ISSN:0301-5696    

DOI:10.1039/FS98015FX001

出版商:RSC    

年代:1980

数据来源: RSC

摘要:

Introductory Lecture BY COURTENAY S. G. PHILLIPS Merton College University of Oxford Oxford OX1 4JD Received 6th January 1981 This meeting is primarily concerned with the interface between physical chemistry and chromatography. Looked at from one side we shall be interested in the manner in which physicochemical techniques and ideas may be used to increase the develop- ment and our understanding of chromatographic methods. Thus the paper by Carley Moroney and Roberts shows how the nature of chromatographic supports can be revealed by appropriate application of photoelectron spectroscopy. The papers by Scott and by Scott and Simpson tell us a great deal about the complexity of liquid-solid surfaces but although these results are of great intrinsic physico- chemical interest the main thrust of their work has been the understanding of h.p.1.c.separations. Again the paper by Knox Kaliszan and Kennedy on enthalpic exclu- sion chromatography may be taken as a neat example of the use of physicochemical logic to extend the boundaries of chromatography as an analytical tool. On the other side and this is to be the main thesis of my own comments the chromatographic methods which have been primarily developed for separation and analysis can be used as simple and remarkably effective techniques for the investigation of a variety of problems of physicochemical interest. T have naturally enquired why I should have been asked to open this meeting. Perhaps it was because I provide a fragile link with the 1949 Discussion of the Faraday Society on Chromatographic Analysis at which I presented my second scientific paper.Perhaps it was because my tutor's tutor was a pupil of Faraday. Perhaps it was be- cause I did not have the courage as so many of you have had to submit a paper for this meeting. However it has been suggested to me that as I have been more fortu- nate than most in benefiting in my research from the physicochemical potential of chromatographic methods I might draw on this experience to illustrate the general field which will be more amply and forcibly brought out in the papers and discussion which are to follow. I can do this with a clearer conscience since the use of chromato- graphic methods to study physicochemical phenomena has been so clearly described and reviewed in the recent book by Conder and Young.' And if I remain open to the accusation of blowing too much my own penny whistle I am at least less likely to run the risk of missing or misrepresenting others.Perhaps I may also sound the antiphon to the remarks of my late lamented mathematical colleague Dodgson about jam to- morrow and jam yesterday but never jam today.* EQUILIBRIA We are today familiar with the use of say electrons neutrons and of a variety of photons in the presence or absence of magnetic and other fields as probes of molecular structure. But molecules themselves are also extremely delicate probes. Moreover these probes can lead directly to the most relevant information for real physico- INTRODUCTORY LECTURE chemical problems.Chromatography of course is often the most practical way of using these molecular probes for it provides us with a range of precise yet rapid methods for the measurement of molecular interactions. The existence of such systems as the Kovkts index illustrates the way in which we regularly use column stationary phases to " feel " the nature of sample molecules. Iremember for example that many years ago we were interested in the identification of new inorganic species such as the simple and mixed hydrides of germanium and silicon of which there turn out to be a very large number indeed only four of which had hitherto been ~haracterised.~ The patterns of retention times which closely followed those of the analogous hydrocarbons enabled us to determine the structures of all these new molecules and to confirm them by a number of auxiliary chromatographic techniques.These included chlorination and separation of the SiCl, GeCl and HCl peaks to determine the stoichiometry use of molecular sieves to distinguish the straight-chain isomers and the combination of a mass-density balance with a thermal- conductivity cell to measure their molecular weights. (In order however to put this into proper perspective it should be mentioned that when we had succeeded in iden- tifying all these new species chromatographically most of my inorganic colleagues were quick to point out that they knew all along that they must be there!) But all this is merely putting chromatography on something of the same level as a tool for molecular structure as infrared spectroscopy was at the end of the last century.The real potential of chromatography in this area has yet to be properly tapped and it is therefore particularly valuable to have this bolder concept explored at this meeting in the most stimulating paper by Kiselev and Poshkus in which they demonstrate how surfaces may be used to reveal remarkable details of the structure of simple mole- cules. The reverse of this the investigation of surface structure is also very neatly illus- trated in the paper by von Rybinski Albrecht and Findenegg which specifically in- vestigates the interaction of adsorbed molecules with one another. In comparison they conclude that the study of the adsorption of single species is only of limited in- terest.However even here it must be noted that the rapidity of chromatographic methods (of which there are of course several e.g. elution on a plateau elution by critical point frontal displacement and thermal desorptionl) can often be of value. Thus Dr. K. F. Scott in my laboratory has recently developed methods for the rapid and automatic generation of adsorption isotherms using a standard chromatograph and a simple mini-computer. These methods are particularly useful to us as we often wish to probe a catalytic surface with a variety of molecules of different shape or chemical character (thus metal sites may be distinguished by hydrogen, exchangeable hydrogen by deuterium-exchange chr~matography,~ while one atom 4.g. Ni in Cu may be identified by Co adsorption).The probing of metals with atomic hydrogen is suggested to us in the paper to be presented by Clifford Gray Mason and Waddicor. I may perhaps give two rather different examples of the way in which such surface studies have assisted us in our investigations of heterogeneous catalysis. In the first we have found that very small changes in overall stoichiometry e.g.in Bi&fOO, can produce very profound changes in catalytic activity and selectivity.6 These result from substantial changes in surface composition which may be shown up for example by p.e.s. but also very dramatically by a complete change in the whole pattern of reten- tion times. Thus on passing from a Bi-rich to a Mo-rich surface of Bi,MoO the relative retention of benzene and hexane changes by a factor of 16.My second ex- ample is taken from work on the catalytic dehydration of alcohols in which the two alternative mechanisms of cis and trans elimination could be distinguished on a model basis by the fact that the transition-state species could differ by one CH group in their C. S. G. PHILLIPS interaction with the s~rface.~ The difference in the retention times of two hydro- carbons on this catalyst thus provides a quantitative estimate of the difference in the activation energies. The delicacy of chromatographic methods means that they can be of particular value in probing weak interactions. The determination of activity coefficients is admirably reviewed for us in the paper by Letcher. On the other hand the nature of solution itself still seems to be a matter of hot dispute which the papers of McCann Purnell and Wellington and of Tiley will provide an opportunity for supporters of regularity or of micropartitioning to show their colours.It is perhaps worth pointing out that molecular interactions can often be highlighted by removing them from solu- tion and thus from the competition of solvent molecules and exposing them on a sur-face. Thus olefins are retarded some 2 or 3 carbon atoms by solution of Ag ions but by more than 50 when AgN03 is adsorbed on a suitable support. Similarly we have found good quantitative evidence for olefin complexes of cadmium when CdF2 is deposited on a surface but not from cadmium-containing solutions.' It seems to me that the most exciting applications of chromatography and molecular probes may well be in the biological field.Thus some years ago we were able to demonstrate that desoxycholic acid in a gas-chromatographic column liquid illustrated some of the characteristics of the bile.9 The paper of Sebille Thuaud and Tillement now opens up for us a fascinating window on this new world of application. KINETICS Since the chromatogram depends on both thermodynamic and kinetic factors it can in turn be used to derive information about diffusion processes and reaction kine- tics as well as equilibria. Indeed chromatographic apparatus may often be con- venient even if no chromatography occurs as is elegantly demonstrated in the paper by Wakeham. The most common method makes use of the plate height.We have at this meet- ing examples of this in the measurement of the diffusion of macromolecules in solu- tion in the paper by Dawkins and Yeadon and in the gaseous diffusion of atomic hydrogen in the paper by Clifford Gray Mason and Waddicor in which the chroma- tography may perhaps be said to have occurred as something of an afterthought. When two competing mass-transfer processes with rather different time-scales can be distinguished the simple van Deemter approach is no longer sufficient and the chromatographic peaks begin to lose their symmetry. The theory of this was worked out by Giddings" in 1963 who also showed that the relative position of the peak maximum would be a function of flow rate for at slow flow rates molecules would spend a higher proportion of their time on the more slowly exchanging sites.We have recently" observed what appears to be a nice example of this phenomenon on a carbonaceous surface laid down on an oxide support as a result of the disproportiona- tion reaction of butadiene. Thus at 300 "Cthis gives normal symmetric peaks for both n-hexane and a branched isomer such as 2,2-dimethylbutane. However at 200 "C while the n-hexane remains normal the 2,2-dimethylbutane peak becomes ab- normally broad asymmetric and with its peak maximum relative to n-hexane sensitive to flow rate. The results fit quite well to a two-site adsorption model in which de- sorption from the slower sites has a half-life of ca. 10 s and extremely well if a few molecules are allowed to explore a third type of site with a longer half-life.The diffi- culty seems to arise when one tries to find a model for these sites which provides slow adsorption for a branched but not for a straight-chain hydrocarbon and the best we have to suggest so far is that there may be an activated rearrangement of the surface INTRODUCTORY LECTURE to better accommodate the more branched molecule. In the presence of even slower adsorption-desorption processes the tail of the peak may become so long and so attenu-ated that it can easily be missed. In such a case the slow process can be shown up most easily by means of stopped-flow chromatography. For example if one takes a column of activated AIL03at say 200 "C and after injection of n-hexane stops this for about half an hour at some point in the column this allows a certain amount of the n-hexane to diffuse slowly into what we presume are fine pores or cavities of an ink-bottle shape with an entry port requiring activation energy.When the gas flow is re- started the bulk of the n-hexane is removed but the desorption of the remainder may be followed in rather minute detail and over some considerable time by periodically stopping the flow allowing some to diffuse out and measuring this as a sharp peak with the detector at a suitable higher sensitivity." The study of the kinetics of chemical reactions may often be considerably assisted by judicious linking of the reactor and the analytical chromatographic column as in the microreactor or sample-vacancy technique^.'^ There are also many occasions when the chromatographic column may itself be used as the reactor.We have an example at this meeting in the paper by Clifford Gray Mason and Waddicor. Infor-mation on the chemical reaction occurring within the chromatographic column can be obtained simply from the loss of reactant or as in diffusion from measurements of peak spreading. However a further feature of the chromatographic process can use- fully be called into play namely the ability to move molecules at will and under precise control through the reactor-chromatographic bed. Reactants individual products and indeed even impurities may all thus be conveniently separated and distinguished from each other particularly by the use of stopped-flow chr~matography.~ Potential inhibitors or cocatalysts may be caused to meet with the reactant at predetermined times and positions in the column.A number of different reactants may be studied simultaneously but at different positions within the reactor. Unwanted isomers may be continuously brought into the most catalytically active part of the column so that a reaction may be driven way beyond its thermodynamic limit for example the al-most complete conversion of n-hexane into 2,2-dimethylbutane.14 In my group we have found that the chromatographic methods can provide a simple means for the rapid investigation of heterogeneous catalytic reactions so rapid in fact that it has become necessary to use on-line computers to keep upwith the flow of data.At the same time catalytic surfaces are being investigated by a variety of molecular probes as I have already discussed. However these probes are not limited to the thermodynamic and diffusional background of the reactions studied. The use of the reactions themselves either with different molecules in the same reaction or with different reactions can also prove to be extremely revealing. Thus by compar- ing the hydrogenolysis of 2,3-dimethylbutane and 2-methylbutane we have been able to distinguish between a surface and a desorption-resorption mechani~m,~~ while Dr. K. F. Scott has been able to use stopped-flow chromatography in systems where the reactant is virtually stationary to measure the distribution across the surface of reac-tion sites of different activity and the manner in which these may be selectively poisoned.16 CHROMATOGR A PHY AND PHY S I CAL CHEMISTRY I have this morning tried to communicate and the rest of this meeting will illumi- nate some of the many ways in which chromatography may be used to unravel prob- lems of real interest in physical chemistry.Indeed in every chromatogram Nature is revealing to us an astonishing wealth of information if only we had like the boy and C. S. G. PHILLIPS the emperor's clothes the eyes and the will to see. Why then one must ask are chromatographic methods so little used apart from their traditional and well-recog- nised function in chemical analysis ? In part this may be due to the complex and often bizarre nature of chromato- graphic stationary phases and operating procedures and from the multiplicity of physicochemical processes which seem to be involved in any chromatographic experi- ment.But as has now been shown by a number of different investigations there is in general good agreement between the physicochemical data obtained by chromato- graphy and by other more traditional methods.' In part this may be due to the curious belief that there must always be something of empiricism and black art in any analytical method. I hope very much that now that so much gas and more recently so much liquid has flowed through high-perfor- mance chromatographic columns for mainly analytical purposes the faith of the Faraday Division in once again casting its benevolent mantle upon us may do some- thing to give chromatography its physicochemical respectability.Finally and once more upon a personal note and as the shadows of life begin to lengthen and policemen and chromatographers get so remarkably young I would like to emphasise the power rapidity and above all the simplicity of the chromato- graphic methods. When I started research some 35 years ago there seemed to be plenty of things well worth investigating with relatively simple apparatus. Today my younger colleagues do not seem to find themselves so fortunately situated and so often are made to feel that nothing worth while can be done without elaborate and expensive equipment. They might be well advised to consider taking up some of the many opportunities which this meeting is about to discuss.J. R. Conder and C. L. Young Physicochemical Measurements by Gas Chromatography (John Wiley London 1979). [See also R. J. Laub and R. L. Pecsok Physicochemical Applications of Gas Chromatography (John Wiley New York 1978)l. L. Carroll Alice through the Looking Glass chap. 5. C. S. G. Phillips P. Powell J. A. Semlyen and P. L. Timms 2. Anal. Chem. 1963 197 202. K. F. Scott and C. S. G. Phillips J. Chem. SOC.,Faraday Trans. 1 1980 76 683. K. F. Scott and C. S. G. Phillips J. Catal. 1978 51 131. A. G. Mitchell P. M. Lyne K. F. Scott and C. S. G. Phillips J. Chem. Soc. Faraday Trans. I 1981 77 in press. 'R. M. Lane B. C. Lane and C. S. G. Phillips J. Catal. 1970 18 281. I. Hadzistelios F. Lawton and C. S. G. Phillips J. Chem. SOC.,Dalton Trans. 1973 2159. A. 0.S. Maczek and C. S. G. Phillips J. Chromatogr. 1967 29 7. lo J. C. Giddings Anal. Chem, 1963 35 1999. G. J. S. Vint and C. S. G. Phillips unpublished work. l2 C. S. G. Phillips M. J. Walker C. R. McIlwrick and P. A. Rosser J. Chromatogr. Sci. 1970 8 401. l3 C. S. G. Phillips and C. R. McIlwrick Anal. Chem. 1973 45 782. l4 C. M. A. Badger J. A. Harris K. F. Scott M. J. Walker and C. S. G. Phillips J. Chromatogr. 1976 126 11. N. D. Perkins and C. S. G. Phillips J. Catal. 1980 66 248. l6 K. F. Scott J. Chem. SOC.,Faraday Trans. 1 1980 76 2065.

