R2, ex at low concentration for the highest molecular-weight sample is mainly due to a change in polyion-counter-ion interaction dynamics (an effect of z,). Similar effects are observed for poly(methacry1ic acid), for which z, is much more easily determined.1° In fig. 4 we show how R2, ex for 23Na varies with the concentration of chondroitin-G. GUNNARSSON A N D H. GUSTAVSSON 0 3 I 1 I 2907 I I I I 0.10 0.0 5 0.15 0.20 c,/mol kg-I FIG. 4.-23Na excess transverse relaxation rates, R2, ex, at 26.5 MHz, plotted as a function of the total concentration of ionic charges in chondroitin-4-sulphate, cp.The points are from experiments and the curve is a theoretical one obtained from eqn (4) and (11) using R2, b- R2, = 259 s-'. R, = 5 A, A = 1 A, Cp/cNa = 0.64, t = 25 O C . 0.5 0.4 pbPB 0.3 0.2 011 0.2 0.4 0.6 0.8 1.0 CplCNa FIG. 5.--Theoretical values of the fraction of bound counter-ions as obtained from P.B. theory,pEB, plotted against cp/cNa at two different DNA concentrations, (- - -) 2 and (-) 15 mmol dm-3. R, = 10 A, bmin = 1.7 A, t = 25 O C . 4-sulphate (CS). The initial solution was 0.175 mol kg-l in charged groups of CS (a = 1) and 0.275 mol kg-l in Na+. Theoretical estimates from P.B. theory are also shown. For CS, R,, R, and Rz, ex are expected to be proportional to p s if x and z, are approximately constant.(There is, of course, no reason to expect that the correlation time remains constant on diluting CS, but at the concentrations studied it may vary2908 ION BINDING T O POLYELECTROLYTES cMg/mmol dm-3 FIG. 6.-23Na excess linewidths, Av,, = R2, ex/^, at 24.5 MHz as a function of the Mg2+ concentration for a 15 mmol dm-3 NaDNA solution. The total concentration of Na+ is 23.8 mmol dmP3. The circles are from the experiments of Bleam et aL9 and the dotted line is from Manning's theory. The curves are obtained from eqn (4) and (1 1) using AVb - AVf = 12 1.5 s-' for A = 3 A (-) and AVb - AVf = 86.3 s-l for A = 7 A (- - - -). Parameters are as in fig. 5. only slightly.) The excess relaxation for both PA and CS undergoes an unexpectedly large decrease at the lowest concentrations.Although we have several suggestions, we cannot give an acceptable explanation of those findings. From the results in fig. 3 and 4 and in table 1, we see that the theory based on the P.B. equation describes the decrease in /? with polyelectrolyte concentration surprisingly well. When comparing experimental values with the theory of Manning it is often assumed that the degree of ion binding is independent of the polyelectrolyte concentration. For infinite cylinders one finds from P.B. theory that /? is independent of concentration only at low polyelectrolyte concentrations.16 A comparison of fig. 3 and 4 reveals that the relative decrease in R2, ex is larger for CS than that for PA. This is a manifestation of the much lower surface charge density of CS.Manning's limiting law predicts that p is independent of salt c0ncentration.l Measurements of &, ex as a function of cp/cNa should therefore give a straight line, provided that R2, b is independent of salt concentration. This is indeed found to be the case in many experiments (for example those of Bleam et al.9 on 23Na n.m.r. linewidths in DNA solutions). To check whether the results from P.B. theory are consistent with these experiments we calculated p b as a function of cp/cNa for DNA at two different concentrations. The results are shown in fig. 5 for two different values of A, 3 and 7A. The experiments of Bleam et al. were performed in the range 0 < cp/cNa < 0.6. The deviation of the theoretical curves from a straight line is not large in this range, whichever value of A is chosen, and probably too small to be observable in experiments.The difference between the curves for different DNA concentrations is likewise not large enough to be observed experimentally. We thus see that theoretical calculations based on the P.B. equation are also consistent with the experiments of Bleam et aZ.9 Bleam and coworkersg~ 21 have also performed measurements on the competition between Na+ and Mg2+ binding to DNA in two different ways. First they haveG. GUNNARSSON AND H. GUSTAVSSON 2909 investigated with 25Mg n.m.r. linewidths the effect on Mg2+ binding of variations in the Na+ concentration.21 They found that a portion of the bound Mg is in fact replaced by Na+, but that there is a fraction of bound Mg which cannot be excluded by added sodium.Secondly they have studied the effect on Na+ binding of additions of Mg2+.9 In fig. 6 we plotted the latter experimental results, and two calculated curves with the P.B. equation (one with A = 3 A and the other with A = 7 A). The theoretical curves have been made to coincide with the experimental point at zero Mg2+ concentration. P.B. theory accounts reasonably well for the experimental results, and the theoretical results are not especially sensitive to the value of A. Thus, again, although absolute values ofp, cannot be obtained with n.m.r. studies, the variation in the fraction of bound ions is elucidated nicely. According to the Manning model, Mg2+ is supposed to exclude Na+ binding completely for 2CMMg/CDNA 2 (1 - bmin/b).The variation in excess 23Na linewidth (shown as the dotted line in fig. 6 ) obtained with the ion condensation model, does not describe the experimental results satisfactorily. From 25Mg n.m.r. studies by Rose et aL2I and ourselves22q23 it is concluded that ' site binding' of Mg2+ dominates unspecific electrostatic attractions. (Site-bound Mg2+ is also of great importance in other polyelectrolytes, e.g. PA and polysa~charides.~~) All cases studied (including the DNA data as presented in fig. 6) are rationalized without recourse to the assumption of site binding of sodium ions. CONCLUSIONS This paper demonstrates that the Poisson-Boltzmann equation rationalizes the 23Na n.m.r. results surprisingly well. Our results therefore support the conclusion of StigterZ4 that the Poisson-Boltzmann equation can be viewed as the second member in a hierarchy of theories, in which the theory of Manning is the first and more approximate one. It is not necessary to introduce any site-binding effects in order to explain the results of the experiments considered above.The overall ion-binding behaviour in these systems is thus determined by long-range electrostatic effects, and site binding is only a secondary effect in sodium-ion binding to polyelectrolytes. G. S. Manning, Ace. Chem. Res., 1979, 12, 443. G. Gouy, J . Phys. Radium, 1910, 9, 451. D. L. Chapman, Philos. Mag., 1913, 25, 475. R. Fuoss, A. Katchalsky and S. Lifson, Proc. Natl Acad. Sci. USA, 1951, 37, 579. T. Alfrey, P. W. Berg and H. Morawetz, J . Polym. Sci., 1951, 1, 543. A. D. MacGillivray, J. Chem. Phys., 1972, 56, 83. R. W. Wilson, D. C. Rau and V. A. Bloomfield, Biophys. J., 1980, 30, 317. S. Forsen and B. Lindman, in Methods of Biochemical Analysis, ed. D. Glick (John Wiley, New York, 1981), vol. 27, p. 289. M. L. Bleam, C. F. Anderson and M. T. Record Jr, Proc. Natl Acad. Sci. USA, 1980, 77, 3085. T. Bull, J . Magn. Reson., 1972, 8, 344. lo H. Gustavsson, B. Lindman and T. Bull, J. Am. Chem. Soc., 1978, 100, 4655. l 2 A. Abragam, The Principles of Nuclear Magnetism (Oxford University Press, Oxford, 1961). l 3 B. Jonsson, H. Wennerstrom and B. Halle, J . Phys. Chem., 1980, 84, 2179. l5 P. Linse, G. Gunnarsson and B. Jonsson, to be published. l6 G. Gunnarsson, B. Jonsson and H. Wennerstrom, J . Phys. Chem., 1980,84, 3114. l7 H. Wennerstrom, B. Lindman, G. Lindblom and G. J. T. Tiddy, J. Chem. SOC., Furaday Trans. I , 1979, W. van Megen and I. Snook, J . Chem. Phys., 1980, 73,4656. 75, 663. S. Engstrom and H. Wennerstrom, J. Phys. Chem., 1978, 82, 2711. to be published. l9 B. Halle, personal communication; B. Halle, H. Wennerstrom, M. Levij, J. de Bleijser and J. C. Leyte, 2o G. S. Manning, J . Phys. Chem., 1981, 85, 870.2910 ION BINDING TO POLYELECTROLYTES 21 D. M. Rose, M. L. Bleam, M. T. Record Jr and R. G. Bryant, Proc. Natl Acad. Sci. USA, 1980,77, 22 P. Reimarsson, J. Parello, T. Drakenberg, H. Gustavsson and B. Lindman, FEBS Left., 1979, 108, 23 H. Gustavsson and B. Lindman, to be published. 24 D. Stigter, J. Phys. Chem., 1978,82, 1603; G. S. Manning has commented on this reference in J. Phys. 6289. 439. Chem., 1978,82, 2349. (PAPER 1 / 1322)