ISSN:0301-5696    

DOI:10.1039/FS9801500007

出版商:RSC    

年代:1980

数据来源: RSC

摘要:

Chromatostructural Analysis (Chromatoscopy) A New Method of Determination of Molecular Structure BY ANDREJV. KISELEV Chemistry Department Lomonosov State University of Moscow and Institute of Physical Chemistry U.S.S.R. Academy of Sciences Moscow U.S.S.R. AND DIONISAS P. POSHKUS Institute of Chemistry and Chemical Technology Academy of Sciences of the Lithuanian S.S.R. Vilnius U.S.S.R. Received 1 lth July 1980 The intermolecular interaction of an isolated adsorbed molecule with a homogeneous flat solid surface and thence the thermodynamic characteristics of adsorption and adsorption chromatography at low (zero) surface coverage depend significantly on the molecular geometry of the adsorbate. Graphitized thermal carbon black (GTCB) is such a homogeneous solid and so may be used to deter- mine some parameters of molecular structure.Use is made of the gas-chromatographically obtained Henry’s constant for adsorption on GTCB with a semi-empirical molecular statistical theory of ad- sorption based on an atom-atom approximation for the potential function for intermolecular adsor- bate-adsorbent interaction. The method is shown to be successful in determining first structural parameters for molecules of hexamethylbenzene indane 2-and 5-methylindanes tetralin and secondly potential-function parameters for internal rotation in the molecules ethylbenzene and diphenyl and its methyl derivatives. By extension of the technique experimentally obtained Henry’s constants for adsorption on ion adsorbent-zeolite (NaX) have been used to determine the quadrupole moment of the cyclopropane molecule.The macroscopic property of a molecule most dependent upon its geometry is its adsorption on a homogeneous flat surface of an inert adsorbent since the molecule is subject only to the molecular field effect of the solid from one side in contrast to the situation in a gas a liquid or within a crystal lattice where effects from all sides are experienced. Hence adsorption on a flat homogeneous surface of a solid body can be an important source of information relating to the structure of adsorbed molecules. This fact has been largely overlooked first because of the inhomogeneity of the porous adsorbents so commonly used for adsorption and secondly because typically adsorp- tion has been measured in experiments starting from rather high surface coverages when intermolecular adsorbate-adsorbate interaction contributes considerably.In recent decades however technical developments have emerged which eliminate any difficulty. Graphitized thermal carbon black (GTCB) which is a dispersed non- porous solid body with a totally homogeneous surface [see reviews (1) and (2)] is now freely available while chromatographs with sensitive detectors can be used to study adsorption at very low surface coverages when adsorbate-adsorbate intermole- cular interaction is negligible. Hence in gas-adsorption chromatography the reten- tion volume for zero sample size from essentially non-adsorbed carrier gas is a mea- sure of the Henry’s constant K, a quantity demonstrably sensitive to the geometry of adsorbate molecules.In capillary columns filled with GTCB for example all the ten isomers of para-dibutylbenzene are easily fully separated. The isomer with the CHROMATOSTRUCTURAL ANALYSIS most branched substituents i.e. para-di-tert-butylbenzene is the first to leave the column and the isomer with most extended substituents i.e. para-di-n-butylbenzene leaves last.3 All five isomers of perhydroanthracene (fig. 1) are also separated as a consequence of the progressive flattening of the m~lecule.~ Ions (ionic crystals and specifically zeolites) and dipoles (hydroxylated surfaces of silica etc.) on the surface of the adsorbent make the Henry’s constant sensitive not only to the geometric structure of the adsorbed molecule but also to specific features of its electron structure such as rigid multipole moments ability for donor-acceptor interaction and proton transfer.Y 4 m 2,a5 Y J ~ -1 ;I L 2 4 6 81012 timelmin FIG.1.-Chromatogram4 of perhydroanthracene isomers on capillary packed column with GTCB for 513 K. Length of column 1.4m internal diameter 0.5 mm; particle size 0.1-0.12mm; carrier gas is hydrogen. 1 cis-syn-cis; 2 cis-anti-trans; 3 cis-anti-cis; 4,trans-anti-trans; 5 trans-syn-trans. Hence we can formulate two strategic appro ache^.^-^ (1) Given the structure of adsorbent and adsorbate we may determine the Henry’s Law constant and related thermodynamic quantities for adsorption at zero surface coverage using the molecular-statistical method.In the following diagram this corres- ponds to moving from left to right. (2) Using the same adsorbent and having measured Henry’s constant for adsorb- ate of an unknown or insufficiently known structure we may conversely determine parameters relating to the adsorbate molecule structure. In the diagram this cor- responds to moving from right to left. A. V. KISELEV AND D. P. POSHKUS structure of molecular-statistical Henry’s molecule and theory of adsorption for constant adsorbent zero surface coverage With a non-specific adsorbent like GTCB the method can thus be used to deter- mine certain geometric molecular parameters whilst with ion adsorbents or adsor- bents with polar groups on their surface the method can alternatively be used to determine some parameters relating to the electronic structure of the molecules.MOLECULAR-STATISTICAL CALCULATIONS OF HENRY’S CONSTANT FOR ADSORPTION OF HYDROCARBONS ON GTCB A semi-empirical molecular-statistical theory of adsorption at zero coverage of hydrocarbons on GTCB2*6-10 has been developed. It is based on molecular statistics and an atom-atom approximation for the potential function @ for intermolecular interaction of the adsorbate molecule to adsorbent. According to molecular statistics the relationship of Henry’s constant to potential energy Q depends on the structure of both the adsorbate molecule and the adsorbent. For adsorption of a three-dimensional (non-linear) quasi-rigid molecule on the basic face of graphite to a good approximation the following equation holds where Qoand Qy are values of @ and its second derivative at an equilibrium distance zo between the molecule’s centre of mass and the surface for fixed Eulerian angles 8 and ty which determine the molecular orientation relative to the mathematically homo- geneous surface.Adsorption on GTCB is undoubtedly non-specific as is indicated for example by the fact that the initial heat of adsorption of ethyl alcohol on GTCB is much lower than is its heat of condensation.2J1 Besides both Henry’s Law constants and the heats of adsorption on GTCB,at low surface coverages decrease in the adsorbate series ethane ethylene and acetylene,” i.e. the reduced number of hydrogen atoms in molecules of ethylene and acetylene compared with the molecule of ethane has greater influence than has the change in electron configuration of the carbon atoms from alkane to alkene and on to alkyne.Non-specific adsorption on GTCB and the linear dependence of heats of adsorp- tion of isolated flat molecules on their polarizability2 indicate an atomic or bond addi- tivity of the energy of intermolecular interaction of molecules with GTCB. Thus to determine the potential function Q one can use the atom-atom approximation @ = CA C VM. . . A (2) where q is the potential energy of intermolecular interaction of an atom of the mole- cule M with an atom of adsorbent A. Our published work shows that the form of atom-atom potential function adopted for the intermolecular interaction is of little consequence in the context of molecular- statistical calculations of although the selection of parameters for these Kl,2p8*12 functions is important.Theoretical and combinatorial expressions allow the deter- mination of q from the properties of adsorbent and adsorbate considered sepa- 16 CHROMATOSTRUCTURAL ANALYSIS rately,2,9,13-15 and values of Kl calculated from the relevant atom-atom potentials q differ but slightly from the corresponding experimentally obtained values. Compari-son of the calculated and experimentally obtained constants (Kl) for a few reference adsorbates then allows one to refine the parameters of the atom-atom potential functions In this way atom-atom potential functions were determined for p.299910 intermolecular interactions of the GTCB carbon atom with the hydrogen atom pH.. . C(GTCB) and the carbon atoms of a hydrocarbon molecule for two of their electron configurations pc(sp3). . . C(GTCB) and q+(sp2) . . . C(GTCB). The possibility of extrapolating the atom-atom potential functions obtained for reference molecules of a given class to other molecules of the same class was subsequently checked. The potential functions p obtained by the above procedure were finally used to calculate Henry’s constants which were in good agreement with the experimental values obtained for adsorption of hydrocarbons of different classes (alkanes cyclanes alkenes aromatic alkylaromatic and other hydrocarbons) on GTCB.2p9*10J”18 Molecular-statistical calculations of thermodynamic characteristics of adsorption for some molecules in other approximations have also been gi~en.’~-~~ DETERMINATION OF GEOMETRIC STRUCTURE PARAMETERS FOR QUASI- RIGID MOLECULES The inverse problem i.e.the determination of some geometric structure para- meters of hydrocarbon molecules using experimentally obtained Kl values for the adsorption on GTCB can now also be solved. To do this values of Kl are calculated as above for the molecule under consideration using a range of values of the unknown structural parameters and preference is then given to those which correspond best to the experimental Kl for different temperature^.^*^'-'^ By this procedure one can determine especially those structural parameters of the molecule upon which the constant Kl is strongly dependent.Specifically this technique can be used for deter- mining valence angles and potential barriers of internal rotation of large fragments of a molecule. Some results obtained by this chromatostructural (chromatoscopic) method are considered below. The detailed structure of tetralin and indane molecules is not well known. The tetralin molecule can be represented as a combination of the benzene and cyclohexene molecules in which case it can be assumed that the common bond has the length of the C ...C bond in benzene and that the C atoms of the cyclohexene ring which are the closest to this bond are located in the plane of the benzene ring. This structural model of the tetralin molecule however does not offer any guidance as to the valence angles of the cyclohexene ring and the extent of deviation of the remaining two C atoms of the cyclohexene ring from the plane of the benzene ring.The best agree- ment of theoretical with experimental values of Kl for tetralin adsorption on GTCB was obtained for the valence angles and deviations h given in table 1 (the two C atoms deviate in different directions from the benzene ring plane).17 Similarly to tetralin the indane molecule can be represented as a combination of benzene and cyclopentene molecules along the C ..C bond. We can also assume that the two carbon atoms of the cyclopentene ring which are the nearest to this bond are located in the plane of the benzene ring. This model however leaves uncertain the position of the fifth carbon atom (dihedral angle cc in fig.2) of the cyclopentene ring. Fig. 2 shows the depend- ence of the calculated value of Kl on this angle cc. Agreement between calculated and experimental values of Kl for indane adsorption on GTCB is obtainedI8 for a = A. V. KISELEV AND D. P. POSHKUS 19 & 3" and this value was used to solve the reverse problem i.e. to calculate Kl for 5-methylindane. The calculation shows good agreement with gas-chromatographic measurements (fig. 3).19 Fig. 4 illustrates results of a similar chromatostructural determination of a for TABLE l.-STRUCTURAL PARAMETERS OF TETRALIN MOLECULES (ATOMS 1 AND 2 FORM THE BOND COMMON TO BOTH RINGS) assumed bond lengths and obtained values of h and valence angles valence angles ~~ C=C in benzene ring 1.399 8 h 0.409 A cZ-c3 Cl-c6 1.504 A (ciC,c, czc1crj 122.5" c3-c4 cs-c 1.515 (c2c3c4,c,c6cs 111.0" c4-c5 1.550 A (C3C4C5 C6C5C4 109.5" C-H in cyclohexene ring 1.093 bi C-H in benzene ring 1.101 A (HCH for C3 and C 108" (HCH for C4 and C5 109.5" (CCC in benzene ring 120" 2-methylir1dane.~~ From the two possible structures for 2-methylindane we now have that one with an axial methyl group may be rejected at once since it yields values for Kl that are far too low to be acceptable.The structure with the equatorially located methyl group in contrast shows good agreement with the experimentally obtained Henry's constant for M. = 11 & 3". The experimental value of Henry's constant Kl for adsorption on GTCB was also used to determine the angle of deviation of the carbon atoms of the methyl groups from the benzene ring plane in hexamethylbenzene (fig.5).7*20The angle determined by this method (ca.10') is in agreement with that obtainedvia the electronographic method (9.9 & 2").21 Having successfully determined the structure of molecules of decalin and tetralin I 4t I I I I I I 5 10 15 20 25 30 FIG.2.-Full curve is for the calculated dependence of Kl for indane adsorption on GTCB (450 K on dihedral angle u. Horizontal dashed line shows the corresponding experimental CHROMATOSTRUCTURAL ANALYSIS w/ 01 I I 0.0020 0.0025 KIT FIG.3.-Dependence of In Kl on l/Tfor 5-methylindane on GTCB. Line is theoretical calculation; dots are experimental and of perhydr0phena:ithrene on the one hand and indane indene and hydroindane on the other we are now working on the important problem of applying chromato- scopy to the determination of the structures of isomers of the carbon skeleton of steroids.Clearly determination of the potentials of intermolecular interaction of oxygen atoms with the C atom of GTCB in different electron configurations would allow extension of the chromatostructural method to steroids proper for aglycone parts of glycosides prostaglandins and many other biologically active substances and their metabolites. The substances available normally contain components which are difficult to identify. Hence the initial problem is to know to which substance or at least set of isomers the Henry's Law constants taken from the chromatogram belong.In the preliminary stages therefore the chromato-mass-spectrometric method is necessary 16 c 5 10 15 20 25 30 ddeg FIG.4.-FulI curve is for the calculated dependence of Kl for 2-methylindane adsorption on GTCB on dihedral angle u for 450 K. Horizontal dashed line is for the corresponding experimental Revised data give u = 11 f3. A. V. KISELEV AND D. P. POSHKUS though it of course meets difficulties when substances have similar mass spectra e.g. isomers of polycyclic hydrocarbons. The distinct difference in the intensities of the peaks of basic fragments and in the spectra of metastable ions has been observed in our laboratory in the case of perhydroanthracene and perhydrophenanthrene isomers.Other methods have to be invoked. r 5 n .5 d W -4 5 10 15 Bideg FIG. 5.-Full curve is for the calculated dependence of Kl for hexamethylbenzene adsorption on GTCB for 500 K on the angle B of deviation of C atoms of methyl groups from the benzene ring plane. Horizontal dashed line is for the corresponding experimental value^.^*'^^*^ DETERMINATION OF THE STRUCTURE PARAMETERS FOR MOLECULES WITH INTERNAL ROTATION Many molecules exhibit internal rotation and in most cases this rotation is hindered. The structural quantities characterising the potential function of inner rotation W of such molecules are an equilibrium angle and the potential barrier or barriers to rotation. These quantities can in fact also be determined from experi- mentally measured values of Kl and their temperature dependence.6 For molecules with internal rotation the Henry’s Law constant is given by where a is the angle of internal rotation K,(a) being Henry’s constant calculated for fixed a.W may have one or more minima and maxima (potential barriers) but for ethyl- benzene for example there is a single barrier Wo. To compare the calculated and experimental Kl values the deviation S is used where Klexpand KIcalcare the experimental and calculated values of Henry’s constant for the column temperature Ti, n being the total number of these temperatures. 6 is CHROMATOSTRUCTURAL ANALYSIS thus the root-mean-square deviation of the experimental from the calculated Kl for different values of W,.Evidently the least 6 corresponds to the best agreement of calculated and experimental Kl values. Fig. 6 shows the dependence of 6 on W for ethylbenzene. The minimum of this curve corresponds to Wo= 1.7 kJ mol-1.20 Fig. 7 illustrates the results of a similar analysis for an isolated molecule of 2,6-dimethykliphenyl which has an equilibrium angle between the planes of the benzene 0.02 h E -5 3 -0.01 I I I 1 1 2 3 W,/kJ mol-' FIG.6.-Dependence of 6 for ethylbenzene on barrier to internal rotation Wo. rings aminand two barriers to internal rotation W, and W,,.*O The method yields amin= 68" Wol> 200 kJ mol-' and W, = 5 kJ mol-I. For diphenyl,6 determination of aminhas proved difficult since the calculated dependence of 6 on aminover a large range of amindoes not show a clear cut minimum.However the value amin= 42"has been obtained by the electronographic method,, and with this value the chromatostructural method then yields for diphenyl W, = 7 and W, z 0.1 kJ mol-I. The controversy aroused by the determination of these parameters by other methods is largely resolved by these results.6 The chromatostructural method described here yields the parameters of W for iso- lated molecules (in vacuo). Knowing these parameters we are clearly now in a posi- tion to calculate molecular conformations in different media particularly in the adsorbed state. CALCULATION OF HENRY'S CONSTANT FOR ADSORPTION IN ZEOLITES AND ESTIMATION OF THE ELECTRIC MOMENT OF A MOLECULE BY THE CHROMATOSTRUCTURAL METHOD The atom-atom (atom-ion) approximation can also be applied to calculate the potential energy of the molecule-zeolite intermolecular interacti~n.~.~~-~~ However in the case of adsorption of non-polar molecules in zeolite one has to take extra account of interaction of the induced dipoles of the admolecule and the electrostatic field of the zeolite.Parameters of the relevant potentials q are first estimated from the properties of the adsorbate and adsorbent taken separately. As in adsorption on GTCB the parameters of the functions 9 are corrected by comparing Henry's constant Kl (referred to 1 g of zeolite of the given composition) calculated via an approximate A. V. KISELEV AND D. P. POSHKUS function p with experimental Kl values for a reference molecule (of the given class of adsorbate in zeolite of the given composition).Corrected atom-ion potential functions for intermolecular interaction with ions I of zeolite NaX of H and C atoms of molecules of alkanes pH. . . I and q9c(sp~)I . . . I are then found using ethane as a reference molecule. Then these potentials are used to calculate Kl for other alkanes and cyclanes on the same zeolite NaX.24-26 Kl amin= 68" I I I J (b) 20: 100 60 70 80 90 umin/deg FIG.7.-Values of dminfor 2,6-dimethylphenyl for different fixed values of the equilibrium angle of benzene ring rotation umin(a) and the corresponding values of internal rotation barriers for flat W, (b),and for perpendicular WO2 (c) location of benzene rings.values calculated as above for methane propane n-butane n-pentane neopentane and for weakly strained cyclanes like cyclopentane and cyclohexane (fig. 8) agree with the corresponding experimental values to within experimental error. Significant disagreement of experimental and calculated values of K has been observed only for cyclopropane. This is obviously associated with the distribution of electron density in this highly strained molecule. When molecules with permanent multipoles adsorb in zeolite their electrostatic orientational interaction with the electrostatic field of zeolite must be taken into CHROMATOSTRUCTURAL ANALYSIS account. In the case of adsorption of ethylene and benzene with large quadrupole moments in NaX orientational electrostatic interactions are calculated with the point quadrupole appr~ximation,~"~~ from the equation (9Q= 2pi Q(3 COS' 8 -l)~-~ (5) I where Q is the.point quadrupole moment of the molecule 0 is the angle between the radius-vector connecting the centre of mass of the molecule with ions of zeolite I and 2 3 lo3K/T FIG.8.-Dependence of In K1on 1/Tfor adsorption of cyclohexane cyclopentane and cyclopropane on zeolite NaX.Lines are calculated results using atom-ion potential (~~(~~3) . . . dots arc experi-mental values. the vector determining orientation of the molecule in the large cavity of zeolite pI being the charge of ion I. Total potential energies of intermolecular interaction of unsaturated and aromatic hydrocarbons with zeolite are then calculated from the equation 28329 where (DA .. . is the potential function for intermolecular interaction of atom A of the molecule with zeolite estimated via the atom-ion approximation from the pro- perties of the adsorbate and zeolite taken separately. Kl calculated as above for ethylene agrees with e~perirnent.~**~ There are no experimental values of Kl for ben- zene but the calculated heat of adsorption of benzene by zeolite NaX is close to the experimental initial heat of adsorption with a zeolite of similar composition. Satisfactory agreement of the calculated and experimental values of Kl for adsorp- tion in zeolites of molecules of known structure obviously then allows extension of chromatostructural analysis to molecule-zeolite systems in general.As already noted in discussing fig. 8 calculation of Kl for cyclopropane using the atom-atom potential v for the sp3 electron configuration of carbon atoms in n-alkanes yielded too low re- sults. The considerable strain in the cyclopropane ring probably makes the electron configuration of the carbon atoms closer to the sp2 configuration a view supported to some extent by results for adsorption on GTCB.2*9 Hence the cyclopropane mole- A. V. KISELEV AND D. P. POSHKUS cule should have multipole moments. If these moments are approximated by a point quadrupole moment located in the centre of the cyclopropane ring then eqn (5) can be used for (DQ. One can in fact define a value for the quadrupole moment of the cyclopropane molecule Q such that molecular-statistical calculation yields Kl coinciding with the experimental value.The relevant value Q 4.10-26 esu slightly exceeds the quadrupole moment of ethylene. PROSPECTS FOR DEVELOPMENT OF CHROMATOSTRUCTURAL ANALYSIS Further development of chromatoscopy with GTCB but more particularly zeolites demands first that the accuracy of experimental Kl must be considerably enhanced. Then we will have access to more accurate parameters for the semi- empirical atom-atom potential functions needed for the molecular-statistical cal- culations of Henry’s constants. Such data will also reveal with greater clarity the influence of molecular structure on intermolecular interaction (e.g. for isomers like anthracene and phenanthrene naphthalene and azulene).Secondly a wider range of atom-atom potential functions should be determined via studies of adsorption on GTCB of not only hydrocarbons but also their derivatives containing halogens oxygen nitrogen sulphur and other elements in different valence states. More specific forms of intermolecular interaction such as the formation of hydro-gen bonds charge-transfer complexes and the like are usually only weakly revealed30 by gas chromatography because of the relatively high temperatures needed to elute large molecules. Liquid chromatography can be helpful in this ~ituation,~’ and the application of chromatoscopy to liquid chromatography thus presents great interest. There are however many problems in developing such a method.First the mole- cular-statistical theory of adsorption from infinitely dilute solutions is as yet insuffi- ciently developed. Secondly Henry’s constants obtained by liquid chromatographic methods are not sufficiently accurate. However the liquid chromatography method can already be applied to find rather simple quantitative correlations of the thermo- dynamic characteristics of adsorption from solution with the change of molecule structure.32 Such empirical correlations must help in the development at the mole- cular level of both semi-empirical calculations of Henry’s constant for liquid chroma- tography of complex molecules and solution of the inverse chromatostructural prob- lem i.e. finding parameters of molecule structure via experimental determination of Henry’s constant by liquid chromatography.N. N. Avgul and A. V. Kiselev Physical Adsorption of Gases and Vapours on Graphitized Carbon Blacks in Chemistry and Physics of Carbons ed. P. L. Walker (Marcel Dekker New York 1970) vol. 6 p. 1. N. N. Avgul A. V. Kiselev and D. P. Poshkus Adcorbtsiya gasov i parov nu odnorodnykh pouerkhnostyakh (Adsorption of Gases and Vapours on Homogeneous Surfaces) (Izadtelstvo Khimiya Moscow 1975). W. Engewald L. Wennrich and J. Porschmann Chromatographia 1978 11,434. A. V. Kiselev V. I. Nazarova and K. D. Shcherbakova Chromatogrphia 1981 14. A. V. Kiselev Chromatographia 1978 11 691. A. J. Grumadas D. P. Poshkus and A. V. Kiselev J. Chem. SOC.,Faraday Trans. 2 1979 75 ’A. 1398. V. Kiselev J. Technol.Biotechnol. 1979 29 673. * A. V. Kiselev and D. P. Poshkus Adv. Colloid Interface Sci. 1978 9 1. A. V. Kiselev and D. P. Poshkus J. Chem. SOC.,Faraday Trans. 2 1976 72 950. lo A. J. Grumadas A. V. Kiselev and D. P. Poshkus J. Chem. SOC.,Faraday Trans. I 1979 75 1281 1288. CHROMATOSTRUCTURAL ANALYSIS A. V. Kiselev Colloques internat. du CNRS Nu. 201 Thermochimie (CNRS Marseilles 1972) p. 487. l2 A. J. Grumadas and D. P. Poshkus Zh. Fiz. Khim. 1979 53 2405. l3 C. Vidal-Madjar L. Jacob and G. Guiochon Bull. SOC.Chim. Fr. 1971 3105. l4 C. Vidal-Madjar and G. Guiochon Bull. SOC.Chim. Fr. 1971 3110. C. Vidal-Madjar M.-F. Gonnord and G. Guiochon J. Colloid Interface Sci. 1975 52 102. l6 W. Engewald E. V. Kalashnikova A. V.Kiselev R. S. Petrova K. D. Shcherbakova and A. L. Shilov J. Chromatogr. 1978 152 453. l7 E. V. Kalashnikova A. V. Kiselev R. S. Petrova D. P. Poshkus and K. D. Shcherbakova Chromatographia 1979 12,799. l8 L. Dimitrov A. V. Kiselev and R. S. Petrova Chromatographia 1980 13. l9 E. V. Kalashnikova A. V. Kiselev K. D. Shcherbakova and S. D. Vasileva Chromatographia 1980 13 493. 2o D. P. Poshkus and A. J. Grumadas J. Chromatugr. 1980 191 169. R. R. Karl J. C. Wand and S. H. Bauer J. Mol. Struct. 1975 25 17. 22 A. Almenningen and 0.Bastiansen Det. Kgl Norske Videns. Selskabs. Skrijter 1958 No 4 3. 23 A. G. Bezus A. V. Kiselev A. A. Lopatkin and Pham Quang Du J. Chem. SOC.,Faraday Trans. 2 1979 75 367. 24 A. V. Kiselev and Pham Quang Du Dokl. Akad. Nauk SSSR 1978,238 284; 1978,241 386.25 A. V. Kiselev and Pham Quang Du Dokl. Akad. Nauk SSSR 1978,243 141. 26 A. V. Kiselev and Pham Quang Du J. Chem. SOC., Faraday Trans. 2 1981 77 1. 27 R. M. Barrer and R. M. Gibbons Trans. Faraday SOC.,1969 59 2875; 1965 61 948. 28 A. G. Bezus A. V. Kiselev and Pharn Quang Du Dokl. Akad. Nauk SSSR 1977,237 126. 29 A. V. Kiselev and Pham Quang Du J. Chem. SOC., Faraday Trans. 2 1981 '77 17. 30 A. V. Kiselev Chromatographia 1978 11 117. 31 V. Ya. Davydov A. V. Kiselev and Yu. M. Sapoznikov Chromatographia 1980 13. 32 A. N. Ageev A. V. Kiselev and Ya. I. Yashin Dokl. Akad. Nauk SSSR 1979,249 377; A. N. Ageev A. V. Kiselev and Ya. I. Yashin Chrornatographia 1980 13.