ISSN:0300-9599    

DOI:10.1039/F19827802901

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. I , 1982, 78,291 1-2919 Infrared Study of the Reactivity of Acetone and Hexachloroacetone Adsorbed on Haematite BY GUIDO BUSCA AND VINCENZO LORENZELLI* Istituto di Chimica, Facolta di Ingegneria, Universita di Genova, Italy Received 9th October, 198 I Infrared spectra of acetone adsorbed at beam temperature on a-Fe,O, show that acetone can chemisorb on Lewis-acid sites and gives, at least partially, enolate anions; these, by aldolic condensation with molecules from the gas phase, produce a chemisorbed form of mesityl oxide. At 523 K acetate ions are formed. Two different forms of trichloroacetate ions are formed on the surface at beam temperature by the adsorption of hexachloroacetone and trichloroacetic acid. This behaviour indicates the presence of pairs of acid-base sites on the surface of haematite, and also shows that its surface hydroxy groups have a lower degree of nucleophilic character with respect to those of other oxides such as aluminas, SnO, and alkaline-earth oxides.In recent years the interaction of several ~ r g a n i c l - ~ and inorganic1$ 5-8 compounds with the surface of haematite (a-Fe,O,) has been studied by infrared spectroscopy, showing the presence of different active sites and the possibility of several mechanisms of adsorption. The present paper reports the results of an infrared spectroscopic study of the adsorption of acetone and hexachloroacetone, and of their transformation products, on the surface of a-Fe,O,, with the aim of identifying cooperative effects in connection with the surface sites.Infrared studies of the interaction of acetone with various supports have shown that this molecule can both physisorbgy lo and chemisorbg on different adsorbants. On some oxide surfaces a number of transformation products, involving cooperative mechanisms, have been recently 0b~erved.ll-l~ The use of ketoneP and perhalogenated acetone9 as probe molecules to study the surface Lewis acidity has also been discussed. EXPERIMENTAL The samples of a-Fe,O, were obtained by vacuum decomposition of pressed discs of goethite (a-FeOOH) (3 x lo4 N cm-,; 30-50 mg cm-2) into the infrared cell. The B.E.T. surface area of the resulting haematite sample was 45.0 m2 g-l. The samples were heated at 673 K in air for 1 h after decomposition and then evacuated for 1 h at beam temperature (b.t.). The resulting surfaces cannot be distinguished spectroscopically from the 'evacuated' surfaces of ref. (6) and (7). Adsorption experiments were also carried out on ' oxygen-rich' surfaces,6* but no difference in the infrared spectra of the adsorbed compounds was detected. Spectra were recorded with a Perkin-Elmer model 521 infrared spectrophotometer. The infrared cell was the same as in ref. (7). The reference-beam attenuation was set up in order to obtain the best apparent transmission in each region for each sample. Real transmission values of the samples were a few percent in the 4000-3000 cm-I region, and ca. l0-15% in the 2000-1200 cm-' region. Acetone, [2H6]acetone, mesityl oxide and trichloroacetic acid were reagent-grade products 291 12912 ACETONE ADSORBED ON a-Fe,O, from Carlo Erba (Milano, Italy); hexachloroacetone was a pure product from Fluka (Buchs, Switzerland).Acetone was dried over calcium sulphate after the use. All liquid adsorbates were purified by freeze-pumpthaw cycles. Trichloroacetic acid was thoroughly degassed before use. RESULTS AND DISCUSSION Fig. 1 shows the spectra of acetone adsorbed on haematite at b.t. A species responsible for the bands at 2960, 2918, 1675, 1435, 1415, 1355 and 1240 cm-I is first formed on the surface [fig. 1 (b)] and is slowly removed by degassing at b.t. [fig. 1 (d)]. The frequencies of these adsorptions can be compared with those of acetone in the gas or liquid phase as reported by Dellepiane and Overend," and this comparison indicates that the species in question is a form of adsorbed acetone which is more perturbed than the purely physisorbed ~ n e .~ ~ lo I I I 1 3600 34003000 2800 1600 1400 1200 wavenumber/cm-' FIG. 1.-Infrared spectra of acetone adsorbed on haematite: (a) initial a-Fe,O, surface; (b, c) after contact with acetone vapour (400 N m-*) at b.t. [(b) 10 min, (c) 30 min]; ( d ) evacuated for 4 h at b.t. The band at 1675cm' which appears to be asymmetric (with a component at ca. 1685 ern-', probably caused by the heterogeneity of the Lewis-acid sites already discussed4), corresponds to the C=O stretching vibration of molecules chemisorbed on Lewis-acid sites and shows a shift in frequency Av = 46-56 cm-I with respect to the gaseous molecule. This value, compared with the corresponding shifts of analogous species on magnesia (32 cm-l),l* titania (51 cm-1)12 and AlCl, (81 ~rn-'),~ defines the relative acidities of these adsorbants, which agree well with the data obtained using the shifts of the 8a band of chemisorbed pyridir~e.~*'~ It is also interesting to compare the shift for chemisorbed acetone (46-56 cm-l) with those previously reported for the C=O stretches of chemisorbed formaldehyde (1 13 c1~1-l)~ and acetaldehyde (88 cm-1)2 on a-Fe,O,, as this reflects the electronic and steric effects of methyl groups on the acid-base interaction of such carbonylic compounds with iron ions.G.BUSCA A N D V. LORENZELLI 2913 A further remark concerns the stretching vibrations of C-H, whose frequencies are lowered with respect to the liquid or gas phase (2960 and 2918 cm-l) probably because of interactions with surface anions, as already noted for analogous species on magnesia14 and zeolites;20 the bands at 1435, 1415 and 1355 cm-l, assigned to CH, deformation vibrations, are only slightly perturbed. I 1 I 1600 1400 1200 wavenumber/cm FIG.2.-Infrared spectra of acetone adsorbed on haematite: (a) initial a-Fe,O, surface; (6) after contact with acetone vapour (133 N m-z, 3 min) at b.t.; (c) evacuated for 1 h at b.t. By increasing the time of contact at b.t. [fig. l(c)], a new species is formed progressively which is more stable to degassing [fig. 1 (d)] and which is characterized by bands at 2955,2910,1670(sh), 1590,1570,1445,1385 and 1370 cm-l. Another species, characterized by a broad absorption at 1540 cm-' and which is also resistant to degassing at b.t., can be identified by its very short time of contact [fig.2(b) and (c)]; however, it also seems to be present under the conditions of fig. I (b) and (c), but is partially masked by the more intense bands mentioned above. These species cannot correspond to forms of acetone absorbed as such. At least two different transformation products are formed by the adsorption of acetone on haematite, one formed immediately and the second more slowly, and only in the presence of acetone vapour. Neither of these products is the acetate ion, as their infrared spectra and thermal behaviour clearly differ from those of the species detected after direct interaction of acetic acid on haematite.1*2 These compounds can be eliminated from the surface by careful degassing at 423 K.However, higher temper-29 14 ACETONE ADSORBED ON a-Fe,O, atures lead to the formation of a new species, probably acetate ions, with a pronounced change in the infrared spectrum. In order to test if such transformation products can be obtained by the condensation of acetone on the surface, infrared spectra of mesityl oxide adsorbed on haematite were also studied. 1 I 1 1 I 3600 3400 3000 2800 1700 1500 1300 w avenum ber/cm-' FIG. 3.-Infrared spectra of mesityl oxide adsorbed on haematite: (a) initial a-Fe,O, surface; (b) after contact with mesityl oxide vapour (133 N m-,, 15 min) at b.t.; ( c ) evacuated for 4 h at b.t.; ( d ) spectrum of mesityl oxide (liquid film). A comparison of fig.3(c), which reports the infrared spectrum of haematite after contact with mesityl oxide vapour and thorough degassing at b.t., with fig. 3 (d), which shows the spectrum of mesityl oxide in the pure liquid, indicates that the strong perturbation in the region of the double-bond stretching vibrations is caused by chemisorption ; however, very small perturbations of the C-H stretching vibration (2950 and 2900 cm-l in the chemisorbed form) and C-H deformation (1465, 1450, 1420, 1380 and 1375 cm-l, practically identical in both liquid and adsorbed species) are detected. From these data it seems reasonable to conclude that mesityl oxide chemisorbs on haematite via coordination of the C=C-C=O conjugate system, probably on Lewis-acid sites, its spectrum being very similar to that of the same molecule chemisorbed on titania (rutile), as reported by Griffiths and Rochester.12 A comparison of fig.1 (c) with fig. 3 (c) allows us to identify the main transformation product of acetone on haematite as an adsorbed form of mesityl oxide. Mesityl oxide is produced industrially by the dehydration of diacetone alcohol, obtained by the base-catalysed liquid-phase aldol condensation of acetone.21 The mechanism of aldol condensation is we11 known, involving as its first step the formation of the enolate anion of acetone.22 It thus seems reasonable to postulate the same mechanism for the formation of mesityl oxide on haematite. On the basisG. BUSCA AND V. LORENZELLI 291 52916 ACETONE ADSORBED ON a-Fe,O, of these considerations, the band at 1540 cm-1 (fig.2) can be assigned to a chemisorbed form of the acetone enolate anion: also other enolate anions show characteristic bands, due to C-C-0 stretchings, in this r e g i ~ n . l ~ y ~ ~ In order to confirm the formation of the acetone enolate anion, adsorption of [2H6]acetone on haematite was also studied. After contact at b.t. new bands at 2950, 2550 (broad and very intense), 2220, 1435, 141 5 and 1360 cm-l were detected, together with bands in the double-bond stretching region (1680, 1590 and 1540 cm-l) corresponding to the vibrations previously assigned to chemisorbed acetone, chemi- sorbed mesityl oxide and chemisorbed enolate anion. The presence of the band at 2550 cm-l, connected with 0-D stretching vibrations, together with those detected at 2950, 1435, 1415 and 1360 cm-l, the first due to C-H stretching and the others to C-H deformations of chemisorbed acetone (see above), indicates that a strong and fast isotopic exchange between acetone methyl groups and surface hydroxy groups takes place, producing undeuterated acetone and surface deuteroxy groups, the band at 2220 being due to C-D stretching vibration of chemisorbed [2H,]a~etone.By increasing the time of contact [fig. 4(d) and (e)] the bands due to both vCH and d,, vibrations decrease in intensity, owing to further isotopic exchange, while the bands at 1590 and 1570 cm-l, which can be assigned to perdeuteromesityl oxide, increase in intensity. Successive long periods of degassing at b.t. leave the bands at 2205, 1670(sh), 1590 and 1570 cm-l of perdeuteromesityl oxide.The results obtained by adsorption of [2H6]a~etone therefore strongly support the previously discussed mechanism of formation of mesityl oxide on the haematite surface, confirming the deprotonating ability of the surface towards acetone, leading to the enolate anion. In order to identify 'the surface sites where such chemisorptive activity takes place, the adsorption of acetone on surfaces with Lewis-acid sites previously poisoned with TABLE 1 .