ISSN:0301-5696    

DOI:10.1039/FS9801500013

出版商:RSC    

年代:1980

数据来源: RSC

摘要:

Study of the Interaction between Adsorbed Hydrocarbon Molecules on Graphitized Carbon using the Chromatographic Step-and-pulse Method VON RYBINSKI ALBRECHT H. FINDENEGG BY WOLFGANG MARTIN AND GERHARD Institute of Physical Chemistry Ruhr TJniversity D4630 Bochum West Germany Received 17th July 1980 The chromatographic step-and-pulse method has been applied to a study of the adsorption of pure gases and gas mixtures on graphitized carbon black. It is shown how the retention time of a small pulse of a vapour B on a step of preadsorbed vapour A is related to the parameters of an equation of state (two-dimensional van der Waals equation) for the adsorbed gas mixture. The method is used to study the adsorption of isomeric C6-alkanes cyclohexane and benzene at 343.2 K.The adsorption isotherms derived for the pure vapours are compared with results obtained by static (gravimetric) measurements. The retention results of B on preadsorbed A are analysed and the best- fit values of the interaction parameter CAB are derived. These values are found to be smaller than the geometric mean of uAand uBfor the pure gases. Gas chromatography has been used essentially in two ways to study the physical adsorption of gases and vapours on solid surfaces (i) the retention time of very small samples of the vapour yields the initial slope (Henry’s-law constant) of the adsorption isotherm. In the case of adsorbents with homogeneous surfaces these studies have been used to investigate the gas-solid interaction potential of isolated adsorbed mole- cules.(ii) Chromatographic studies at higher concentrations of the vapour in the mobile phase can be used to determine adsorption isotherms up to higher surface coverages. The elution method frontal-analysis method and the step-and-pulse method have been applied for this purpose. In all these cases the pressure gradient along the column represents a major problem in the quantitative analysis of the ex- perimental data at higher concentrations of the vapour.2 The application of gas- chromatographic methods to adsorption studies of single gases therefore remains of limited interest since these measurements can now be made accurately by static met hods. When turning to the adsorption of gas mixtures a different situation arises. Static methods of studying the simultaneous adsorption of two components involve the measurement of two quantities viz.amount and composition of the adsorbate as a function of two independent variables pressure and composition of the gas phase. So far only a few studies of this kind have been sufficiently accurate for a test of theoretical models of mixed-gas ad~orption.~.~ A gas-chromatographic method to measure the infinite dilution activity coefficients of an adsorbed gas in the presence of a preadsorbed gas was used by Sloan and mull in^.^ However their method is valid only for negligible pressure gradient along the column. The experimental conditions under which such low pressure gradients can be achieved (short column large particle size low flow rate) lead to poor accuracy of the results.6 Recently Dondi et aL7 used the step-and-pulse method to determine single-gas adsorption isotherms of benzene CHROMATOGRAPHIC STUDY OF MIXED GAS-ADSORPTION and cyclohexane on graphitized carbon black considering in detail the finite pressure drop along the column.In the present work we extend this formalism to the adsorp- tion of binary gas mixtures. The theoretical basis of our analysis is outlined in the next section. It is shown how the retention time of a pulse of component B on a step of component A is related to the parameters of a two-dimensional equation of state for adsorbed gas mixtures. The method has been applied to binary mixtures of isomeric C,-alkanes including cyclohexane and benzene adsorbed on a low-surface-area graphitized carbon black.The adsorbates were chosen in order to study the influence of molecular shape on the pure-gas and mixed-gas adsorption parameters. THEORETICAL GENERAL RELATION FOR THE RETENTION IN GAS ADSORPTION CHROMATOGRAPHY The general theory of the step-and-pulse method has been derived by Valentin and Guiochon.6 Its application to the determination of pure-gas adsorption isotherms was outlined and discussed by Dondi et aL7 Here we summarize the application to two-component gas adsorption. In the present work the column containing the solid adsorbent is equilibrated with an adsorbable vapour A which is added to the inert carrier gas. When a constant mole fraction xAhas been established throughout the gas phase the partial pressure pAdecreases along the column in proportion to the total pressurep.As a consequence points along the column correspond to different points of the adsorption isotherm of pure component A. When a small sample of a second adsorbable vapour B is now injected two chromatographic signals will appear at the column outlet. One of these signals (retention time tA) is caused by the perturbation of the adsorption equilibrium of component A and is essentially the same as in the case of pure component A. The other signal (retention time tB) arises from the ad- sorption of component B in the presence of component A. It is convenient to intro- duce the retention quantities RA and RB where tM is the retention time of an inert gas measured at the same experimental con- ditions as tA and tg,pis the volume flow rate of the mobile phase at column outlet and column temperature T,msasis the area of the adsorbent in the column and j is related to the ratio of inlet pressure to outlet pressure PI=pi/po,by .3Pi-1 J=zpj51' The retention quantity R is related to the derivative of the adsorption isotherm of pure component A W. VON RYBINSKI M. ALBRECHT AND G. H. FINDENEGG RBis related to the initial slope of the partial isotherm of component B at given partial pressure (and hence given surface coverage) of component A (arB/ @B)pA,pB+() by The significance of the quantities drA/dpA and (arB/ apB)pB-Fo is illustrated in fig. 1. In eqn. (3) and (4) P is the reduced column pressure P =p/po. Hence the experi- mental values of RA(xA)and RB(xA) are related to average values of the corresponding PA PA FIG.1.-Schematic diagram of the adsorption of a binary gas mixture.isotherm derivatives for the range of vapour pressures pa = xAp,from column inlet to column outlet. It is not possible therefore to derive the shape of the adsorption isotherms from the experimental values of RA(x)and RB(x),but the parameters of any given isotherm equation can be determined by a least-squares fit of experimental R(xA)values for a range of mole fractions xA. For this purpose the right-hand sides of eqn (3) and (4) have to be inverted by expressing P as a function of the surface con- CHROMATOGRAPHIC STUDY OF MIXED GAS-ADSORPTION centration of the adsorbed components.Analytical isotherm equations of pure gases are usually given in the form pi =f(t?,) where Bi is the fraction of surface covered by component i = ei = ri/rm,iriN,,. (5) Inserting the isotherm equation pi I=f(Si) into eqn (3) leads to the relation7 The corresponding expression for RB(xA) refers to the situationp -+ 0; thus we have Taking P2and dP from this relation we find with Eqn (8) and (9) may be used to derive best-fit values for mixed-gas adsorption isotherms when the partial isotherms of the two components are given in the form pi =f,(Si €Ij). VAN DER WAALS EQUATION FOR MIXED-GAS ADSORPTION Several two-dimensional equations of state have been proposed in the literature to represent the physical adsorption of pure gases on homogeneous chemically inert surfaces.8 The most widely used of these is the two-dimensional van der Waals equation where a = 1/NT is the area per adsorbed molecule a and /3 are the two-dimensional van der Waals parameters.For mixtures the following combining rules for these parameters are usually adopted where xi and xi = 1 -xi are the mole fractions of the two components in the adsorbed layer and uAB is a parameter characteristic of the A-B interaction. By sub- stituting these expressions into the equation for phase equilibrium between the adsorbed layer and an ideal-gas phase the following relations are obtained for the partial isotherms of the two components9 W. VON RYBINSKI M. ALBRECHT AND G. H. FINDENEGG Here Kiis the Henry's law constant of component i and we have used the relation In the step-and-pulse method the concentration of the second component (B) is kept much smaller than the concentration of the first component (A).If KA and KB are of similar magnitude this condition implies that x 4x1 N 1 and & < 0,; thus the partial isotherm for component A reduces to the isotherm equation of pure A In the expression forp we substitute &/x," = OAPB/PAX~and neglect x& as compared with x~~AB; this leads to the partial isotherm of component B in the limit pB-f 0 Eqn (16) represents the van der Waals expression forf(0,) which can be inserted into eqn (6) to derive best-fit values of the parameters KAand aA/PAfrom a set of experi- mental values of RA(xA). Information about the mixed-gas adsorption parameters can be extracted from the retention quantity RB(xA).Eqn (16) and (17) yield the following expression for Here the Henry's-law constants KAand KB have been expressed by the retention quan- tities of the pure components in the analytical limit; viz. which follows from eqn (3) (5) and (16); similarly RO,B= rm,JKB;furthermore in the exponential of eqn (18) rm,B/rm,A has been substituted for PA/PB. Eqn (1 8)can be inserted into eqn (8) to derive best-fit parameters for the adsorption of gas mixtures from a set of experimental values of RB(xA). The numerical procedure is outlined in the next section. EXPERIMENTAL CHROMATOGRAPHIC MEASUREMENTS The gas-chromatographic apparatus with diffusion cell and saturation cell for the adjust- ment of the concentration of vapour A in the carrier gas has been described previously.1° A known weight (m,N 1 g) ofgraphitized carbon black (Sterling FT-G a = 11.1 m2g-l) was packed into the adsorption column (2 mm i.d.length 0.4 m). Adsorbed impurities were removed by flowing helium through the bed for 24 h at 400 "C. Helium (purity 99.996%) was used as the carrier gas. The hydrocarbons used as adsorbates had a purity of at least 99%. A representative chromatogram is shown in fig. 2. At first the pure carrier gas is displaced by a mixture of carrier gas and vapour A (mole fraction xA). When a steady signal has been attained at the column outlet (step height hA,s)the detector signal is compensated to zero CHROMATOGRAPHIC STUDY OF MIXED GAS-ADSORPTION and the amplifier is set to highest sensitivity.A small amount of vapour B is injected at time r = 0; two positive signals are then obtained (retention times tA and tB). For all experiments the pulse heights hA,pand hB,pwere < 5% of the step height hA,s. The residence time of a pulse of inert gas tM,was determined using methane. The mole fraction xAwas calculated from the saturation pressure of A pL( F), at the temperature of the saturation h t h =O rB I 1 I I r=O +t FIG.2.