-INFRARED BANDS (cm-l) AND THEIR ASSIGNMENTS TO ADSORBED SPECIES ON a-Fe20, mesityl acetone acetic oxide adsorption acid adsorption at 523 K adsorption2 acetone adsorption at b.t. acetone physisorbed chemisorbed enolate mesityl mesityl acetate acetate acetone acetone ion oxide oxide ions ions assignments - 2960 - 2955 2950 2940 - 2918 - 2910 2900 2880 1670sh - 1590 1570 - - 1540 - - - - - - - 1415 { ::;z 1445 1385 1360 1355 1370 1240 - - - 1665sh vc=c-c=o - - vc-c-0 1560 1590 - - 1 - - 1540 Vas co; - 1440 1440 Vsym co; 1540 1465sh 1450 1420sh 1380 1375G .BUSCA AND V. LORENZELLI 2917 a stronger base (such as pyridine) has also been studied. No bands due to any form of acetone were detected on such surfaces at pressures of 1-5 Torr (1 33-665 N m-2), even after a long period of contact. At higher pressures (10 Torr = 1.33 kN m-,) a broad band appears at 1690 cm-l, together with a second band at 1360 cm-l and a shoulder on the lower-frequency side of the 19b band of chemisorbed pyridine (i.e. at ca. 1415 cm-l). All these new features are very similar to those of acetone physisorbed on ~ i l i c a ~ ~ ~ ~ and disappear on a short degassing at b.t., indicating the existence under these conditions of physisorbed acetone.[They are probably also present under the conditions of fig. 1 (b), but are masked by the more intense bands of the previously identified species.] The absence of chemisorbed acetone and of its transformation products on haematite poisoned by pyridine indicates that the formation of enolate anions needs the cooperative action of surface oxygen atoms or hydroxy groups on acetone previously chemisorbed on cationic sites. The fact that no mesityl oxide is formed on a surface where only chemisorbed acetone and enolate ion are present (i.e. with no gas-phase acetone), as shown in fig. 2(b) and (c), indicates that enolate ion should react with a molecule of gaseous acetone to give first the aldol condensation and then dehydration of the intermediate diacetone alcohol.This proposed mechanism involves the presence on the surface of pairs of acidic and basic sites, Fobably iron and oxygen ions, which interact at b.t. with the same acetone molecule. On the basis of the results described above, the a-Fe,O, surface shows behaviour with respect to the acetone molecule which is similar to that of rutile,12 while on other oxides (A1203,11 Sn02,24 alkaline-earth including MgO, where different results 1 1 1 1600 1400 1200 wavenumber/cm-' FIG. 5.-Infrared spectrum of acetone adsorbed on haematite: (a) initial a-Fe,O, surface; (b) after contact with acetone vapour (400 N m-2, 15 min) at 523 K.2918 ACETONE ADSORBED ON a-Fe,O, have been obtained with differently prepared 25) chemisorbed acetates are formed at room or beam temperature.However, even on haematite, new features appear if the contact is made at 523 K (fig. 5). In this case the main product which resists degassing at 473 K shows two broad and very intense bands at 1540 and 1440 cm-l together with bands at 2940 and 2880 cm-l, due to C-H stretching vibrations. Similar bands in the vco stretching region are also detected after adsorption of [2H,]acetone at 523 K, together with a broad adsorption at 2550 cm-l, due to surface deuteroxy groups formed under these conditions, and a band near 2200 cm-l with a shoulder near 2100 cm-l, due to C-D stretching vibrations.Both their spectroscopic features and their stability on the surface indicate that these species are acetate ions (already observed on the same surface after the adsorption of acetic acid and other C2 organic compounds2) and [2H,]acetate ions, respectively. From our data it is not possible to establish if acetate ions are obtained through an oxidation mechanism (also involving the formation of an oxidized C, species, probably formate or carbonate ions or CO,) or through an acetal-like species (formed by nucleophilic attack of an OH species on the electrophilic carbonyl carbon of chemisorbed acetone, and production of methane); this has been shown to occur even under milder conditions on aluminas,ll SnO, 2 4 9 26 and MgO.14 1800 1600 1400 1200 wavenum ber/cm -l FIG.6.4nfrared spectra of hexachloroacetone and trichloroacetic acid adsorbed on haematite : (a) a-Fe,O, surface; (b) after contact with hexachloroacetone vapour (133 N m-,, 15 min) at b.t.; (c) another sample after contact with trichloroacetic acid vapour (66 N m-e, 30 min) at b.t. In order to confirm the behaviour of ketones on the haematite surface we have also studied the adsorption of hexachloroacetone ; this cannot undergo enolization owing to the absence of active hydrogen atoms in the alpha position, but is more active towards nucleophilic attack, owing to the higher polarization of its C=O bond by the inductive effect of the chlorine atoms. The spectrum of hexachloroacetone adsorbed on a-Fe,O, is shown in fig. 6(b). The two bands at 1770 and 1745 cm-l,G .BUSCA AND V. LORENZELLI 2919 very similar to the C=O stretching vibrations of this molecule in the liquid phase or in are due to weakly perturbed hexachloroacetone. The species responsible for the two intense bands at 1605 and 1385 cm-l is predominant at low pressures, while a new species, responsible for an analogous pair of bands centred at 1670 and 1360 cm-l, is formed progressively at higher pressures. An analogous situation is detected after the adsorption of trichloroacetic acid [fig. 6 (c)]. These species absorb in the same regions as the metal trichlor~acetates~~ and can be identified as two different types of trichloroacetate ions. The species responsible for the lower frequency- difference bands, which is formed first and is more stable to degassing, is probably bidentate or bridged; the other is monodenate.28 Trichloro- and trifluoro-acetate ions were also observed as the main product of hexachloro- and hexafluoro-acetone adsorption on alkaline-earth oxides,25 Ti02,2e SIIO,~~ and The different behaviour of acetone and hexachloroacetone, which agrees with their differing abilities to undergo nucleophilic attack at a C=O bond, can be taken as an indication that, on haematite also, acetate and trichloroacetate ions are formed by a mechanism involving nucleophilic attack of the surface hydroxy groups.The difference in the behaviour of acetone on haematite with respect to that of the same molecule on other oxides (e.g. aluminas, SnO, and MgO) already remarked upon, indicates that the surface hydroxy groups have a smaller degree of nucelophilic character on this oxide.l6 C. H. Rochester and S. A. Topham, J. Chem. SOC., Faraday Trans. 1, 1979, 75, 1259. V. Lorenzelli, G. Busca and N. Sheppard, J. Catal., 1980, 66, 28. G. Busca and V. Lorenzelli, J . Catal., 1980, 66, 155. G. Busca and V. Lorenzelli, Mater. Chem., 1981, 6, 175. G. Busca and V. Lorenzelli, Mater. Chem., 1980, 5, 213. F. Al-Mashta, N. Sheppard, V. Lorenzelli and G. Busca, J. Chem. SOC., Faraday Trans. I, 1982,78, 979. G. Busca and V. Lorenzelli, J . Catal., 1981, 72, 303. A. Bertoluzza, G. B. Bonino, G. Fabbri and V. Lorenzelli, J. Chim. Phys., 1966, 63, 395. H. Knozinger, H. Krietenbrink, H. D. Muller and W. Schulz, Proc. VZth Znt. Congr. Catal. (The Chemical Society, London, 1976), p. 183. * V. Lorenzelli, G.Busca, F. Al-Mashta and N. Sheppard, J . Catal., 1981, 72, 389. lo R. P. Young and N. Sheppard, J. Catal., 1967, 7, 223. l 2 D. M. Griffiths and C. H. Rochester, J . Chem. SOC., Faraday Trans. 1, 1978, 74, 403. l 3 0. Koga, T. Onishi and K. Tamaru, Lr. Chem. SOC., Faraday Trans. I, 1980, 76, 19. l 4 J. A. Lercher, H. Noller and G. Ritter, J . Chem. SOC., Faraday Trans. 1, 1981, 77, 621. l5 W. Schulz and H. Knozinger, J . Phys. Chem., 1976, 80, 1502. l6 M. L. Hair and J. D. Chapman, J . Phys. Chem., 1965,69, 3949. G. Dellepiane and J. Overend, Spectrochim. Acta, 1966, 22, 593. H. Miyata, Y. Toda and Y. Kubokawa, J . Catal., 1974, 32, 155. N. E. Tretiakow and V. N. Filimonov, Kinet. Katal., 1973, 14, 803. 2o G. Senkyr and H. Noller, J . Chem. SOC., Faraday Trans. 1, 1975, 71, 997. 21 K. Weissermel and A. J. Arpe, Industrial Organic Chemistry (Verlag Chemie, Weinheim, 1978), p. 247. 22 C. K. Ingold, Structure and Mechanism in Organic Chemistry (Cornell University Press, Ithaca, N.Y., 23 K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds (Wiley, New 24 E. W. Thornton and P. G. Harrison, J. Chem. SOC., Faraday Trans. 1, 1975, 71, 2468. 25 N. E. Tretiakow and V. N. Filimonov, Kinet. Kutal., 1970, 11, 815. 26 P. G. Harrison and E. W. Thornton, J . Chem. SOC., Faraday Trans. I, 1976, 72, 1313. 2 i E. Spinner, J . Chem. SOC. 1964, 4217. 28 G. B. Deacon and R. J. Phillips, Coord. Chem. Rev., 1980, 33, 227. 29 D. M. Griffiths and C. H. Rochester, J. Chem. SOC., Faraday Trans. I , 1977, 72, 1913. 1953), p. 680. York, 3rd edn, 1978), p. 249. (PAPER 1 / 1577)

ISSN:0300-9599    

DOI:10.1039/F19827802911

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. 1, 1982, 78, 2921-2928 Tracer Diffusion in Methanol and Acetonitrile under Pressure BY ROBERT L. HURLE AND LAWRENCE A. WOOLF* Research School of Physical Sciences, Australian National University, Canberra, A.C.T. 2600, Australia Received 9th November, 198 1 Tracer-diffusion measurements have been made with a diaphragm cell at pressures up to ca. 260 MPa for acetonitrile in methanol and methanol in acetonitrile at 283, 298 and 313 K and up to 300 MPa for carbon disulphide in acetonitrile at 298 K. A correlation treatment used previously for self-diffusion in pure fluids has been extended to cover this type of tracer diffusion. Analysis of the data in acetonitrile indicates that dipoledipole interactions between the tracer species and solvent may have a significant effect on the diffusion.As a result of work carried out over the last decade, there is now a reasonable quantity of accurate data available for self-diffusion under pressure in a number of liquid systems covering a range of molecular sizes and interactions. The rough hard-sphere theory developed by Chandler1 has proved a valuable method of summarising and interpreting the data; it has been useful also for liquids composed of non-spherical molecules and even in associated systems where the model was not expected to apply. A correlation treatment of self-diffusion data due to Dymond2 has also proved useful, particularly since it can be used outside the density range of the hard-sphere model. Both approaches provide an estimate of the equivalent hard-sphere diameter of the diffusing particle.Chandler’s model has also been used for mutual diffusion in systems at atmospheric pres~ure.~ However, Dymond and W001f4 have recently used the rough hard-sphere model for a qualitative interpretation of tracer-diffusion data for benzene, toluene, benzo-a-pyrene, carbon disulphide and acetonitrileinn-hexane at pressuresup to400 MPa. The results seemedinaccord with the theory but a quantitative comparison was hampered by the small amount of computer simulation results available for smooth hard-sphere model systems of two components in which the molecules have a significantly different size ratio. This paper reports tracer-diffusion measurements which have been made up to ca. 260 MPa for acetonitrile in methanol and methanol in acetonitrile at 283, 298 and 3 13 K and up to 300 MPa for carbon disulphide in acetonitrile at 298 K.Because the equivalent hard-sphere diameter ratios, G ~ / C T , , where 2 denotes the tracer and 1 the solvent species, are closer to unity than was generally the case for Dymond and Woolf, the data for these systems enable a more quantitative test than was possible in their systems. It has also been possible to extend Dymond’s correlation approach for self-diffusion to interpret the tracer data. EXPERIMENTAL The experiments were all made with the high-pressure diaphragm-cell technique using radioactive tracers; the technique and the counting procedures have been described previ~usly.~* Pressures were measured to kO.4 MPa and temperatures to kO.01 K.Details of the chemicals 292 12922 TRACER DIFFUSION IN CH30H A N D CH,CN used and the densities needed for the experimental measurements and interpretation of the results have been given by Hurle and Woolf for methanol' and acetonitriles and by Woolf for carbon di~ulphide.~ RESULTS The experimental results are given in tables 1 and 2. They are fitted within the estimated experimental error of +2% by the equation 3 i-1 In (D/ 1 0-9 m2 s-l) = A , + Ai( p/MPa)i + A4( 1 03K/T) + A5( 1 03K/T) (p/MPa). (1) The coefficients of the equation are given in table 3. TABLE 1 .-TRACER DIFFUSION OF ACETONITRILE IN METHANOL UNDER PRESSURE T / K p/MPa D/lO-g m2 s-l T/K p/MPa D/10-9 m2 s-I 283.2 0.1 26.2 27.0 57.6 95.6 145.6 198.1 252.5 298.2 0.1 14.6 33.8 34.8 61.8 83.1 2.73" 2.42 2.44 2.20 1.91 1.67 1.43 1.32 3.43" 3.28 2.96" 3.02 2.69 2.54 298.2 106.9 137.0 186.3 218.8 255.4 313.2 0.1 22.3 51.6 54.7 93.6 142.2 204.1 271.1 2.27 2.11 1.87 1.76 1.61 4.25" 3.86 3.45 3.35 3.05 2.64 2.28 1.97 a Mean of two experiments.DISCUSSION Self-diffusion results in liquids can be correlated quite well over the entire density range by using the measured self-diffusion coefficient D1(l) to define a reduced diffusion coefficient2 where n is the number density, the Enskog dilute fluid self-diffusion coefficient D, is given by (nD,), = (3/8)(RT/xM)a ay2 and the volume of close packing is V, = N0;/2/2 with (T, the molecular diameter. Eqn (2) may be extended to the tracer-diffusion coefficient of a solute 2 in a solvent 1 by recognising that the diffusion is a limiting case of mutual (binary) diffusion so that the mutual-diffusion coefficient D,, -+ Di(1) as n2 -+ 0.The reduced tracer-diffusion coefficient is then defined as (3) (4) W1)= [n&(l)/(n~,)oI (V/ V,Y (2) D,*(1) = [~~;(1)/Wl2)01 (V/ v,F (nD12)o = (3/8) [RT(Ml+ M2)/(2nM,M2)14 0 2 where the low-density Enskog mutual-diffusion coefficient is given byR. L. HURLE AND L. A. WOOLF TABLE 2 .-TRACER DIFFUSION IN ACETONITRILE UNDER PRESSURE 2923 ~ _ _ _ _ _ _ _ _ _ T/K p/MPa D/10-9 m2 s-l T / K p/MPa D/10-9 m2 s-l 283.2 298.2 298.2 0.1 18.3 60.8 102.6 167.9 209.3 275.3 0.1 16.3 38.5 69.4 90.1 0.1 56.9 139.1 236.5 306.2 methanol 3.94" 298.2 3.59 3.24 2.76 2.42 3 13.2 2.16 1.86 4.90a 4.57 4.19 3.70 3.47 carbon disulphide 4.91a 3.76 2.83 2.31 1.86 144.4 188.7 236.9 0.1 13.0 37.0 38.1 69.3 106.9 158.0 224.6 272.8 2.95 2.70 2.30 5.67a 5.38 4.98" 4.98 4.30 3.90 3.37" 2.75 2.53 a Mean of two experiments.TABLE 3.-cOEFFlCIENTS OF EQN (1) coefficient l*CH,OH/CH,CN 14CH,CN/CN30H C35S2/CH3CN A0 5.259 10sA2 6.318 1 0 3 ~ ~ - 7.337 1O9A3 - 7.455 A4 - 1.100 1 O ~ A , 9.710 r.m.s.d. m2 s-l) 0.056 5.650 1.596 7.457 19.857 - 4.384 -5.991 -6.555 - 34.832 - 1.316 0 0 0 0.031 0.032 and = Na13/d2. In this way the correlation approach makes allowance for the classical smooth hard-sphere dependence of the tracer-diffusion coefficient on mass, size and temperature but it does not impose any other specific constraint. The analogy with the self-diffusion correlation procedure is maintained by v?rying o2 with temperature so as to bring the D,*(l) values for each tracer species close to a common curve.This procedure has been applied to the data for tracer diffusion in acetonitrile with the results shown in fig. 1. (Included in fig. 1 are some recent results for the tracer2924 TRACER DIFFUSION IN CH,OH AND CH,CN 0.3 D* 0.2 0.1 I I I I I I I I 1.5 1.6 1.7 1.8 V l v, FIG. 1.-Reduced tracer-diffusion coefficients in acetonitrile: V, CS, at 298 K; 0, CH,OH at 283 K; A, CH,OH at 298 K; 0, CH,OH at 313 K; 0, H,O at 298 K; broken line: reduced self-diffusion coefficient of acetonitrile. diffusion of water in acetonitrile obtained by A. J. Easteal in these laboratories.) The 0, values for acetonitrile were those determined previously from self-diffusion data the oz values of 0.427 nm for carbon disulphide and 0.242 nm for water at 298 K were also from self-diffusion lo For methanol the oz value at 298 K (0.364 nm) was assumed to be that for self-diff~sion;~ for the measurements at 283 and 313 K the o2 values (0.390 and 0.360 nm, respectively) were determined arbitrarily as those giving the best agreement between the D,*(l) for those two temperatures with the D,*(l) at 298 K.The relative positions of the D,*(1) curves in acetonitrile show differences much greater than would be expected on the basis of the different sizes and shapes of the various molecular species. The correlation approach assumes that interactions between the molecules are only of the van der Waals type. However, methanol, acetonitrile and water have permanent dipole moments (9.7 x 11.3 x and 6.3 x C m, respectively) and carbon disulphide a quadrupole moment (14 x C m).It seems likely, therefore, that the intermolecular attractive forces due to dipole-dipole interactions (quadrupole-dipole for CS,/CH,CN) are responsible for the major differences in the diffusion of the various molecular species in acetoni trile. In methanol the D,*(l) values for CH,CN at 283,298 and 313 K were brought close to a common curve by fixing the o2 value at 298 K identical to that found for the0.; D* 0.1 5 0.1 R. L. HURLE AND L. A. WOOLF 2925 I I I I I ’ I I I I I I I I 1.7 1.8 1.9 2 .o V’V0 FIG. 2.-Reduced tracer-diffusion coefficients of acetonitrile in methanol: 0 , 2 8 3 K; 0 , 2 9 8 K; A, 313 K; 0, reduced self-diffusion coefficient of methanol at 298 K.TABLE 4.-TRANSLATIONAL-ROTATIONAL COUPLING CONSTANTS O A , , FOR TRACER DIFFUSIONa TIK tracer solvent 283.2 298.2 313.2 CH,CN CH,OH 0.24 (fO.01) 0.28 (fO.01) 0.31 (kO.01) (CH,OH CH,OH 0.17 0.21 0.24) CH,OH CH,CN 0.51 (f0.02) 0.54 (k0.02) 0.56 (f0.04) (CH,CN CH,CN 0.50 0.53 0.54) cs2 CH3CN - 0.64 (k0.06) - a a(CH,OH) = 0.364 nm; a(CH,CN) = 0.409 nm; a(CS,) = 0.427 nm. self-diffusion of acetonitrile at the same temperature and determining a 0, value at 283 and 3 13 K which best approximated coincidence of the D,*( 1). The resulting curve shown in fig. 2 lies some distance from, but is parallel to, the D:(l) curve for self-diffusion in methanol. It would be wrong to attribute strong physical significance to the 0, values for acetonitrile in methanol obtained in this way.Moreover, note that the effect of temperature on the hydrogen-bonded structure of methanol is, by this procedure, included in the 6, values. Consequently the range of values for 0, of 0.465 nm at 283 K, 0.409 nm at 298 K and 0.380 at 313 K reflects what is believed to be a rapid change with temperature of the methanol hydrogen bonding.’ ROUGH HARD-SPHERE MODEL The rough hard-sphere model of self-diffusion1 is readily extended in principle to tracer diffusion of solute 2 in solvent 1 but the resulting equation is severely handicapped in its application by the extremely limited molecular-dynamics simulation data available for the diffusion coefficient (D12)SHS of two-component systems2926 TRACER DIFFUSION I N CH,OH AND CH,CN composed of hard spheres.In the liquid region of most interest those data effectively concern only those systems where (a) the diameter of the tracer molecule (a,) does not exceed that of the solvent molecule (a,), (b) the molar mass of the tracer molecule (M,) is the same as that of the solvent (M,) and (c) the reduced density of the solvent n* = n,@, where n, is the number density, is between 0.88 and 0.94 (data are also available for n* = 0.47). Czworniak et aL3 have represented the data as a correction factor with (D,,)E the Enskog dense-fluid expression for the mutual-diffusion coefficient. For tracer diffusion of component 2 in solvent 1 C = 0.840 - (q - 0.463) [7.69 + 32.3 (y~ - 0.463)] + In (M2/M1) x [0.299-0.165 In (M,/M,)]-0.200 (a,-a,)/a,+0.336 [In (M2/M1)] x (a, - a,)/a, - L7.80 In (M,/M,) + (5.33/0,) (a, - a,) (q - 0.463)] (8) where q = q1 = ~n10;/6.Since n2 = 0 where g12(0) is the unlike radial distribution function at contact, which for n2 = 0 reduces to Eqn (7)-(10) reproduce the simulation data for (D12)SHS within ca. 6% for M2/M, close to 1 with a2/a1 between 0.5 and 1 and q between 0.46 and 0.49 (for self-diffusion the range of q is 0.37-0.49). By analogy with the rough hard-sphere model of self-diffusion, the tracer-diffusion coefficient of rough hard spheres is Values of the rotational-translational motion coupling factor 'A,, are given in table 4 with some corresponding data for self-diffusion. The values of ai were those determined from self-diffusion meas~rements;~-~~ the ratios 0,/0, were all within the limits of eqn (8).For CH3CN (2) in methanol (l), M,/M, is 1.28, for CH,OH (2) in acetonitrile (l), M,/M, is 0.78. The relative constancy of 'A,, for those systems is satisfactory and well within the error (& 10-15%) of the molecular-dynamics results on which eqn (8) is based. For the carbon disulphide tracer data, however, the ratio M,/M, is 1.85, which is well outside the limit of eqn (8) and the values of 'A,, vary from 0.54 at y~ = 0.41 to 0.70 at q = 0.48. Consequently only the three values in the known density range of eqn (8) (0.46 < q < 0.49) have been used to obtain the 'A,, given for carbon disulphide/acetonitrile in table 4; even with that limitation the 'A,, values may well be unreliable. Under these circumstances it is useful to examine the data using the qualitative approach adopted by Dymond and W001f4 for tracer diffusion in n-hexane.Dymond and Woolf examined the experimental and theoretical variation of the ratio D:(l)/Dl(l), where Dl(l) is the self-diffusion coefficient of the pure solvent 1R. L. H U R L E AND L. A. WOOLF 2927 I 0.5 0.55 0.6 0.65 Voi 1' FIG. 3.-Ratio of solute (2) tracer-diffusion coefficient to solvent (1) self-diffusion coefficient at 298 K: solid lines, 1, (ITJIT~ = 0.75, M,/M, = 1); 2, (1, 0.1); 3, (1, 1); 0, methanol in acetonitrile (0.89, 0.78); 0, CS, in acetonitrile (1.04, 1.85). at the same density as the tracer measurement of DZ(1). They introduced the appropriate Enskog expressions for the tracer- and self-diffusion coefficients to obtain with The theoretical variation of D;( l)/Dl( 1) [ie.the r.h.s. of eqn (12)] with &/ V is shown in fig. 3 assuming O A l z = A , , at constant M,/M, and a2/01; the solid curves are based on computer simulation data for mutual and self-diffusion, (D/DE),, and (D/D E)ll, respectively, for which smooth hard-sphere theory provides that 'A,, and A , , must be unity. The inference from the theoretical curves is that in the liquid region Dz(I)/D,(l) is relatively insensitive to small changes in M,/M, at constant 02/01 but more affected by changes in 02/01 at constant M,/M,. For real systems, however, O A 1 2 and A,, are net necessarily unity or equal. Consequently, the exact position of DZ(l)/D,(l) for such systems on graphs such as fig. 3 will depend on the oA,,/A1, ratio, and there is an indication that for some systemsl0 'A,,/A,, > I for o,/o, - 1 and M,/M, - 1.For CS,/CH,CN, o,/o, = 1.04 and M,/M, = 1.86, while for CH,OH/CH,CN the corresponding ratios are 0.89 and 0.78. It would be expected that the values of the ratio of diffusion coefficients for methanol would be much closer to the curve o,/o, = 0.75, M J M , = 1 than would the ratios for CS, in acetonitrile if smooth hard-sphere behaviour were followed in the two tracer systems. But the two sets of ratios shown in fig. 3 for experiments in those systems are close to coincidence. The inference to be drawn is that the O A 1 2 value for CH,OH/CH,CN is smaller than that for CS,/CH,CN, reflecting more translational-rotational coupling. This qualitative result is in agreement with the relative values of the rough hard-sphere parameters given in table 4 which were determined using eqn (1 1).In summary, it may be said that the correlation of the data using the reduced2928 TRACER DIFFUSION I N CH,OH AND CH,CN tracer-diffusion coefficient suggests a qualitative means of establishing the relative effects of dipole-dipole interactions on diffusion. The rough hard-sphere approach provides some indication of the translational-rotational momentum exchange between the solute and solvent species but the numerical values of the O A , , parameter are of dubious reliability because of the extremely limited simulation data on which they are based. Until more simulation results are available it is pointless to speculate on whether or not the O A , , values give any indication of the influence of attractive interactions between the diffusing particles. We thank A. J. Easteal for permission to use his data for tracer diffusion of water in acetonitrile. R. L. H. thanks the Australian Government for a Commonwealth Post-graduate. Research Award. D. Chandler, J. Chem. Phys., 1975, 62, 1358. J. H. Dymond, Physica, 1974, 75, 100. K. J. Czworniak, H. C. Andersen and R. Pecora, Chem. Phys., 1975, 11, 451. J. H. Dymond and L. A. Woolf, J. Chem. SOC., Faraday Trans. I , 1982, 78, 991. L. A. Woolf, J. Chem. SOC., Faraday Trans. I , 1975, 71, 784. R. Mills and L. A. Woolf, The Diaphragm Cell (A.N.U. Press, Canberra, 1968). R. L. Hurle and L. A. Woolf, J. Chem. SOC., Faraday Trans. I , 1982, 78, 2233. L. A. Woolf, J. Chem. SOC., Faraday Trans. 1, 1982, 78, 583. ' R. L. Hurle and L. A. Woolf, J. Chem. SOC., Faraday Trans. 1, 1982, 78, to be published. lo L. A. Woolf and K. R. Harris, Chem. Phys., 1978, 32, 349. (PAPER 1 / 1736)