-Chromatogram for a step of cyclohexane (A) and a pulse of n-hexane (B). cell Tsc,using the vapour-pressure equation of the pure vapour," and from the total pressure in the saturation cell The pressures at column inlet and column outlet pi and p" were measured by mercury manometers; the ratio PIwas always between 1.4 and 2.0.The error in the experimental quantities causes a maximum relative error in the retention R of &3.5%. Results obtained with different columns (particle sizes 0.1250.1 50 and 0.200.30 mm re-spectively) agreed within 2%. Details of the experiments are given elsewhere.12 NUMERICAL PROCEDURE FOR PARAMETER FITTING The experiments yield the retention times tA and tB for given concentrations xA; RAand RBare obtained from the measured quantities by eqn (1) and (2),respectively. From a set of experimental RA(xA) data best-fit parameters of the isotherm equation of pure vapour A [eqn (16)] can be calculated; from these and a set of RB(xA) data the additional parameters of the partial isotherm equation of vapour B [eqn (17)] are then obtained (see below).As the initial (xA-+ 0) value of the retention RO,Aand RO,B,can be measured accurately (plateau values in fig. 3) the constants KA and KB in eqn (16) and (17) are expressed by r,,m,A/RO,A and rm,B/RO,B, respectively. Hence the isotherm equation for pure A [eqn (16)] contains the two adjustable parameters K~ and Tm,A= 1/NPA. Once these parameters for pure components A are known the equation for B in the mixture eqn (17) also contains only two adjustable parameters uiz. NAB and I'm,B= l/NbB. The fitting procedure for these two sets of para- meters proceeds in an analogous manner. Let us denote the adjustable parameters of a given isotherm equation by u and Tm. The best-fit values of cr I? are found by calculating the sum of the square deviations S(a r,) = V'X [R~(xA) -RC(XA)'JZ (20) for a range of values a and rmand drawing contour lines of S(u r,) in the two-parameter diagram of a as a function of rm.I3 The minimum of the deviation function S(u,I?,) corres- W.VON RYBINSKI M. ALBRECHT AND G. H. FINDENEGG ponds to the best-fit values of the two parameters; the mutual correlation between cc and I?,, and hence the physical significance of the resulting best-fit values can be judged from the shape of the S(a,r,,Jcontour diagram. In eqn (20) RE(xA)denotes an experimental retention value as obtained by eqn (1) or eqn (2); the corresponding value RC(xA)is calculated by eqn (6) or eqn (8) respectively with the chosen parameter values a and Tm.In these equations the integration limits 6; and 62 are derived from the given partial pressures pA = xApat column inlet and column outlet by solving the implicit isotherm eqn (16) by an iterative procedure; for the resulting values of 6A the relative error in pAwas made less than The integration of eqn (6) and (8) was then performed numerically again with a relative error d low5. Details of the computer program are given elsewhere.lZ RESULTS AND DISCUSSION We have made a study of the adsorption on graphitized carbon black of the five isomeric C,-alkanes (n-hexane 2-methylpentane 3-methylpentane 2,2-dimethyl- butane 2,3-dimethylbutane) cyclohexane and benzene. The pure vapours as well as binary mixtures of two vapours in the limit of the step-and-pulse method (pB-f 0) have been studied.In this paper some of the results are presented with the emphasis on a critical evaluation of the new method. All of the results refer to a common temperature of 343.2 K. Fig. 2 shows a chromatogram for a step of cyclohexane (A) and a pulse of n-hexane (B). The first peak can be attributed to the displacement of adsorbed A by the adsorption of B; so long as the basic condition of the step-and-pulse method is met uiz. hB,p S 0.05 h,,, the retention time fA is found to be independent of the nature of component B and to agree well with the retention time of a pulse of A on a step of A. The negative signal following the peak of A is a consequence of the mass balance of this component. The retention time tB and the ratio of the peak heights of the two components hA,p/hB,p,depends on the nature of B and also on the step height hA,s.Table 1 shows that for a given component A (cyclohexane) the ratio of pulse heights TABLE 1.-RATIO OF PEAK HEIGHTS hA,p/hB,p,AS A FUNCTION OF STEP HEIGHT (MOLE FRACTION XA)FOR PREADSORPTION OF CYCLOHEXANE (A) AND TWO COMPONENTS B xA/1o-3 n-hexane (B) XA110-3 2,3-dimethyl butane (B) -1.1 0.9 2.4 1.4 2.2 0.23 5.5 1.6 5.2 0.28 9.7 1.6 8.9 0.36 13.4 2.9 12.3 1.4 18.1 11 - h,/h increases with increasing mole fraction x,; hence the amount of A which is dis- placed by a given amount of B increases with increasing surface coverages. This effect is more pronounced for n-hexane than for 2,3-dimethylbutane (which is less strongly adsorbed than n-hexane but more strongly than cyclohexane; cf.fig. 3). Fig. 3 shows the dependence of the retention R of all hydrocarbons studied on the vapour phase concentration of n-hexane. For low xAthe retention R(x,) attains a constant plateau value R which corresponds to the retention of B on the homogeneous patches of the graphite basal plane in the analytical limit. R is a measure of the strength of interaction of adsorbed molecules with the solid. With increasing concen- CHROMATOGRAPHIC STUDY OF MIXED GAS-ADSORPTION tration xAthe retention RB(xA)increases and passes a maximum. This behaviour can be attributed to attractive lateral interactions between the test molecule B and surround- ing adsorbed molecules A.For the branched C6-isomers and cyclohexane this maxi- mum is found at higher bulk concentrations xA than for n-hexane itself (B = A). The retention of benzene on preadsorbed n-hexane does not exhibit such a maximum; -0.3- -3 I& e” E 0.2-* N * I E c-( s -e I E \ % 0.1 --lo-& I o-~ 10-2 XA FIG.3.-Retention R(xA) of small samples of the isomeric hexanes cyclohexane and benzene as a function of mole fraction xAof n-hexane in mobile phase. The adsorbent is Sterling FT-G graphit-ized carbon black column temperature 343.2 K PI = 2.0. this finding indicates that the lateral interaction of n-hexane with the aromatic mole- cules are weaker than with the aliphatic C6-isomers.We now turn to a quantitative analysis of the retention data in terms of the two- dimensional van der Waals (Hill-de Boer) equation as outlined earlier in this paper. Fig. 4 shows the experimental retention data R:(xA) of n-hexane on Sterling FT-G (corresponding to the data of the top curve in fig. 3) and the function R2(xA)calculated by eqn (6) with the pure-gas van der Waals functionf(OA) given by eqn (16) and ex- pressing KA by Tm,*and the experimental RO,A value [eqn (19)]. The curve exhibited in fig. 4 corresponds to the best-fit values of xA and rm,A as listed in the first line of table 2. The deviation function s(2&A/pA has a well-defined minimum with nearly-circular contour lines which means that the values of the two parameters are not strongly correlated and can be determined with some confidence.The results for pure cyclohexane can also be represented by the two-dimensional van der Waals equation but some of the branched isomers do not conform to this model equally well.12 W. VON RYBINSKI M. ALBRECHT AND G. H. FINDENEGG xA/10-2 FIG.4.-Experimental and calculated retention data of pure n-hexane adsorbed on Sterling FT-G graphitized carbon black; parameters listed in table 2. The chromatographic adsorption results for the pure components have been com- pared with static adsorption measurements on the same systems'* which were obtained by a gravimetric 171ethod.l~ Fig. 5 shows the results of the gravimetric study of the n-hexane/Sterling FT-G system. The two curves represent two-dimensional van der Waals isotherms with best-fit values of aA and rm,A as derived from the gravimetric data (a) and from the gas-chromatographic data (b).In the fitting procedure of the static measurements allowance has been made for a small positive increment of rA at pA= 0 to account for adsorption on high-energy sites of the graphitized carbon. Correspondingly in the fitting procedure of the gas-chromatographic measurements I 20 LI 0" .E G 10 1 .o 2.o 3.O rA/1Od6 rnol m-2 FIG.5.-Comparison of the gravimetric (a) and chromatographic (b)results of the n-hexane/Sterling FT-G system. Parameters of curve (a) RO,*= 0.258 x rnol m-' mbar-' rm,A = 4.09 x mol m-2 uA= 3.19 x J m2,TA(pA= 0) = 0.05 x mol m-'. Parameters of curve (b) see table 2.CHROMATOGRAPHIC STUDY OF MIXED GAS-ADSORPTION the value of Rohas been identified with the plateau value of R(xA)around xA = and the higher values of R near xA = lop5were attributed to the heterogeneity of the adsorbent. The initial slope of the isotherm obtained by the static and dynamic methods then agrees within 3%; the best-fit values of aAagree within 1%; but the monolayer capacity rm,A which follows from the static measurements exceeds the chromatographic value by ca. 8% (cf. fig. 5). The reason for this discrepancy in Tm between the static and dynamic results (which has also been found for other adsorp- tives) is not known. The experimental retention data R;(xA) obtained for a pulse of B on a step of A were analysed in terms of eqn (8) with the function g(eA) given by eqn (18).The 0.080 p -x I I I I 0.050' 2 .o 4 .O 6.O 8.0 xA,40-3 FIG.6.-Experimental retention data R;(xA)of cyclohexane (B) on preadsorbed n-hexane (A) and the corresponding functions R;(xA)for the three choices (a)-(c) of the parameters listed in table 2. parameters Ro,Aand RO,Bwere again taken from the experimental R(xA)at xA2 and the parameters aA and rm,A from the analysis of the single-gas gas-chromato- graphic results. The parameters KAB and rm,B could then be derived from the R;(xA) data as outlined in the experimental section. Results of such an analysis with n- hexane as the preadsorbed component A are summarized in table 2. In this table three different sets of the parameters NAB and rm,B are listed for each component B viz.(a> QAB =.\/a,aB; rm.B = r<B (b) ~AB best fit; rm,B = rmlB (c) NAB and rm,B best fit. Here aiand rm*,i denote the best-fit values derived from the gravimetric study of the pure gases i. Fig. 6 shows the experimental retention data R;(xA)for cyclohexane (B) on preadsorbed n-hexane (A) and the corresponding functions R;(xA)for the three choices (a)-(c) of the parameters listed in table 2. The simple mixing rule (a)yields systematic deviations in RZ at large concentrations x,; taking the pure gas parameter rtB and fitting aABalone [curve (b)]yields an aAB value slightly lower than the geo- metric mean of aAand as. If both parameters are fitted [curve (c)] the resulting Tm,B TABLE 2.-PARAMETERS OF EQN (18) FOR THE RETENTION OF HYDROCARBONS (COMPONENT B) ON STERLING FT-G WITH PREADSORBED n-HEXANE (COMPONENT A) AT 343.2 K.PARAMETERS RO,AAND RO,BFROM EXPERIMENTS AT LOW XA; PARAMETERS XA XB AND rm,A FROM ANALYSIS OF PURE GAS ADSORPTION; PARAMETERS CAB AND rm,B BY THREE DIFFERENT PROCEDURES (a)-(C)(SEE TEXT). component B &,A RO B XA RB rm,A rm,B ~AB mol m-2 rnbar-l J m2 mol m-2 10-39 Jm2 kAB method n-hexane 0.253 3.17 3.77 2-methylpentane 0.253 0.145 3.17 2.87 3.77 4.43 3.02 -4.43 2.47 0.18 -1.82 4.72 3-methylpentane 0.253 0.129 3.17 3.12 3.77 4.45 3.14 -4.45 3.03 0.04 -3.04 3.72 2,3-dimethylbutane 0.253 0.102 3.17 2.89 3.77 4.74 3.03 -4.74 2.83 0.07 -3.64 3.33 2,2-dimethylbutane 0.253 0.062 3.17 3.42 3.77 4.67 3.29 -4.67 3.OO 0.09 6.04 2.78 -cyclohexane 0.253 0.060 3.17 3.56 3.77 5.21 3.36 -5.21 3.31 0.01 Z 4.12 3.71 -benzene 0.253 0.175 3.17 1.09 3.77 6.50 1.86 -6.50 1.78 0.04 z -Q 6.92 1.78 Q CHROMATOGRAPHIC STUDY OF MIXED GAS-ADSORPTION is lower and aAB is larger than in (a).However the differences between the three curves are insignificant at most of the experimental concentrations xA. Fig. 7 shows the same sort of graph as fig. 6 for benzene (B) on n-hexane (A). In this case the three sets of parameters (a)-(c) are rather similar (table 2). For both systems the deviation function S(aAB rm,B) exhibits a flat minimum in the direction of the para- meter indicating that the variation of the monolayer capacity rm,B has little * 0.101 I 1 2.0 4 I.O 6.0 xA/lo-3 FIG.7.-Similar plot to fig. 6 for benzene (B) on preadsorbed n-hexane (A). influence on the function hence for B on preadsorbed A the parameter Tm,* cannot be determined with significant accuracy. In order to obtain aAB from the experimental results the fitting procedure (b)is therefore preferred. The best-fit values of aAB obtained by procedure (b)are smaller than the geometric mean of aAand aB for all systems listed in table 2. The deviation from the geometric mean is usually expressed by a parameter kAB:5 The values of k, for the present systems are also given in table 2. Results of further static and chromatographic adsorption measurements for the pure vapours and chromatographic results for mixtures of these vapours will be published.l5 This work has been supported by a Research grant from the Minister fur Wissen- schaft und Forschung des Landes Nordrhein-Westfalen.A. V. Kiselev and Y. Y. Yashin Gas-Adsorption Chromatography (Plenum Press New York 1969). P. Valentin and G. Guiochon Sep. Sci. 1975 10 245 271 289. R. 0.Friederich and J. C. Mullins Ind. Eng. Chem. Fundam. 1972,11,439. P. G. Hall and S. A. Muller J. Chem. SOC. Faraday Trans. 1 1978 74 948 2265. E. D. Sloan and J. C. Mullins Ind. Eng. Chem. Fundam. 1975 14 347. P. Valentin and G. Guiochon J. Chromatogr. Sci. 1976 14 56 132. 'F. Dondi M.-F. Gonnard and G. Guiochon J. Colloid Interface Sci. 1977 62 303 316. A. Patrykiejew M. Jaroniec and W. Rudzinski Chem. Eng. J. 1978 15 147. W.VON RYBINSKI M. ALBRECHT AND G. H. FINDENEGG S. E. Hoory and J. M. Prausnitz Chem. Eng. Sci. 1967 22 1025. loW. von Rybinski and G. H. Findenegg Ber. Bunsenges. Phys. Chem. 1979 83 1127. B. J. Zwolinksi and R. C. Wilhoit Handbook of Vapor Pressures and Heats of Vaporization of Hydrocarbons and Related Compounds (College Station Texas 1971). Landolt-Biirnstein Zahlenwerte und Funktionen aus Physik Chemie Astronomie Geophysik und Technik (Springer Berlin 1960) 11. Teil Bandteil a. l2 W. von Rybinski Dissertation (Ruhr-Universitat Bochum 1980). l3 A. Piechocki Staatsexamensarbeit (Ruhr-Universitat Bochum 1977). l4 S. Bliimel Diplomarbeit (Ruhr-Universitat Bochum 1980). l5 W. von Rybinski and G. H. Findenegg to be published.