ISSN:0300-9599    

DOI:10.1039/F19827802921

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1982, 78, 2929-2935 Kinetics of the Oxidation of DL- 1 -Phenylethanol by Tris (2,2'-bipyridine)nicke1(111) Ions in Aqueous Perchlorate Media BY DAVID Fox AND CECIL F. WELLS* Department of Chemistry, University of Birmingham, Edgbaston, P.O. Box 363, Birmingham B15 2TT Received 1st December, 1981 The product of the oxidation of DL-1-phenylethanol by [Ni(bipy),13+ is acetophenone, and it is found that the ratio I A[Ni"'] I// A[acetophenone] 1 = 2, analogous to the stoichiometry found for the oxidation of secondary alcohols by aqua-cations. The rate of the oxidation is first-order in [Ni(bipy)i+] and first-order in [alcohol] with the second-order rate constant independent of acidity. It is concluded that the oxidation is an outersphere reaction not involving the preliminary removal of a bipyridine molecuie from the Ni"' and that none of the alcohol is converted to an oxidatively inert form cia a solvent-sorting equilibrium involving protons.The values found kinetically for the equilibrium constant K , for this solvent-sorting involving phenylethanols are compared with K , values for other alcohols. The enthalpy and entropy of activation are compared with values for AH* and AS* for the oxidation of a range of substances by [Ni(bipy),13+. Unfortunately a detailed investigation of the kinetics of the oxidation of simple aliphatic alcohols by [Ni(bipy),13+ is not possible since their rates are comparable to the rate of oxidation of water by [Ni(bipy),13+. Such kinetic data for simple secondary alcohols oxidized by a cation without water molecules in its first coordination sphere would have afforded an interesting comparison with the data available from the kinetics of the oxidation of such secondary alcohols by aqua-cati0ns.l [Ni(bipy),I3+ has been found to oxidize hydrogen peroxide,2 bromide ions,3 azide ions4 and benzyl alcohol5 in outersphere reactions without the removal of a bipyridine molecule from the Ni3+, and it has been shown4 that they involve a genuine cationic oxidation rather than a free-radical oxidation involving a Ni**-bipyridine radical complex.Unlike the oxidation of benzyl alcoh01,~ where the stoichiometry was assumed by the analogy with the oxidation of p-benzohydroquinone by [Ni(bipy),13+ and the oxidation of benzyl alcohol by permanganate ions,6 product yields can be determined for the oxidation of 1 -phenylethanol by [Ni(bipy),13+. The investigation of the stoichiometry of this latter reaction, together with a detailed kinetic investigation of the reaction, is reported here.EXPERIMENTAL MATERIALS [Ni(bipy),13+ was prepared as described previo~sly.~-~ A 5 x lop4 mol dm-3 solution of [Fe(bipy),12+ was prepared by dissolving the calculated weight of AnalaR F,eSO, - 7H20 and AnalaR 2,2'-bipyridine in water. Solutions of sodium perchlorate were prepared by the accurate neutralization of AnalaR HC10, with AnalaR sodium carbonate: the solution was boiled to expel CO, and then filtered through a Whatman no. 42 paper. The reagents needed for the estimation of acetophenone were prepared as described previ~usly.~ DL- 1 -Phenylethanol was purified by shaking with a solution of iron(I1) sulphate, and the alcohol layer was washed four 95 2929 FAR 12930 OXIDATION OF DL- 1 -PHENYLETHANOL times with distilled water and fractionally distilled.The procedure used' to remove trace carbonyl impurities from simple aliphatic alcohols could not be applied here owing to the elevation of the boiling point on addition of 2,4-dinitrophenylhydrazine. Water was distilled once in an all-glass still. AnalaR perchloric acid was used. PROCEDURE Rates were determined by following changes in optical density at 360 nm in the thermostatted cell housing of a Unicam SP500 series 2 spectrophotometer. To maintain a constant temperature, water was circulated from a thermostat or a water+alcohol mixture from a cryostat depending on the temperature required : normally the difference between inlet and outlet temperatures did not exceed 0.1 OC.Initial concentrations of [Ni(bipy),13+ were always ca. 5 x low5 mol drn-,. Ionic strength was maintained at I = 2.00 mol dm-, by the addition of sodium perchlorate. Yields of acetophenone were measured spectrophotometrically at 450 nm as the acetophenone 2,4-dinitrophenylhydrazone anion7 after neutralizing the perchloric acid in the reaction mixture and distilling off the acetophenone in a closed system under high vacuum. [Ni(bipy)g+] were determined by sampling into a solution containing [Fe(bipy),12+ and measuring the change in optical density at 522 nm. RESULTS AND DISCUSSION STOICHIOMETRY It was not possible to determine the consumption ratio I A[NiIII] I// A[PhEtOH] I (PhEtOH = DL-1-phenylethanol) since the reaction was too slow with NilI1 in excess, the rate in these conditions becoming comparable with that of the [Ni(bipy),13+ + water reaction.Interference from the latter reaction becomes negligible with [PhEtOH] in high excess over the [Ni(bipy)i+]. Under these latter conditions, the ratio IAINilI1]l/ IA[acetophenone]l can be determined when all the [Ni(bipy),13+ is allowed to react. Values of this ratio for varying acidities are given in table 1, which also shows that TABLE 1 .-VALUES OF I AINil"] [/I A[acetophenone] I AT VARIOUS VALUES OF [HClO,] AND UNDER VARYING CONDITIONS I AINilll] ] I A[acetophenone] 1 [HC1041 /mol dm-, conditions 2.00 in air 1.80 1.00 in air 1.88 0.40 under N, 1.92 2.00 under N, 2.00 the ratio is unchanged by the removal of all the oxygen by nitrogen. We conclude therefore that the overall reaction is represented by eqn (1) 2Ni111 + PhEtOH --+ 2Ni11 + CH,COPh + 2H+ (1) which is analogous to the oxidation of simple aliphatic secondary alcohols by aqua-cati0ns.l RATES OF OXIDATION AT 9.2OC [PhEtOH] was always maintained in large excess over [Ni(bipy)i+] to suppress the attack of [Ni(bipy),I3+ on the solvent water molecules.Plots of log,, (optical density) against time were always linear. Fig. 1 shows a linear plot of the pseudo-first-orderD . FOX A N D C. F. WELLS 293 1 0 1 2 3 4 5 [PhEtOHl/10-2 mol dm-3 FIG. 1 .-Plot of the pseudo-first-order rate constant against concentration of 1-phenylethanol at 9.2 OC with [HCIO,] and ionic strength = 2.00 mol dmP3.rate constant k,, calculated from the slopes of such plots, against [PhEtOH] in 2.00 mol dm-3. This plot passes through the origin. It is concluded that the reaction is first order in both [Ni(bipy)i+] and [PhEtOH]. Values for k, and the second-order rate constants, k,, are collected in table 2. The values of k , show that the rate is independent of the acidity. VARIATION OF RATE WITH TEMPERATURE Table 2 contains values for k, obtained from the slopes of linear plots of log,, (optical density) against time for varying [PhEtOH] and varying acidity at 3.6, 17.1, 25.5 and 32.8 O C . The derived values of k,, also given in table 2, confirm that k , is invariant with acidity at constant temperature and is unaffected by the removal of oxygen by nitrogen. The mean values of k , with standard errors are also given in table 2.These latter values are used to plot log,, k , against the reciprocal of the-absolute temperature, which gives a straight line (fig. 2). From the least-squares treatment of these data, the enthalpy of activation AH* = 77f2 kJ mol-1 and the entropy of activation AS* = - 2.4 A 5.7 J K-l mol-1 were calculated. MECHANISM OF THE OXIDATION The rate of reaction is first order in [Ni(bipy)i+], first order in [PhEtOH] and independent of acidity in the temperature range 3.6-32.8 O C . To conform to the observed stoichiometry (l), the rate-determining step will be reaction (2) Nil1'+ PhEtOH 2 NilI + PhC(CH,)OH + H+ Nil1'+ Phe(CH,)OH 2 Nil1 + PhCOCH, + H+ (2) followed by the very rapid reaction ( 3 ) ( 3 ) with k , 3 k,.The independence of the rate with acidity shows that this oxidation is similar to the oxidations of H202,, Br-,3 N;4 and benzyl alcohol5 in being an outersphere reaction without removal of a bipyridine ligand during the oxidation, as the removal of the bipyridine ligand is acid-depender~t.,.~ Solvent-sorting between the simple aliphatic alcohols and water molecules in the solvated proton has been found to influence the oxidation of these substrates by aqua-cations1 and by the photoexcited anthraquinone- 2-sulphonate anion.8 This introduces an acid dependence on the rate of oxidation, with 95-22932 OXIDATION OF D L - 1 -PHENYLETHANOL TABLE 2.-vALUES FOR k, AND k, FOR VARYING [PhEtOH], [HClO,] AND TEMPERATURE WITH IONIC STRENGTH = 2.00 mol dm-3a (UNDER NITROGEN) ~ ~ ~ ~ ~ [HClO,] [PhEtOH] k* k2 mean k , T/OC /mol dm-3 / mol dm-3 / 1 0-3 s-l / 10-l dm3 mo1-1 s-1 /dm3 mol-1 s-l 9.2 9.2 9.2 9.2 9.2 9.2 9.2 9.2 9.2 9.2 9.2 3.6 3.6 3.6 3.6 3.6 3.6 3.6 17.1 17.1 17.1 17.1 17.1 17.1 17.1 17.1 17.1 17.1 17.1 17.1 25.5 25.5 25.5 25.5 25.5 25.5 25.5 25.5 25.5 25.5 25.5 25.5 32.8 32.8 32.8 32.8 32.8 32.8 32.8 32.8 32.8 32.8 2.00 2.00 2.00 2.00 2.00 1.60 1.60 1 .oo 1 .oo 1 .oo 0.40 2.00 2.00 1.60 1.60 1 .oo 1 .oo 2.00" 2.00 2.00 2.00 2.00 2.00 1.60 1.60 1.60 1.60 1 .oo 1 .oo 0.40 2.00 2.00 2.00 2.00 2.00 1.60 1.60 1.60 1 .oo 1 .oo 1 .oo 0.40 2.00 2.00 2.00 2.00 2.00 1.60 1.60 1 .oo 1 .oo 1 .oo 10.6 15.9 26.4 39.6 53 21.1 26.4 15.9 21.1 26.4 53.0 26.4 39.6 26.4 39.6 26.4 39.6 39.6 6.6 13.2 26.4 39.6 53.0 13.2 26.4 39.6 53.0 13.2 26.4 53.0 6.6 13.2 26.4 39.6 53.0 13.2 26.4 39.6 26.4 39.4 52.8 39.6 3.96 7.9 10.6 13.2 15.8 7.9 13.2 10.6 15.8 21.1 0.51 0.75 1.32 2.00 2.70 1.10 1.32 0.81 1.10 1.38 2.54 0.65 0.98 0.67 0.97 0.65 0.97 0.95 0.88 1.76 3.52 5.1 6.9 1.76 3.58 5.1 6.7 1.82 3.46 6.9 1.90 3.92 7.6 11.2 15.4 3.92 7.8 11.4 7.5 12.0 15.4 11.6 2.76 5.8 7.5 9.0 10.8 5.8 8.8 7.5 11.0 15.0 0.24 0.236 0.250 0.253 0.255 0.261 0.250 0.255 0.26 1 0.261 0.240 2.51 k0.09 x 0.123 0.124 0.127 0.123 0.123 0.123 0.120 1.23A0.02 x lop2 0.67 0.67 0.67 0.64 0.65 0.67 0.68 0.64 0.53 0.69 0.66 0.65 1.44 1.49 1.44 1.41 1.45 1.49 1.48 1.44 1.42 1.52 1.46 1.47 3.49 3.67 3.54 3.41 3.42 3.67 3.33 3.54 3.48 3.55 0.351 kO.011 6.640.2 x 0.146 & 0.003D.FOX AND C . F. WELLS 2933 1.5 l k I \ 3.2 3.3 3.1 3.5 3.6 3.7 lo3 KIT FIG. 2.-Plot of the logarithm of the second-order rate constant for ionic strength = 2.00 mol dm-3 against the reciprocal of absolute temperature. the alcohol present in the solvated proton being oxidatively inert.s However, when the extent of the hydrolysis of the aqua-cation is sufficiently high, as with Mnit in reaction (4) Mnit MnOHii + H& with MnOHii oxidatively inactive, the acid-dependence derived from this tends to cancel out that which derives from the solvent-sorting equilibrium (5) (4) Kh ROHsolv + ~ ~ + ~ ~ z ~ ~ z ~ s o l v * W+(HzO)z-1 ROHIsolv + H2Osolv ( 5 ) but when Kh is low enough an acid dependence results, as with Cog;.' However, if reaction (4) is absent and K, = [{H+(H,O),-, ~ ~ ~ ~ , o , v 1 / ~ ~ ~ + ~ ~ 2 ~ ~ z > , , , , 1 [ROHSOIVI is very low, the rate of oxidation will also be acid-independent.Unfortunately, the concentration of PhEtOH cannot be made high enough for one to perform the equilibrium measurements to produce a value for K,. However, values of K, for simple aliphatic alcohols are increased by electron-releasing substituents and decreased by electron-withdrawing substituent~,~ l1 and it isconcluded that the electron-withdrawing phenyl group12 reduces K , for 1-phenylethanol, as with benzyl alcoh01,~ to a value which is negligible compared with propan-2-01. On the other hand, K, for phenol is high enough to meas~re,~ owing to the electron-releasing effect of the 7c electrons of the phenyl group operating in opposition to the electron-withdrawing inductive effect : this electron-releasing effect cannot operate with 1 -phenylethanol and benzyl alcohol, and hence the rate of oxidation of both by the non-hydrolysed [Ni(bi~y)~]~+ is independent of the acidity.Solubility measurements on benzyl alcohol in low concentrations of mineral acid have been compared13 with solubility measurements at very high concentrations of mineral acid and with the direct determination of pK at very high concentrations of mineral acid. However, the above discussion shows that the solvent-sorting with benzyl alcohol and water in the neighbourhood of the proton cannot be involved in the low concentrations of mineral acid used, as was suggested,13 and the solubility in this region must be explained by other effects.The same is true for benzoic acid, also used in the solubility meas~rements,~~ as the inductive effect of the phenyl group will ensure that its K , value is considerably less than K , z 0.1 dm3 mol-1 found for2934 OXIDATION OF D L - 1 -PHENYLETHANOL acetic acid:9 as explained above, the higher Kc (phenol) = K, (ethanol) arises from opposing electron-withdrawing and -releasing effects in phenol. Differences between these solvent-sorting equilibria at low concentrations of mineral acid and the direct pK determinations in very high concentrations of mineral acid arising from the difference in the two types of media have been discussed In their more general discussion of K, values these authors13 ignore the good agreement obtainedg3 lo between values for K, from spectrophotometric and kinetic determinations for acetone and methyl acetate as well as for ethyl acetate and the good agreement found between K, values for acetone obtained from a variety of techniques.1° Their assertion13 that the medium effects on K, and the pK ofp-nitroaniline in high alcohol concentrations, expected from spectral changes for p-nitroaniline in various solvents,l* have not been observed is not supported by the results of our extensive investigationsg* l1 of such effects in low concentrations of mineral acid. It is perhaps not surprising if non-oxygen containing molecules13 also participate in solvent exchanges in the solvation shells of ions.In table 3 the values of AH* and AS* are compared for a range of substrates with [Ni(bipy)J3+. The values for 1 -phenylethanol are in close agreement with those for benzyl alcohol.The oxidation of hydrogen peroxide and these two alcohols is very TABLE 3.-cOMPARISON OF ENTHALPIES AND ENTROPIES OF ACTIVATION FOR THE OXIDATION OF SUBSTRATES BY [Ni(bipy),13+ IN AQUEOUS PERCHLORATE MEDIA AH* AS* substrate /kJ mol-I /J K-’ rno1-I H202 3 8 f 2 - 126+7 Br- 60 f 4 - 3 f l l N, 36+3 -3+10 CH,CH(C,H,)OH 77+2 - 2.4+ 5.7 C,H,CH20H 77k 1 2.0 1.3 similar in that each produces a radical and a proton as well as Nil1 in the rate-determining step. As AS* is closely connected with configurational changes, AS,* (the contribution to the entropy of activation resulting from the release of solvated water molecules in the change of charge on the Ni species on going from the initial state to the transition state) should be about the same for all three substrates (as it is for Br- and N;, where similar charge changes occur), and AS: (arising from the restrictions of water molecules in the solvation of the released proton) would also be expected to be about the same, making the total value of AS* the same for all three.However, AS* for hydrogen peroxide is much more negative than the values for the two alcohols. Both these alcohols possess phenyl groups, and the electron transfer which produces the charge change in Ni may come from the 71 system of these phenyl groups, the proton being released to the solvent in a later step in contrast to the simultaneous electron and proton transfer with H,O,. The positive charge produced in the phenyl group in the rate-determining step with the two alcohols will be diffuse and make a much smaller negative contribution to AS* through restriction of water molecules than AS:, resulting from the release of a proton to solvent with H,O,.Hence AS* values in table 3 for the two alcohols are closer to those for Br- and N;, which have no proton to lose, than to that of H,O,, which has to lose a proton to the solvent in the oxidative step.D. FOX AND C . F. WELLS 2935 C. F. Wells and G. Davies, Trans. Faraday SOC., 1967, 63, 2737; C. F. Wells, C. Barnes and G. Davies, Trans. Faraday SOC., 1968, 64, 3069; C. F. Wells and C. Barnes, J. Chem. SOC. A , 1968, 1626; C. F. Wells and M. Husain, Trans. Faraday Soc., 1970, 66, 679, 2855; C.F. Wells and C. Barnes, J. Chem. SOC. A , 1971, 430; R. Varadarajan and C. F. Wells, J. Chem. Soc., Faraday Trans. 1, 1973,69, 521 ; C. F. Wells and A. F. M. Nazer, J. Chem. SOC., Faraday Trans. I , 1976, 72, 910; C. F. Wells and D. Fox, J. Inorg. Nucl. Chem., 1976, 38, 287. C. F. Wells and D. Fox, J. Chem. Soc., Dalton Trans., 1977, 1498. C. F. Wells and D. Fox, J . Chem. SOC., Dalton Trans., 1977, 1502. J . K. Brown, D. Fox, M. P. Heyward and C. F. Wells, J. Chem. Soc., Dalton Trans., 1979, 735. D. Fox and C. F. Wells, J . Chem. SOC., Faraday Trans. I , 1982, 78, 1525. K. K. Banerji and P. Nath, Bull. Chem. Soc. Jpn, 1969, 42, 2038. C. F. Wells, Tetrahedron, 1966, 22, 2685. * C. F. Wells, Discuss. Faraday Soc., 1960, 29, 219, 255; Trans. Faraday Soc., 1961, 57, 1703, 1719; J. Chem. Soc., 1962, 3100. C. F. Wells, Trans. Faraday Soc., 1965, 61, 2194; 1966, 62, 2815; 1967, 63, 147; Hydrogen-bonded Solvent Systems, ed. A. K . Covington and P. Jones (Taylor and Francis, London, 1968), pp. 323-334; J. Phys. Chem., 1973, 77, 1994, 1997. C. F. Wells, J. Chem. SOC., Faraday Trans. I , 1973,69, 984; 1974,70, 694; 1975,71, 1868; 1976,72, 601 ; 1978,74,636,1569; 1981,77,1515; Adv. Chem. Ser., 1979,177,53; G. S . Groves and C. F. Wells, unpublished results. lo C. F. Wells, J. Chem. Soc., Faraday Trans. I , 1972, 68, 993. l2 K. B. Wiberg, Physical Organic Chemistry (Wiley, New York, 1964), p. 415. l3 F. Joo and M. T. Beck, Acta Chim. Acad. Sci. Hung., 1979, 100, 421. l4 A. I. Kiss and G. Horvath, Acta Chim. Acad. Sci. Hung., 1964, 42, 15. (PAPER 1 / 1868)