ISSN:0301-5696    

DOI:10.1039/FS9801500025

出版商:RSC    

年代:1980

数据来源: RSC

摘要:

Photoelectron Spectroscopic Study of the Surfaces of some High-performance Liquid Chromatography Substrates BY ALBERTF. CARLEY AND M. WYNROBERTS LEEMORONEY Department of Chemistry University College Cardiff CF1 1 XL Received 4th September 1980 An X-ray photoelectron spectroscopic study of seven different substrates used in high-perform- ance liquid chromatography is described. The substrates investigated are RP2 RP8 Permaphase ODs Permaphase ETH Spherisorb ODs Spherisorb-5-Amino and Spherisorb-5-Nitrile; in addi- tion and for comparative purposes only core-level spectra have been obtained for samples of quartz and standard silica. Wide-scan spectra in the electron binding energy range (0 to ca. 1000 eV) pro- vided a chemical characterisation of the respective “ surface regions ” i.e.within a depth of ca. 20 A. In addition to the anticipated photoelectron peaks owing to silicon oxygen and carbon clear evidence for the presence of sodium and titanium was obtained with some of the substrates. In the case of Spherisorb-5-Amino and Spherisorb-5-Nitrile high resolution N(1s) spectra are reported and two distinct nitrogen species are shown to be present. Estimates of the concentrations of the various species (nitrogen sodium and titanium) present within the “ probing depth ” are reported. Some general comments are made on the significance of these results to the surface modification of silica substrates used in h.p.1.c. and also on the liniitations of the present approach. The purpose of this work was to explore what information could be obtained from photoelectron spectroscopy on the nature of the surfaces of a series of solid substrates used in h.p.1.c.columns. The spectroscopic data for the chemically modified sub- strates are compared with data obtained with silica quartz and alumina. As far as we are aware there have been no previous studies aimed specifically at elucidating at the atomic level the chemical nature of such substrate surfaces. We regard the pre- sent paper to be exploratory in nature. Photoelectron spectroscopy and in particular X-ray photoelectron spectroscopy (X.P.S.) has emerged over the last decade as an important experimental approach for defining the nature of solid surfaces. By monitoring the kinetic energy EK,of photo- electrons produced by for example Al(Ko() radiation (hv = 1486 eV) the binding energy (EB)of the electrons within the atom involved in the photoemission process may be determined [eqn (l)] The binding energy EBis usually equated with the energy of the corresponding level in the neutral atom through Koopmans’ theorem (the “ frozen orbital ” approxinia-tion) which ignores relaxation of the other electrons in the atom (or molecule) during the photoemission process.A photoelectron spectrum therefore consists of a series of discrete peaks superimposed on a background of inelastically scattered photo- electrons. The value of EBis referred to the Fernii level of the spectrometer but spectra are calibrated with respect to a peak whose binding energy is accurately known and of general acceptance.Two such peaks are the C(1s) and Au(4f) peaks at 285 and 83.7 eV respectively. The binding energy value provides us immediately with information P.E.S. STUDY OF H.P.L.C. SUBSTRATES on the chemical nature of the " surface region ". We illustrate what is meant by " surface region '' by making recourse to eqn (2) Y = Y,(1 -which gives the photoelectron yield Yd from a solid of thickness din terms of Y, the photoelectron yield from an " infinitely thick " solid and the electron escape depth A. Using eqn (2) we can show that ca. 80% of the photoelectron signal is generated from a depth below the surface of 2A. A typical value of 3 is ca. 13 A. The contribution of the signal from the outermost surface layer is ca. 30% of the total intensity.It should be emphasised that these " model calculations " are for a flat atomically smooth solid surface and are included here to make clear what is meant by the " sur-face sensitivity of X.P.S.". Although there are limitations to the experimental approach when we attempt to quantify data obtained with " rough " surfaces X.P.S. nevertheless provides unique information on the chemistry of the outermost layer of a solid. The identity of the surface atoms can be ascertained unambiguously and under favourable conditions shifts in the binding energy values can be used to provide clues as to the chemical environment of the particular atom. Estimates can also be made of the atomic con- centration of each element present from a knowledge of the peak intensities photo- ionization cross-sections and electron escape-depth data.In some cases shifts in binding energy do not result in the emergence of a distinct second new peak and a mere broadening of the peak occurs. Therefore the " full width at half maximum height " (f.w.h.m.) value is a parameter which particularly if followed up by a curve-fitting procedure can allow detail to be extracted from a composite and particularly wide photoelectron peak. EXPERIMENTAL All the X.P.S. data reported were obtained using a Vacuum Generators (ESCA-3) instru- ment which has been described previous1y.l The samples were mounted in two ways (a) by pressing into a thin piece of indium foil and (b) by spreading over double-sided Scotch tape. The first method enabled more reliable calibration of the binding-energy scale and also the generation of better peaks from which intensities could be determined.It however suffered the disadvantage that the indium photoelectron peaks obscured the sodium and titanium peaks. The " Scotch tape " mounting can give rise to silicon and carbon peaks but enables emission from sodium and titanium to be observed. The photoelectron spectra obtained using both methods of sample mounting were internally consistent. Seven different substrates based on silica have been studied RP2 which is reported to have a C2 species bonded to the surface; RP8 which is reported to have a C8 hydrocarbon bonded to the surface; Permaphase ODs; Permaphase ETH; Spherisorb ODs; Spheri-sorb-5-Amino and Spherisorb-5-Nitrile.RESULTS AND DISCUSSION WIDE-SCAN SPECTRA The experimental procedure with all the samples was to obtain first a wide-scan spectrum up to ca. 700 eV binding energy (i.e. in the kinetic-energy range 800-1500 eV). This enabled the elements present within the " surface region " to be identified. The next stage was to obtain higher-resolution spectra of those regions of the spectrum that were considered worthy of more detailed study. In fig. 1 is shown a wide-scan spectrum plotted as a function of electron binding energy for a sample of Spherisorb-5-Amino. The spectrometer parameters are as A. F. CARLEY L. MORONEY AND M. W. ROBERTS I I I I I I I 0 100 200 300 400 500 600 binding energy/eV FIG.I .-Wide-scan spectrum from Spherisorb-5-Amino (analysing energy = 100 eV f.s.d.= 3 x lo4 c s-9. stated in the legend to the figure. The main peaks present have been assigned to photoemission from the Si(2p) Si(2s) C(ls) N(Is) Ti(2p) and O(1s) electron energy levels. On the other hand Permaphase ODS showed (fig. 2) the presence of only silicon carbon oxygen and sodium. Pure alumina as used as a column packing in chromatography also showed clear evidence for both titanium and sodium and a spectrum (fig. 3) is included here since the data may be of general relevance for under- standing the role of the surface or in particular partitioning at the surface in chroma- tography. Wide-scan core-level spectra were also obtained for RP-2 RP-8 Permaphase ODs Permaphase ETH Spherisorb ODs Spherisorb-5-Nitrile quartz and a standard V -T--7-~ I -0 100 200 300 400 500 600 700 binding energy /eV FIG.2.-Wide-scan spectrum from Permaphase ODS (analysing energy = 100 eV f.s.d.= 3 x lo4 c s-1). P.E.S. STUDY OF H.P.L.C. SUBSTRATES I I I I I I 0 100 200 300 460 5b0 600 700 800 binding energy/eV FIG.3.-Wide-scan spectrum from alumina (analysing energy = 100 eV f.s.d. = 3 x lo4c s-'). sample of silica. In table 1 are summarised the binding energies of the photoelectron peaks corresponding to the various elements found to be present i.e. silicon carbon oxygen nitrogen sodium and titanium. The unsuspected elements titanium and sodium were found in both RP-2 and RP-8 sodium in Permaphase ODS and Perma- phase ETH and titanium in both Spherisorb-5-Nitrile and Spherisorb-5-Amino.Also reported in table 1 are the respective f.w.h.m. values of the Si(2s) C(1s) and 7-7-7 , :-r-1 1 I I I 1 I 530.9 535.0 binding energy/eV FIG.4.-O(ls) spectrum from the RP-2 substrate (analysing energy = 20 eV f.s.d. = 1 x lo4c s-'). A. F. CARLEY L. MORONEY AND M. W. ROBERTS 43 TABLE OF BINDING ENERGIES, 1.-SUMMARY F.W.H.M. VALUES AND PEAK INTENSITY RATIOS species normali sed intensity rat i0s sample bonded to EB (f.w.h.m.)/eV relative to Si(2p) surface Si(2s) C(1s) O(1s) N(ls) C(1s) O(1s) N(ls) Na(ls)Ti(2p) 153.7 283.8 532.2 RP-2 c2 (3.7) (2.8) (2.8) -0.43 7.81 -0.2 0.1 153.9 284.1 532.5 RP-8 cs (3.7) (2.8) (2.8) -1.20 7.34 -0.2 0.1 154.3 284.6 532.6 Permaphase C18 (3.7) (2.6) (3.1) -6.42 9.58 -1.0 -ODS Permaphase -glycidoxy-154.0 285.1 532.4 ETH propyl (3.4) (3.5) (2.6) -3.58 11.53 -0.8 I 154.3 284.3 532.6 Spherisorb ODS C18 (3.6) (3.2) (2.9) -0.59 7.54 -0.1 Spherisorb-5-153.4 284.4 532.1 398 Amino -NH2 (3.4) (3.5) (2.8) 400.4 0.31 7.50 0.15 -0.1 Spherisorb-5-154.1 284.2 532.2 398.0 Nitrile -CN (3.6) (4.5) (2.9) 0.36 8.52 0.10 -0.1 154.0 284.5 532.3 quartz -(3.1) (2.3) (2.2) -1.60 8.14 --153.7 284.0 531.7 silica -(2.9) (2.5) (2.1) -0.79 7.34 --Assuming Si(2p) = 103.0 eV for Si02(Nordberg et al.Inovg. Chern. 1970 9 2469). O(ls) peaks. The binding energies have been calculated with reference to the Si(2p) value of 103.0 eV in quartz. HIGH-RESOLUTION SPECTRA The O(1s) spectra were generally uninformative with binding energies close to 532 eV for all the samples.The f.w.h.m. values vary from 2.1 eV (for silica and quartz) to between 2.8 and 3.1 eV for all the other substrates. A typical O(ls) spec- trum for the RP-2 substrate is shown in fig. 4. The C(1.s) spectra for RP-8 Perma- phase ETH Spherisorb-5-CN and Quartz are shown in fig. 5. We draw particular attention to the C(ls) spectrum for Permaphase ETH [fig. 5 spectrum (b)]which is characterised by a very large f.w.h.m. value (ca. 4.5 eV); the other Spherisorb sub- strates and Permaphase-ETH and f.w.h.m. values > 3 eV. Two Spherisorb substrates showed N(1s) peaks arising from nitrogen " atoms " bonded to the surface in one case as an amino group and in the other as a cyano group.Only these two samples in fact showed any evidence of intensity in the N(1s) spectral region and the high-resolution spectra are shown in fig. 6. Clearly there are significant differences in the N( 1s) profiles observed with Spherisorb-5-Amino and Spherisorb-5-Nitrile. P.E.S. STUDY OF H.P.L.C. SUBSTRATES 280.0 285.0 290.0 bndingi energy/eV FIG.5.-C(ls) spectra from (a)RP-8 substrate (b)Permaphase ETH (c) Spherisorb-5-CN (d)quartz (analysing energy = 20 eV f.s.d. = 3 x lo3c s-’). ASSIGNMENT OF N(l.5’) AND C(l.5’)SPECTRA In the N(1s) spectrum for Spherisorb-5-Nitrile [fig. 6 spectrum (a)] there are obviously two components one with a peak at a binding energy of 397.5 eV and the other the major component of the profile at ca.398.5 eV. We suggest that the cyano 395.0 400.0 405.0 binding energy/eV FIG.6.-N(1s) spectra from (a) Spherisorb-5-Nitrile (6) Spherisorb-5-Amino (analysing energy = 50 eV f.s.d. = 3 x lo3c s-’). A. F. CARLEY L. MORONEY AND M. W. ROBERTS species (CN) has the higher N(1s) value while the surface species responsible for the peak at 397.5 eV is a nitrogen adatom. There is considerable and unequivocal evi- dence2 for assigning an N(1s) value of ca. 397 eV to nitrogen adatoms chemisorbed on solid surfaces. Furthermore with the progressive addition of ligands to the adsorbed " nitrogen atom " (NH NH2 NH3) the N( 1s) value increased in approximately 1 eV steps.3 We therefore attribute the 398.5 eV peak to surface-CN species. In the case of the Spherisorb-5-Amino substrate the N( 1s) profile [fig.6 spectrum (b)]also shows two distinct components with peak maxima at binding energies of ca. 398.5 and 400 eV. We have therefore conclusive evidence for two kinds of " nitro-gen " present at the surface and the most likely are NH and NH species. Extensive studies of the adsorption of ammonia and hydrazine on solid surface^^.^ have estab- lished that NH2 and NH species are characterised by N(1s) values of ca. 399.5 and 398 eV respectively. The f.w.h.m. value of ca. 4 eV is also conclusive evidence for at least two nitrogen species being present. SURFACE ELEMENTAL COMPOSITION In addition to the observed binding energies and f.w.h.m. values reported in table 1 we include the normalised intensity ratios [relative to Si (2p)l of the C(ls) O(ls) N(ls) Na(1s) and Ti(2p) peaks.It is interesting to note that for substrates RP-2 Spherisorb ODs Spherisorb-5-Amino and Spherisorb-5-Nitrile quartz and silica the O(ls)/Si(2p) ratios fall within the range 7.5-8.5 (table 1) indicating a consistency for the 0:Si atomic ratios. Clearly for such substrates as Permaphase ETH and Permaphase ODS where substantial modification of the surface by long-chain hydrocarbons had occurred we would expect a significant variation in the observed 0:Si ratio. To convert peak intensities e.g. the oxygen and silicon peaks into atomic ratios nO/nsiwe have used eqn (3) where A is the area of the relevant photoelectron peak p the photoionization cross- section of the particular electron shell E the respective kinetic energies of the photo- electrons and p an asymmetry factor.To confirm the validity of our procedure we TABLE 2.-ATOMIC RATIOS o/si FOR SILICA AND QUARTZ CALCULATED FROM EQN (2) USING USING TWO SETS OF PHOTOIONISATION CROSS-SECTION DATA AND Si(2p) AND Si(2s) PEAKS A B O(lS)lSi(2P) O(1s)/Si(2s) O(1s)/Si(2p) O(ls)/Si(2s) silica 1.6 2.0 1.1 1.8 quartz 1.8 2.2 1.2 2.0 A using photoionisation cross-section data of Thomas and co~orkers.~ B using photoionisation cross-section data of Sc~field.~ analysed first (as a model system) the photoelectron spectra for silica and quartz. We have calculated the atomic ratios 0:S using the O( Is) Si(2p) and S(2s) peak areas and two sets of photoionization cross-section values those determined experimentally by Thomas and co~orkers,~ and also the calculated values of Scofield5 (table 2).The model calculations indicate clearly that it is preferable to make use of O(ls)/Si(2s) peak P.E.S. STUDY OF H.P.L.C. SUBSTRATES ratios rather than O(ls)/Si(2p) ratios since only then do we observe the expected value of ca. 2.0 for the 0:Si atomic ratio. There is little to choose between the two sets of cross-section data (A and B) but the values of 1.6 1.8 1.1 and 1.2 based on the Si(2p) peak areas clearly give an unacceptably low 0:Si ratio. In addition to silicon and oxygen carbon nitrogen sodium and titanium were shown to be present in the surface region Na in RP-2 RP-8 and the two Permaphase substrates; Ti in RP-2 and RP-8 and the three Spherisorb substrates and nitrogen ia the Spherisorb-5-Amino and Spherisorb-5-Nitrile substrates.Carbon was present in all cases. If we assume that the Na Ti and N are present at the surface then we can estimate their respective surface concentration 0using eqn (4) where the subscripts m and s refer to the adsorbed layer and substrate respectively Y is the observed signal strength from the atomic level monitored ,u is the appropriate sub-shell photoionisation cross-section vsis the number density of the relevant sub- strate atoms and 07 is the angle between the electron take-off direction and the surface normal. TABLE 3.-sURFACE CONCENTRATIONS OF Na Ti AND ‘N’ SPECIES (loi5Cm-2) CALCULATED BY MEANS OF EQN (4) sample Na Ti N RP-2 0.1 0.1 -RP-8 0.1 0.1 -Permaphase ODS 0.6 -Permaphase ETH 0.5 Spherisorb ODS -0.1 -Spherisorb-5-Amino -0.1 0.5 Spherisorb-5-Nitrile -0.1 0.4 In table 3 are summarised our results.The two Permaphase substrates contain appreciable surface concentrations of sodium (0.6 x 10’’ cm-2 for Permaphase EDS and 0.5 x lo1’ cm-2 for Permaphase ETH) RP-2 and RP-8 contain appreciably less sodium 0.1 x 10’’ cm-2 but some titanium is present (0.1 x 1015cm-2) as is also the case with the three Spherisorb samples (0.1 x 10’’ cm-2). Both titanium and sodium were also found to be present in the A1203sample (fig. 3). Nitrogen was found only with the Spherisorb-5-Amino and Spherisorb-5-Nitrile substrates at concentra- tions of ca. 0.5 x cm-2 and 0.4 x 10‘’ cme2 respectively.GENERAL COMMENTS The object of this study of various h.p.1.c. chemically bonded substrates by X.P.S. was to ascertain whether or not this approach could provide evidence for the atomic nature of the respective surfaces and therefore show how the surfaces had been modified by replacing surface hydroxyls with various polar or hydrophobic groups. The surface analytical data showed some interesting and unexpected features with significant concentrations of sodium and titanium present with some substrates. Both these elements were also present in very significant quantities in the surface of “pure ” alumina. The sodium was assigned by characteristic Na(1s) and Auger peaks. By making use of photoionization cross-section data estimates were made of their A.F. CARLEY L. MORONEY AND M. W. ROBERTS respective concentrations and in general when present the concentrations were in the range (0.1-0.5) x lo1’ atom cm-2. Definitive evidence was obtained from N(1s) spectra of Spherisorb-5-Amino and Spherisorb-5-Nitrile for surface nitrogen species ; the estimated “ nitrogen concentrations ” were 0.5 x and 0.4 x lo” ern-' res-pectively. The high-resolution spectra indicate the presence of two distinct types of nitrogen species; one of these is obviously related to the replacement of the hydroxyl group the other may be associated with the surface titanium present. It is also interesting to note that the surface concentration of hydroxyl groups pre- sent on the silica gels is estimated to be between 0.2 x lo1’ and 0.4 x 10‘’ cm-2 which suggests a high degree of OH replacement in Spherisorb-5-Nitrile and Spheri- sorb-5-Amino substrates.It should be emphasised that in estimating surface concentrations we have assumed a flat surface when there is substantial evidence for the usefulness and validity of eqn (4). However for surfaces of unknown topography there must of necessity be doubt regarding the accuracy of the calculated values6 Only in the case of N(1s) spectra was there obvious evidence for two different “ surface bonded ” species. The O(1s) and C(1s) spectra were broad and uninforma- tive with little or no evidence for “ chemical shifts ” from one sample to another. Information from these spectra was therefore limited to the trivial observation that C(ls) intensities generally increased with the Rumber of carbon atoms present in the bonded species and in only one case (Permaphase ETH) was there evidence for two different carbon species [fig.5 curve (b)]. The photoemission intensity from the Si(2s) is relatively low with Al(Kx) radiation and there would be advantages in using Zr(Lol) radiation where a distinction is possible’ between differently bonded silicon atoms. Studies of standard silica and quartz samples showed that it is preferable to make use of Si(2p) peaks for the quantitative estimate of silicon. That inner core- levels are preferable in quantitative studies seems to be of general significance.’ We are grateful to Prof. J. H. Purnell Dr. C. F. Simpson and Dr. R. A. C. Gray who supplied us with the substrates used in this study.C. R. Brundle M. W. Roberts D. Latham and K. Yates J. Electron Spectrosc. 1974 3 241. K. Kishi and M. W. Roberts Surf. Sci.,1977 62,252; A. F. Carley and M. W. Roberts Proc. R. Soc. London Ser. A 1978 363 403. M. H. Matloob and M. W. Roberts J. Chetn. Res. (S),1977 336. S. Evans R. G. Pritchard and J. M. Thomas J. Electron Spectrosc. 1978 14 341. J. H. Scofield J. Electron Spectrosc. 1976 8 129. Y. M. Cross and J. Dewing Surface and Interface Analysis 1979 1 26. ’J. E. Castle and R. H. West Surface Reactivity and Catalysis Group Meeting Cardiff 1980. ’A. F. Carley S. Rassias and M. W. Roberts J. Chern. Res. CS) 1979 208.

ISSN:0301-5696    

DOI:10.1039/FS9801500039

出版商:RSC    

年代:1980

数据来源: RSC

摘要:

The Silica-gel Surface and its Interactions with Solvent and Solute in Liquid Chromatography BY R. P. W. SCOTT? Chemical Research Department Hoffman-La Roche Inc. Nutley New Jersey 071 10 U.S.A. Received 27th August 1980 The multilayer formation of water on silica gel is discussed and experiments indicating the exist- ence of three layers of water are described. The interaction of active silica gel with solvent is also considered. Activated silica gel as used in chromatography appears to contain one strongly hydrogen- bonded water molecule per silanol group which can dispersively interact with non-polar solvents form- ing a monolayer. In contact with a polar solvent however the hydrated silanol group can hydrogen- bond to the polar solvent and form a strongly held solvent layer on top of which a bilayer of polar solvent can form by polar interactions with the first layer.The mechanism of solute interaction with activated silica can be described as follows. Solutes interact with the multilayer surface in a chroma- tographic column in two ways. If the solvent layer is weakly held by dispersive forces as in the case of a non-polar solvent the solute can displace the solvent layer and interact directly with the hydrated hydroxyl groups. If on the other hand the solvent is polar and is strongly held by hydrogen-bonding forces to the hydrated silanol group solutes may associate directly with the polar solvent layer but not displace it unless the solute has a polarity similar to the solvent in which case it is consequently eluted at a high k' value.At low concentrations of polar solvent only a small amount of the second layer of weakly held solvent is formed and thus the interaction of a solute with the surface will be with the primary layer of polar solvent. Under such circumstances changes in retention resulting from changes in solvent composition will reflect changes in solute interactions with the mobile phase and not with the surface of the stationary phase. Such a system has been examined; it was shown that the probability of polar interactions in the mobile phase were directly related to the concentration of polar solvent and this was substantiated by results obtained from the examination of liquid/liquid distribution systems. Evidence was also provided that indicated that the magnitude of polar interactions was related to the polarizability per cm3 of the interacting substances.Silica gel is an amorphous silica first prepared by Graham1 in 1861. Today most silica gels are prepared by reacting sodium silicate with hydrochloric acid or by decomposing pure silicon tetrachloride with water.2 The retention characteristics of silica result from the unique nature of its surface which is determined by its method of formation. When a solution of sodium metasilicate is treated with hydrochloric acid monomeric silicic acid is farmed which immediately starts to polymerize to form macromolecular silicic acid as a colloidal solution. The macromolecules are irregu- lar three-dimensional networks of Si04 tetrahedra and their size controls the surface area and pore volume of the silica.When polymerization is complete the solution gels with the OH groups on the surface of the elementary particles condensing fusing the primary particles together. During subsequent syneresis mother liquor exudes from the gel and the hydrogel contracts forming a firm gel which is then washed free of sodium salts. During heating at 120 "C,further condensation occurs between the particles and the hydrogel is converted to the hard silica known as xerogel. The pore size pore volume and surface area of silica are strongly affected by the 7 Present address Perkin-Elmer Inc. Norwalk Connecticut U.S.A. 50 SOLUTE-SOLVENT INTERACTIONS ON SILICA GEL conditions of preparation. The pH of the rea~tion,~ the pH of the gelling medium 4-8 and subsequent treatment of the hydrogel' are some factors that aEect the properties of the gel.Thus when examining the characteristics of a silica and thc nature of the solute and solvent interactions with the surface the same silica gel must be used or results cannot be correlated with one another. THE PROPERTlES OF SILlCA GEL Silica gel is a coherent aggregate of elementary microparticles roughly spherical in shape having diameters ranging from 10 to 10 000 A. These microparticles are not those used for packing columns each of which contains thousands of submicro- particles fused together to provide the matrix from which the macroparticle is made. For a given silica with a specific surface area and porosity the elementary particles have a more restricted range of diameters e.g.907 of the primary particles ofa silica with a mean pore diameter of 60 A will have diameters lying between 5 and 500 A. The nature of the surface water and hydroxyl group on silica has been the subject of much controversy. According to de Boer and VleeskenslOv" a silica dried at 120 "C under atmospheric conditions has lost all physisorbed water and still contains all surface hydroxyls whereas higher temperatures partially deplete the surface of OH groups. Lange12 suggests that strongly physisorbed water (e.g. in very nar- row micropores) is only removed at 180 "C but Young and Bursh13 infer that at 180 "Csome chemically bound water can be lost. Conversely Fripiat and Uytter- hoevenI4 believe that bound water can only be removed at 300 "C or more.Fraissard H I 3rd layer of weakly adsorbed water loss between room temperature and 70 OC ONH maximum loss at 40 'C reversible removed by dry solvents i A I 2nd layer of weakly absorbed water loss complete at 120 OC maximum loss at H/0 I00 "C,reversible removed by dry solvents H 1st layer of strongly hydrogen bonded water loss commences at 200 "C and appears I complete at 650 "C reversible NOT removed by solvents 0\H I k silanol groups lose water to produce siloxyl groups commences at 450 "C complete I at 1100 "C. Loss is irreversible 0 I Ti\* 0 1 FIG.1.-Schematic representation of water bound to a siianol group. et aZ.15 and Scott and Traiman16 consider that temperatures of 600 "C are needed to remove all molecular water.Both authors distinguish between " adsorbed " water (which by definition is removed below 150 "C) and " constitutional " or hydrogen bonded water which is only removed between 400 and 600 "C or even higher. The surface of silica is depicted in fig. 1. The multilayer adsorption of water on silica was discussed by Anderson and Wickersheim" in 1964 Mitchell in 1966" and later by Linsen in 1970.19 More recently Scott and TraimanI6 provided evidence of multilayer formation of water and Scott and Kucera verified that bilayer adsorption R. P. W. SCOTT I I L 1 I 0 200 400 600 800 1000 temperature/"C FIG.2.-Curves relating weight loss (- -) and differential weight loss (-) of silica to heating temperature (taken from thermogravimetric curve).of the polar solvents ethyl acetate tetrahydrofuran and methyl ethyl ketone took place on silica.20*21 The multilayers depicted in fig. 1 suggest three sources on the silica from which water will evolve on heating. Evidence of this is demonstrated by thermogravimetric analysis provided slow heating rates are used to obtain maximum resolution between the water evolved from the different sources. In fig. 2 a thermogram is shown for a 16 mg sample of Partisil20 and includes the differential curve obtained from the nor- mal curve by computation. The sample was heated from 25 to 1000 "C at 1 "C min-' and it is seen that there are indeed three distinct temperature ranges over which water is evolved.The initial loss occurs at 100 "C and is complete at 200 "C. Subsequent to treatment at 200 "C the water loss again increases to a maximum at 400 "C and is virtually complete at 800 "C. Increases in temperature above 800 "C results in fur- ther loss giving a maximum at 950 "C which falls to zero above 1000 "C. The % w/w water lost on heating at 110 and 1000 "C is shown in table 1. The weakly held TABLE 1.-LOSS OF WEIGHT (% W/W) OF SILCA GEL ON HEATING AT 110 AND 1100 "c 1 2 mean loss on heating to 110 "C 4.91 4.67 4.79 loss on heating to 1100 "C 8.63 8.50 8.57 loss on heating from 110 to 1100 "C 3.72 3.83 3.78 (by difference) SOLUTE-SOLVENT INTERACTIONS ON SILICA GEL water lost at 110 "C amounts to 4.8%w/w. The total loss at 1000 "C was 8.6% w/w and by difference the loss between 110 and 1000 "C was 3.8% w/v.The weakly held water is removed by solvent extraction and can be determined by Karl Fischer titration. The free water content by Karl Fischer and by solvent extraction is shown in table 2. The water loss at 110 "C is the same (within experimental error) as that 2.-wATER CONTENT OF SILICA GEL BY KARLFISCHER TABLE DETERMINATION AND SOLVENT EXTRACT ION water content (%) method of measurement 4.93 Karl Fischer 4.96 Karl Fischer 4.78 4.31 extraction by dry ethyl acetate and by Karl Fischer determination extraction by dry THF and byKarl Fischer determination removed by solvent extraction. Thus activation at 150 "C is equivalent to activation by dry solvents but when used in chromatography the silica will still contain water that would be lost between 200 and 1000 "C.If silica is equilibrated with solvents containing traces of water the second layer of water may be replaced. Wet solvents are sometimes employed to improve peak symmetry but the silica will be deactivated and retention will be reduced. Water lost between 200 and 800 "C and between 800 and 1000°Cis derived from two different sources; the latter from the condensation of surface hydroxyl groups to siloxyl groups. The condensation requires considerable energy and would probably be accompanied by destruction of the silica matrix and a reduction in surface area as noted by Vleeskens22 and Scott and K~cera.'~ The latter authors showed that reten- tion also decreases on heating above 600 "C indicating that some silanol groups are removed.Uytterhoeven et aLZ4showed that the total hydroxyl content of silica is located at the surface when silica is heated to 600 "C or above for extended periods of time. Scott and Traiman16 monitored the elimination of silanol groups from silica on heating. Silica was heated at different temperatures for 2 h and subsequently reacted with dimethyl octyl silyl chloride for five days. After work up and drying the carbon content of the silica was determined by microanalysis. The results are shown in fig. 3. As the monofunctional silyl reagent reacts with one silanol group stoichiometrically the carbon content was directly proportional to the number of silanol groups reacted.It is seen from fig. 3 that the condensation of silanol groups commences at 400 "C but even at 600 "C only 10% of the silanol groups have been eliminated. Comparing the curves shown in fig. 2 with those in fig. 3 the water lost above 800 "C is due to condensation of silanol groups whereas the loss between 200 and 800 "C was derived from strongly held or hydrogen bonded water. Scott and Traiman by infrared measurements showed that the loss between 200 and 800 "C was of strongly held water. They pressed silica equilibrated at room temperature with an atmosphere containing 50% humidity into a bromide disc and obtained the typical infrared spectrum shown in fig. 4. It is generally recognized that the broad band at 3400 cm-' is due to free water.25 The area of the band between 3000 and 4000 cm-l was measured and the disc then heated at 25 "C intervals to 400 "C the area of the water absorption bands being measured at each interval.The results are shown in fig. 5 as curves relating water adsorption peak areas against temperature. Heating R. P. W. SCOTT to 150 "C results in a rapid loss of weakly held free water but subsequent to treatment at 150 "C water is lost at a much slower rate; this loss appears to be complete at 600 "C. The value of 600 "C is obtained by extrapolation because potassium bromide melts above 400 "C. It is possible to show that little or no adsorption occurs at these 15-n .3 3 x v .--3 10-._ m c c 2 0 "0 100 200 300 400 500 600 700 800 900 temperat ure/"C FIG.3,-Graph of percent (wlw) carbon content of silica against temperature of activation for different silica samples reacted with dimethyloctylsilyl chloride.wavelengths after 600 "C or more,21 but due to scattering the spectra exhibit consider- able " noise." From table 2 the loss of 3.78% of water on heating the silica from 100 to 1000 "C would represent the loss of one molecule of water from each hydroxyl group and the loss of one molecule of water from the condensation of two hydroxyl groups. Thus there are 8.5 x lo2' hydroxyl groups present per gram of silica which 0.200 r 0.400 0.500 0.600 0.700 0.800 0.900 1 .oo t I 2.00 I 1 I I I I I 4000 3500 3000 2500 2000 1800 1600 1400 1200 1000 BOO 600 wavenumber/cm- FIG.4.-Infrared spectra of silica gel.would include all hydroxyl groups including those contained in the very small inacces- sible and sealed pores. If the hydroxyl groups constitute the adsorption sites on the silica then 8.5 x lo2' represents the theoretical maximum number of sites available for adsorption. In practice a number less than this would be chromatographically SOLUTE-SOLVENT INTERACTIONS ON SILICA GEL 0 100 200 300 400 500 600 temperature/"C FIG.5.