ISSN:0300-9599    

DOI:10.1039/F19827802929

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J . Chem. SOC., Furuduy Trans. I, 1982, 78, 2937-2945 Kinetics of the Electrochemical Evolution of Isotopically Enriched Gases Part 1.-180160 Evolution on Platinum in Acid and Alkaline Solution BY CHRISTINE R. CHURCHILL AND D. BRYNN HIBBERT* Department of Chemistry, Bedford College, Regent's Park, London NW 1 4NS Received 1 1 th December, 198 1 A method is described which follows, by mass spectrometry, the kinetics of la0160 evolution from an electrode surface enriched with lSO. The kinetics on a platinum surface in acid and alkaline electrolyte were consistent with a mechanism of oxygen evolution which involves successive oxidations on a single platinum atom. The total number of platinum sites responsible for oxygen evolution was less than the total number of atoms at the surface.The electrochemical evolution of oxygen from metals occurs on an oxide-covered surface,l but the exact nature and participation of this surface in the reaction has yet to be fully interpreted. Rosenthal and Veselovski9 first demonstrated the existence of surface oxides during oxygen evolution by means of .an l80 tracer. They showed that whilst surface oxygen compounds formed between 0.8 and 1.2 V (us. SHE) do not take part in the process, higher oxides formed at potentials > 1.4 V do participate. Several authors have postulated schemes involving reactions of higher oxides but no direct evidence has been forthcoming. Tseung and JasemlO have correlated, with some success, the potentials of the metal/metal-oxide couple or lower-metal-oxide/higher-metal-oxide couple with the onset of oxygen evolution.Further work on NiCo,O," has confirmed that Ni3+ (possibly Ni4+) and Co4+ are formed at potentials at which oxygen is evolved. The importance of this approach to electrolytic devices is obvious; no matter how efficiently oxygen is evolved on a particular surface, if the highest metal oxide is not formed until a potential is reached which is substantially above E* for oxygen evolution (1.23 V at 25 "C), then a voltage loss is inevitable. In the case of platinum, Hoarel agreed that unless the platinum electrode is anodically polarised beyond the equilibrium potential of the PtO/PtO, couple, oxygen evolution cannot proceed. In an earlier work12 the use of I*O to observe the kinetics of evolution of oxygen on NiCo,O, was demonstrated. Here, using an improved experimental technique and an extended kinetic interpretation which allows for two intermediate oxides, information is obtained regarding the total number of sites, the extent of higher oxide formation and the general mechanism.THEORY FRACTION OF IN THE EVOLVED GAS A T THE SURFACE A simple mechanism for oxygen evolution on a metal in oxidation state, N , MN via higher oxidation states N + I, N + IT is MN+-OH -+ MN+IOH+e MNflOH+-OH -+ MN+110+e+H20 2M A'+ I 1 0 --+ 2MN+0, 29372938 1s0160 EVOLUTION ON PLATINUM for reactions in alkaline media, and MN + H,O -+MN+IOH+H++e (1 b) MN+IOH+H20 -+ MN+IIO+H,O++e (2 b) 2MN+110 -+2MN+0, (3 b) for reactions in acid media. The initial oxidation state of the metal ( N ) may refer to a bare metal surface ( N = 0), or to an oxide covered surface ( N > 0).For example, on platinum PtO, (i.e. N = 4) is the stable oxide on which oxygen is evolved. If the reactions are carried out in electrolyte containing a fractionf, of l80, in the steady state a fractionf-, of MlsOH and MlsO will be found at the surface. Evolution of oxygen on such an enriched electrode in an electrolyte having a normal ls0 content leads to a progressive depletion of la0 from the surface and the appearance of ls0l6O and 1s0180 in the gas phase by reactions such as MlsOH+-OH -+ M180+H20+e (4) MlsO + ML60 -+ 2M + la0160. ( 5 ) 2M180 + 2M + lsO1sO (5’) may be neglected. In addition, if the overpotential for oxygen evolution is high, the back reactions of (4) and (5) may be neglected.Thus the reaction scheme comprising reactions (4) and ( 5 ) is one of consecutive first-order processes giving rise to 180160 in the gas phase. Let a and b be the initial enrichment of l80 in the surface oxide layer as M180H and M180, respectively, and let x, y and z be the enrichment of ls0 at time t as M180H, M180 and l80l6O, respectively. The kinetic equations are thus If the surface enrichment is small, reactions such as dx/dt = - k , ~ dy/dt = k,x- k,y dz/dt = k, y where k, and k, are first-order rate constants for reactions (4) and (5). In this derivation no account is taken of kinetic isotope effects and single values of k, and k, are assumed. The solution for y is y = exp (-k5t) [k,a/(k,-k,)I ([exP ((k5-k4)t)-lI+b} ( 6 ) for k4 # k5 and y = (kat+b) exp (-kt) (7) for k, = k, = k. The fraction of l80 in the evolved oxygen (fi8) is the ratio of the rate of evolution of l80 as l80l60 and the total rate of evolution of oxygen atoms.Thus (8) fis = (d~/dt)/(Li/2F) = k5y/(Li/2F) where i is the galvanostatic current, F is the Faraday constant and L is Avogadro’s number. The rate constants k, and k, are dependent on the current flowing and the total number of sites at the electrode surface. The rate of reaction (4) for all oxygen atoms must be iL/2F atom s-l and therefore the rate of reaction of M180H is (iLI2F) (fMoH), where fMoa is the fraction of MOH which is MlsOH. In terms of MlaOH (x) and the total MOH (x,), the rate is (iL/2F) (x/x,). Thus dx/dt = - iLx/(2FxT). (9)C. R. CHURCHILL AND D. B. HIBBERT 2939 Comparison with the treatment above shows that k , = iL/(2F+).If yT is the total MO, then by similar reasoning k , = iL/(2FyT). Substituting values of k , into eqn (8) gives where y is given by eqn (6) or (7). (10) f 1 8 = Y / y , FRACTION OF "0 IN THE EVOLVED GAS AT THE MASS SPECTROMETER In an experiment the fraction of lS0 in the evolved gas cannot be measured at the electrode surface but some distance away where it is diluted by a purging gas. It is possible to allow for this in the kinetic equations and obtain an expression for the experimental rn/e 34 signal. Let the evolved oxygen and a purge gas expand into a volume which may accommodate n moles of an ideal gas at a total rate v mol s-l. The rate at which all l80 is introduced into the volume is k,y [eqn ( 5 ) ] plus a background component vb.The natural abundance of l80 is 0.002, so vb = ( i L / 2 F ) x 0.002 atom s-l. The rate at which l80 is removed from the volume is the rate at which all gas flows through (v) multiplied by the fraction of l80 in the volume, which is n18/n, if 4 8 is the total in the volume. Therefore dn,,/dt = k , y + vb - (n,,/n) (1 1) where u/n (= k,) is a first-order rate constant which determines the rate at which the mass spectrometer 'sees' changes in l80 at the electrode surface. Eqn (11) may be solved using the expression for y , eqn (6), IS = (Vb/kn) [1 -exP ( - k n t)I 4- [ak4k5/(k5-k4)1 [exP (-k4 t)-eXP ( - k n t)I/(kn-k4) (12) +[kti/(kn-ktJI ( b - [ak,/(k,-k,)l} [exp (-k,t)-exP ( - k , t ) l for k , # k, and n,, = (vb/kn)[ 1 -exp (- k , t)] + [ k / ( k , - k)] {akt - [ka/(kn - k)] + b} exp (- k t ) + [k/(kn - k)] [ka/(kn - k ) - b] exp ( - k , t ) (1 2') for k, = k , = k .The mass spectrometer signal of m/e 34 ( S ) is proportional to the fraction of l80 in the volume at constant pressure = K(n18/n> (1 3) where K is the constant of proportionality. In a background experiment in which there is no enrichment (a and b both zero) (14) Data from enriched and background experiments will thus allow calculation of the unknowns in eqn (12)-(14). S = ( K / n ) ( ~ , / k n ) [1 - ~ X P (-knt)l* EXPERIMENTAL MATERIALS Platinum black electrodes on 80 gauge platinum mesh screens were prepared by electrolysis of a solution which contained 5 g PtC1, in 0.1 dm3 water.13 The electrodes were formed into a cylinder to provide the maximum area in a given volume of cell.The surface areas of the platinum electrodes were measured by hydrogen stripping14 and oxygen charging curves.I6 All reagents were of AnalaR grade. The acid electrolyte was 1 mol dmV3 sulphuric acid and the alkaline electrolyte was 5 mol dmF3 potassium hydroxide, made up in doubly distilled, deionised water. Enrichment to ca. 3% l80 in alkaline electrolyte and 4% l80 in acid electrolyte was accomplished by the addition of suitable amounts of 20% l80 H,O (B.D.H.) to a stock solution2940 l8ol60 EVOLUTION ON PLATINUM of appropriate concentration of either the acid or the alkali. The isotopic content of the electrolyte was determined by evolving oxygen in the electrolyte and measuring the ratio of m/e 32 and 34 at the steady state.APPARATUS The enrichment of a surface with lSO was performed in a thermostatted (20f 1 "C) two-compartment cell, continuously purged with nitrogen. The cell used to monitor the sub- sequent evolution of l80 in normal electrolyte is shown in fig. 1. The aim of the experiment is to detect l80 near to the electrode with as little dead space as possible. The probe to the mass spectrometer (P) was placed in the path of the outgoing gases just above the surface of the FIG. 1 .-The electrochemical cell. For description see text. electrolyte. Calcium chloride in the probe removed water vapour and also cut down the volume of the probe. Nitrogen was introduced into the working electrode compartment at (I). The arrangement shown allows the majority of the gases in the cell to pass out through an air-lock (11); only a small fraction was diverted into the mass spectrometer via a leak valve (L).The counter-electrode was a high-surface-area platinum electrode (C) and the potential of the working electrode (W) was measured against a dynamic hydrogen electrode (DHE) reference electrode (R) via a Luggin capillary. The DHE itself was standardised against a bubbling hydrogen electrode. No account was taken of the ohmic drop in the electrolyte. METHOD The platinum electrode was first reduced by evolving hydrogen on it at 0.5 A. The current was reversed and oxygen was then evolved in enriched electrolyte at constant current (usually 0.5 A) for periods of time between 15 min and 6 h. The electrode was removed, washed in distilled water for periods up to 1 h and introduced into the cell of fig.1 at open-circuit voltage. Nitrogen purging was maintained at a constant rate (ca. 5 times the rate of oxygen evolution) throughout the experiment. When the m/e 34 peak had fallen to an acceptably low value (< 1 % of the normal background), showing that the small amount of air introduced with the electrodeC. R. CHURCHILL A N D D. B. HIBBERT 294 1 had been purged away, a constant current (0.25-0.75 A) was passed through the cell by a Weir constant current supply or a Thompson Associates potentiostat operating in a galvanostatic mode. The quadrupole mass spectrometer (Vacuum Generators Anavac 2) operating at to mbar continuously monitored m/e 34 which was displayed on a Y-t recorder.This procedure will be referred to as an enriched run. The peak reached a steady state corresponding to the normal background value in < 10 min. The current was switched off and when the peak height had fallen back to its zero value the current was switched on again and m/e 34 monitored with time. From this second run the background 1s0160 evolution was determined. It is possible to ignore the small amount of l80 which can be introduced into the electrolyte during the enriched experiment. This can be no more than of the background and would be expected to be some orders of magnitude less. During the experiment the potential of the working electrode was continually monitored. The accuracy of the method was tested by a series of blank experiments in which two background runs were performed consecutively, and others in which the electrode was simply soaked in enriched electrolyte before being washed etc.in the manner described above. A computer program written in FORTRAN v fitted background data, taken at 3 s intervals, to eqn (14) by a least-squares procedure. Using the values of k , and K / n so obtained, plus a calculated value of ub, the data of an enriched run were fitted to eqn (12) and (1 3). Thus a, b, xT and y , were determined. In addition, an experiment was performed in which the potential of the electrode was measured during oxygen evolution in the cell of fig. 1 and at open circuit for some time after the current was switched off. Potential measurements were also made for an electrode after it had been washed in the manner described above, following oxygen evolution in enriched electrolyte. RESULTS SURFACE AREA OF THE ELECTRODE The surface area of the platinum electrode determined by hydrogen stripping and by oxygen charging curves agreed within 10%.A typical surface area of a 1 cm x 1 cm mesh determined by these methods was 2000 cm2. l80 ENRICHMENT A typical fitted background run is given in fig. 2. k , was of the order of 0.0142 s-l for an evolution current of 0.25 A. The values obtained for k , in a series of experiments were consistent with the estimated volume of the dead space (n) and the flow rate of the gases (u). A typical enrichment curve (enriched run minus background run) for evolution in alkali is given in fig. 3. The results obtained from acid and alkali solutions were of the same orders of magnitude.For the same 5.0 m2 platinum electrode, values of the constants in eqn (6) were: in alkali, xT = (1.09 k 0.12) x 1019, yT = (1.02k0.13) x lOl9, a = (1.48k0.46) x 10l6 and b = (1.42+ 1.26) x The errors in these values represent the standard errors of six experiments. For experiments in acid the constants were xT = (0.99k0.08) x 1019, yT = (0.80+0.03) x 1019, a = (1.48 kO.19) x 10l6 and b = 6.02 x 10l2. The number of platinum atoms on this electrode from oxygen charging experiments was 5.76 x 1019. All blank experiments showed that no enrichment of lSO could be imparted to an electrode by soaking in enriched electrolyte alone. VOLTAGE MEASUREMENTS Fig. 4 shows the potential of an electrode during oxygen evolution and at open circuit in the cell for up to 90 min.The continuous line shows the effect on the open circuit potential of removing the electrode from the cell and washing it for 90 s before re-introduction into the cell.2942 l8ol60 EVOLUTION O N PLATINUM ’ j{ , , , , , , 20 40 60 80 100 120 140 160 time/s FIG. 2.-Mass spectrometer signal (circles) for m/e 34 after the start of oxygen evolution on the Pt electrode in normal alkaline electrolyte. The continuous line is a fit to eqn (14). DISCUSSION TOTAL NUMBER OF SITES Two facts emerge from these experiments concerning the number of sites on the platinum surface responsible for evolution of oxygen in alkali. First, the numbers of sites generating the higher oxides MOH and MO [x, and y , of eqn (9) and (lo)] are the same.This is consistent with consecutive oxidation on a single platinum atom. Secondly, the values of xT and yT are ca. 1/5 of the value determined for the total number of sites by oxygen charging in alkaline solution. It may be impossible to form a further complete layer of oxygen on an already oxidised surface or reaction between certain high oxide pairs [reaction (3)] may be unfavourable on particular crystal planes. Arguments similar to the latter were proposed by Yeung for the very low surface oxygen coverage found during oxygen reduction on N ~ , ~ , S ~ , , , C O O , . ~ ~ RESIDUAL “0 I N THE SURFACE It was found that after the electrode was taken out of the enriched electrolyte, whatever the period elapsed (practically this was not less than 3 min), no MIsO remained in the surface, i.e.b = 0. ‘MO’ is considered to be an unstable oxide which is only formed at high anodic potentials and which decomposes by reaction (3). Its lifetime on removing the anodic potential would be short, in accordance with the experimental findings. In the absence of any mechanism which removes Ml80H, the initial MlsOH [a of eqn (6), (7) etc.] should be ca. 3% of xT in alkaline electrolyte and ca. 4% of xT in acid electrolyte. The values for a found in experiments with both acid and alkaline electrolyte were twenty times less than this figure. The loss of MlsOl b L 13 12 11 10 h 9 - 8 - - .d c e v d ; 7- ._ 6 - 5 - QJ --. 4 - C. R. CHURCHILL AND D. B. HIBBERT - - - - 3 - 2 - 1 - 0 2943 I,,,,,,,, 0 20 40 60 80 100 120 140 160 time/s FIG.3.-Difference between m / e 34 signal for oxygen evolution on the Pt electrode enriched with l80 and the background experiment (circles). The continuous line is a fit to eqn (1 2) and (1 3). Washing time was 90 s. The total elapsed time between evoltution in enriched and normal electrolyte was 3 min. may arise from chemical decomposition of MOH, exchange of I 8 0 with an underlying oxide layer or exchange of la0 with the electrolyte. It is also possible, as Laitinen and Enke have suggested, that evolution of oxygen, whilst being preceded by oxide formation, occurs via unstable adsorbed intermediates on the strongly bound oxide 1ayer.I' The potential of the electrode some minutes after cessation of evolution of oxygen and in a nitrogen purged solution (fig.4) is well above the ultimate steady-state potential. The chemical decomposition of the intermediate oxide MOH is thus relatively slow and would not cause the observed loss of M-180H. Exchange with an underlying oxide layer may be facile, but if the evolution of oxygen takes place on a metal surface, any oxide formed would have the isotopic content of the enriched solution. Exchange of oxygen with the normal electrolyte offers the most likely explanation of the apparent loss of l80. Reaction between the intermediate oxide MOH and adsorbed water by proton hopping M1*OH + M"OH, + M180H, + M160H (1 5 ) would diminish adsorbed M180H.2944 1s0160 EVOLUTION ON PLATINUM \ \ \ \ \ \ \ 11 ' * 3 t --- - - --- 1 . 2 1 2 1 0 60 120 180 240 jj 5LOO FIG.4.-Potential (us. DHE) of the platinum electrode in alkaline solution at 20 O C . I, Potential during steady-state evolution of oxygen at 0.25 A. 11, Open-circuit potential of electrode in cell purged with N, (dashed line). The continuous line shows the effect of removal from the cell for 90 s and washing in distilled water. timels THE NATURE OF THE OXIDES ON PLATINUM The potentials associated with the formation of oxides of platinum, measured against a hydrogen electrode in the same solution, arels PtO 0.98 V, PtO, 1.05 V and PtO, ca. 2.0 V. The stable form of the oxide in the presence of oxygen is PtO,. There is some doubt about the exact potential of the formation of PtO3,l8g l9 but it is known to decompose to PtO, and oxygen. We therefore assign the initial state of the platinum surface as PtO, with transition to PtO, as the mechanism of oxygen evolution.The fall in open-circuit potential with time shown in fig. 4 occurs as the intermediate oxide PtO, -OH decomposes back to PtO,. The mechanism of oxygen evolution is therefore PtrVO, + -OH -+ PtVO,OH + e (16) PtVO,OH + -OH + PtVIO,O + e + H,O (17) 2PtVI0,O + 2PtIV0, + 0,. (1 8) Another question concerning this mechanism is to what extent is the reactive oxygen atom of eqn (1 8) part of a recognisable species PtO,? A highly mobile adsorbed atom may not be correctly represented as PtO,. Further isotopic studies on the exchange between the oxygen atoms in PtO,-O may give the answer. HoareZ0 has shown that the formation of a Pt-0 'alloy' during strong anodisation results in a reversible oxygen potential of 1.229 V.He ascribes irreversible behaviour on platinum which is not saturated with oxygen to the action of a local cell at the platinum surface. It may be, therefore, that it is not correct to write the surface species as distinct oxides, although the dissolution of oxygen into the platinum lattice may occur concurrently with oxygen evolution. SENSITIVITY The use of l80 as a tracer for oxygen evolution studies reveals the exceptional sensitivity of the method. In this work an enrichment of ca. 10l6 atoms was determined. The lower limit for the enrichment (a) was found experimentally to beC. R. CHURCHILL AND D. B. HIBBERT 2945 ca. 10% of the normal abundance of ls0 in the total number of sites (xT). For xT = 1.1 x 1019 sites the limit would be 2.2 x 1015 atoms.(The sensitivity of the mass spectrometer eventually limits the total amount of l80). In terms of the rate at which l 8 0 is evolved, it is desirable for k, and k , to be < 0.2 s-l. For a current of 0.1 A, k, = 0.2 s-l gives xT = 1.5 x lOls atoms. The absolute sensitivity of the mass spectro- meter is not a limiting factor, and if the evolution of oxygen occurred from an electrolyte containing no l80, several orders of magnitude would be added to the sensitivity. C. C . is supported by an S.E.R.C. CASE award with British Gas. D.B.H. thanks the University of London Central Research Fund for an equipment grant. J. P. Hoare, The Electrochemistry of Oxygen (Wiley Interscience, New York, 1968). A. Damjanovic and B. Jovanovic, J. Electrochem. Soc., 1976, 123, 374. A. C. C. Tseung and S. Jasem, 151st Meeting of the Electrochemical Society, Philadelphia, 1977, Extended Abstracts no. 351. S. E. S. El Wakkad and S. H. Emara, J . Chem. Soc., 1952, 461. A. Damjanovic, A. T. Ward and M. O’Jea, J . Electrochem. Soc., 1974, 121, 1186. T. R. Hoar, Proc. R. Soc. London, Ser. A, 1933, 142, 628. A. K. N. Reddy, M. A. Genshaw and J. O’M. Bockris, J . Chem. Phys., 1968, 48, 671. N. I. Rosenthal and V. 1. Veselovsky, Dokl. Akad. Nauk SSSR, 1956, 111, 637. .1 A. Hickling, Trans. Faraday SOC., 1945, 41, 333. lo A. C. C. Tseung and S. Jasem, Electrochim. Acta, 1977, 22, 31. l 1 A. C. C. Tseung and S. Jasem, J. Electrochem.Soc., 1979, 126, 1353. l 2 D. B. Hibbert, J . Chem. Soc., Chem. Commun., 1980, 203. l 3 G. J. Hills and D. J. G. Ives, J. Chem. Soc., 1951, 305. lo A. C. C. Tseung, P. R. Vassie and B. S. Hobbs, Symp. Electrochem. Eng., ed. J. D. Thornton (Institute of Chemical Engineers, London, 1971), vol. 1, p. 123. l 5 M. Breiter, C. A. Knorr and W. Volkl, Z . Elektrochem., 1955, 59, 681. K. Y. Y. Yeung, Ph.D. Thesis (The City University, 1979). l7 H. A. Laitinen and C. G. Enke, J . Electrochem. Soc., 1960, 107, 773. l6 M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions (Pergamon Press, London, l9 C. Marie, C. R . Acad. Sci., 1907, 145, 117. *O J. P. Hoare, J . Electrochem. Soc., 1978, 125, 1768. 1 966). (PAPER 1 / 1924)

ISSN:0300-9599    

DOI:10.1039/F19827802937

出版商:RSC    

年代:1982

数据来源: RSC