-Graph of relative area of water adsorption peaks between 4000 and 2000 cm-' for silica gel heated at different temperatures. available as a proportion of the hydroxyl groups would be in pores inaccessible to most solutes and solvents. The loss of 4.79% of water on heating the silica gel to 110 "C represents 16.1 x 1020molecules which constitute approximately two further layers of weakly held water molecules.INTERACTION OF NON-POLAR SOLVENT WITH ACTIVATED SILICA-G E L SU R FA CE S Silica gel is used with two types of interactive solvents non-polar and polar which are usually employed as solutions in hydrocarbons. A cNoroform + heptane solvent in contact with silica would produce a layer of chloroform on the surface the coverage depending on the strength of the chloroform solution. Scott and Kucera20 deter- mined the adsorption isotherms of three non-polar solvents butyl chloride chloro- form and benzene between a heptane solution and a silica surface (Partisil20). The adsorption isotherms are shown in fig.6 and the results of curve-fitting the data to the monolayer function of the Langmuir isotherm equation are given in table 3. d I concentration of solvent in n-heptane (% wiv) FIG.6.-Langmuir adsorption isotherms for three non-polar solvents. 0,chloroform; x butyl chloride; 0,benzene. Data curve fitted to the Langmuir function Y = x/(A + Bx). R. P. W. SCOTT TABLE 3.-RESULTS FROM CURVE FITTING THE DATA FROM THE ISOTHERMS FOR BENZENE BUTYL CHLORIDE AND CHLOROFORM ON SILICA GEL TO THE LANGMUIR FUNCTlON J' = X/(A +BX) mass of solvent no. of molecules on surface when molecular on surface when index of completely weight of completely solvent determination A B covered/g g-' solvent covered benzene 0.999 45.54 12.30 0.0813 78.1 6.3 x lozo butyl chloride chloroform 0.999 0.996 56.60 50.56 9.59 8.48 0.1043 0.1179 92.6 119.4 6.8 x 1020 6.0 x lozo mean 6.4 x lozo The values for the index of determination indicate that the surface is covered by a monolayer according to the Langmuir isotherm function and the monolayer of each solvent contains approximately the same number of molecules uiz.6.4 x lo2'. If the active sites that cause the adsorption are considered to be the hydrated hydroxyl groups on the silica surface this figure compares with the total number of hydroxyl groups on the surface of 8.5 x lo2'!obtained from the thermogravimetric data but it must be remembered that the value of 8.5 x lo2' includes those hydroxyl groups in- accessible to the solvents given above. INTERACTION OF POLAR SOLVENTS WITH SILICA-GEL SURFACES The interaction of a polar solvent with the silica surface could differ from that of a non-polar solvent as hydrogen bonding can occur with the surface water molecule.Thus a layer of polar solvent molecules would be more firmly held and be analogous to the second layer of water molecules that form on the silanol groups and in fact a second layer of polar solvent might form complementing the third layer of water. Bilayer sdsorption of polar solvents on silica has been experimentally demonstrated. Scott and KuceraZ1 determined the adsorption isotherms of ethyl acetate tetrahydro- furan and methyl ethyl ketone on silica shown in fig. 7. The results did not fit a q , .-$ -I 0 I 0 20 30 concentration of polar solvent in n-heptane (% w/w) FIG.7.-Adsorption isotherms for different polar solvents on silica gel 0, ethyl acetate; x ,methyl ethyl ketone; 0 tetrahydrofuran.Data curve fitted to the bilayer Langmuir-type function Y = A -(A + ABx/2)/(1+ BX + CX'). SOLUTE-SOLVENT INTERACTIONS ON SILICA GEL Langmuir isotherm function for monolayer formation but gave an accurate fit to the Langmuir-type bilayer adsorption isotherm and the constants for the curve fit are shown in table 4. Each layer of the bilayer system contains approximately the same TABLE4.-RESULTS FROM CURVE-FITTING THE DATA FROM THE ISOTHERMS OF TETRAHYDROFURAN METHYL ETHYL KETONE AND ETHYL ACETATE ON SILICA GEL TO THE BlLAYER FUNCTION Y = A -(A + ABxI2)/(1 + Bx + CX2) mass of solvent on surface when no.of molecules solvent A B C completely covered by a monolayer only /g g-' molecular on surface when weight of completelysolvent covered by a monolayer only ethyl acetate methyl ethyl ketone 0.1931 0.1724 14.0 22.6 1.96 3.29 0.0965 0.0862 88.1 72.1 6.6 x lozo 7.2 x lozo tetrahydrofuran 0.1660 39.5 2.38 0.0830 72.1 mean 6.9 x lozo 6.9 x 1020 number of solvent molecules viz. 6.9 x lo2' per g of silica which compares well with the mean value of the number of solvent molecules per monolayer of the non-polar solvents. The isotherms of the individual layers of ethyl acetate are shown in fig. 8 and it is seen that the first layer is almost completely formed when the solvent concen- tration is only ca. 0.8% w/v. The curve in fig.7 strongly suggests bilayer formation on silica gel; however rlI m 01 Y 1 4 concentration of ethyl acetate in n-heptane (% w/v) FIG.8.-Composite adsorption isotherm for ethyl acetate on silica gel. Data curve fitted to the bilayer Langmuir-type function Y == A -(A + ABx/2)(1 + BX + CX'). (a) composite isotherm; (b) monolayer isotherm; (c) bilayer isotherm. there are two alternative explanations to be considered before bilayer formation is fully confirmed. One alternative explanation highly improbable but theoretically possible is that the shapes of the curves shown result from non-ideal behaviour of the polar solvents in heptane. This is unlikely as the concentration range examined was R. P. W. SCOTT between lov4and g where the solutions are sufficiently dilute to expect ideal behaviour.Secondly if non-ideality did exist it would be remarkable indeed that the non-ideality would produce divergence from the normal adsorption isotherms in such a way as accurately to fit a bilayer isotherm for all three quite different sol- vents. The distribution of each solvent ethyl acetate tetrahydrofuran and methyl ethyl ketone between n-heptane and water over the considered range 0-3% w/v has been determined and the results obtained are shown in fig. 9. Linear curves are concentration of solvent in n-heptane (;{ wiv) FIG.9.-The distribution of ethyl acetate tetrahydrofuran and methyl ethyl ketone between water and n-heptane. (a) methyl ethyl ketone I.D. 0.999; slope 1.327. (6)tetrahydrofuran I.D.1.000; slope 0.745. (c) ethyl acetate I.D. 0.990; slope 0.355. obtained relating the concentration of the solvent in each phase and the curves extra- polate to the origin within experimental error. It follows that the simple distribution law is obeyed and the solvents in both phases behave in an ideal manner.26 The second alternative explanation would be that the adsorption curves shown in fig. 7 result from inhomogeneity of site activity. However this possibility is eliminated by the monolayer adsorption curves obtained for non-polar solvents (including the polar- izable solvent benzene) and the fact that there are approximately the same number of molecules in each monolayer of a non-polar solvent as there are in each layer of the bilayer formed by a polar solvent.The surface of silica under various chromatographic conditions is summarized in fig. 10. Silica gel activated by heating to 150 "C or with dry solvents is depicted in fig. lO(a). One water molecule is hydrogen bonded to each silanol group which acts as the active site. Active silica gel in contact with a non-polar solvent is depicted in fig. 10(b) where a monolayer of non-polar solvent is adsorbed on the silica. Each silanol group is hydrogen-bonded to water which in turn interacts with a non-polar SOLUTE-SOLVENT INTERACTIONS ON SILICA GEL solvent molecule by dispersive forces. In contact with a low concentration of polar solvent (e.g. ca. 0.5% w/v) a monolayer of polar solvent is hydrogen-bonded to the water molecule which in turn is hydrogen-bonded to the silanol groups as depicted in fig.lO(c). The layer of polar solvent is complementary to the second layer of water in fig. 1. Finally if the concentration of polar solvent is high enough a second layer u FIG.10.-Multilayer formation on activated silica gel when in contact with different solvents. (a) activated silica gel; (b) activated silica gel in contact with non-polar solvent; (c) activated silica gel in contact with polar solvent at low concentrations; (d) activated silica gel in contact with polar sol-vent at high concentrations. 0hydrogen-bonded water ; B hydroxyl group; ti4 non-polar solvent held by dispersion forces ; polar solvent hydrogen bonded to water; interacting with polar solvent. of solvent is formed by polar interactions with the first layer of polar solvent as de- picted in fig.lO(d). The second layer of weakly held polar solvent is complementary to the third layer of water shown in fig. 1. The silanol groups are the original sites of adsorption which are probably deactivated first by a layer of water then by a layer of non-polar or polar solvent and finally (if a sufficient concentration of solvent is used) by a second layer of polar solvent. INTERACTION OF SOLUTES WITH A SILICA SURFACE WHEN IN CONTACT WITH SOLVENT Solute molecules can interact in two ways with a silica surface covered with solvent molecules. If the solvent molecules are weakly held the solute may displace the sol- vent molecule and interact directly with the hydrated silanol groups as is illustrated in fig.ll(a) where the solute molecules (X) displace the solvent molecules (0)and interact directly with the hydrated silanol groups. If the solvent molecules are strongly held by hydrogen bonding then a solute molecule can associate directly with the solvent molecule in much the same way as the second layer of polar solvent is formed as shown in fig. 1l(b) where the solute molecule X interacts with the solvent molecule 0 but does not displace it. If the second layer of the polar solvent is near completion then both displacement and association can take place. The outer layer of weakly held polar solvent can be dis- R. P. W. SCOTT placed by a solvent molecule which can then associate with the first layer of polar solvent as shown in fig.ll(c). Finally if the interactive forces of the solute with respect to the hydrated silanol groups are sufficiently strong then the solute can dis- place the first layer of polar solvent. For this to happen the polarity of the solute must be similar to that of the polar solvent and under chromatographic conditions such a solute would be eluted at a very high k' value. X I 000000000 0 7 0000 xoooo X FIG.11.-Different types of interactions of a solute with a silica surface. X solute; 0,solvent. (a) Interaction by displacement. (b) Interaction by association. (c) Interaction by association and displacement. INTERACTIONS OF A SOLUTE ON THE SILICA-GEL SURFACE WHEN IN CONTACT WITH A NON-POLAR SOLVENT Interactions of the type shown in fig.1l(a)have been verified. Silica activated at 200 "C was brought into equilibrium with a solution of approximately 15% butyl chloride in heptane. By analysing the solvent mixture the quantity of butyl chloride on the silica gel could be determined. Aliquots of 250 mm3 of anisole a solute that would be eluted from a chromatographic column employing the same solvent mixture at a k' of 4.2 were added sequentially and samples of the solvent taken and analysed after each addition. From the change in concentration of butyl chloride and anisole relative to the total quantity of each substance added to the system the amount of butyl chloride and anisole on the silica gel could be calculated which is shown in fig. 12 as graphs relating the mass of anisole and butyl chloride adsorbed on the silica in g g-' against the concentration of anisole in the solvent mixture.It is seen that as the anisole concentration is increased anisole is progressively adsorbed on the surface of the silica and simultaneously the butyl chloride is continuously displaced. INTERACTIONS WITH SILICA GEL IN THE PRESENCE OF POLAR SOLVENTS The interactions of a solute with the silica-gel surface when in contact with polar solvent is described in fig. 1l(b). Under these circumstances the solute can interact directly with the layer of polar solvent without displacing it. Scott and Kucera20 employed a solvent mixture containing 0.350/ w/v ethyl acetate in heptane in place of SOLUTE-SOLVENT INTERACTIONS ON SILICA GEL the butyl chloride mixture in a similar experiment to the one above where the mono- layer of ethyl acetate is virtually complete and very little of the bilayer is formed.Four different solutes were used anisole eluted at a k’ of 2.4 nitrobenzene eluted at a k’ of 4.7 m-dimethoxybenzene eluted at a k’ of 10.5 and benzyl acetate eluted at a k’ of 27 from a column operated with the same solvent. Each of the solutes was pro- gressively added to the silica gel/solvent system the concentration of both solute and polar solvent determined after each addition and thus the mass of solute and solvent 0.100 1 I I I I 0 I 2 3 concentration of anisole (% w/v) FIG.12.-The adsorption of (0)anisole on silica gel from a solvent mixture of butyl chloride and n- heptane and the desorption of (0)butyl chloride.Mean composition of the solvent 15.7% w/v butyl chloride. adsorbed on the silica surface could be calculated. The results are shown in fig. 13 and 14. In fig. 13 it is seen that the addition of anisole and nitrobenzene does not dis- place any of the ethyl acetate from the stationary phase but associates directly with it. This would be expected because the polarity of the ethyl acetate is significantly greater than anisole and nitrobenzene and thus would be more strongly held on the surface of the silica. In fig. 14 it is seen that m-dimethoxybenzene is also interacting with the ethyl acetate on the surface of the silica and is still not displacing it. However in fig. 14 it is seen that the solute benzyl acetate which has a polarity similar to that of ethyl acetate behaves differently.The addition of benzyl acetate is accompanied by an increase in ethyl acetate concentration in the mobile phase and consequently a decrease of ethyl acetate on the stationary phase. Thus the benzyl acetate is suffi- ciently polar to displace the ethyl acetate in the first layer and associate directly with the hydrated silanol groups. The situation in fact depicted by benzyl acetate in fig. 14 is a similar effect to that shown in fig. 12 where the anisole displaces the weakly held butyl chloride from the activated silica surface. R. P. W. SCOTT .E" 500-25e I 0 250 5bO 750 1000 1250 1600 mass of solute added/mg FIG.13.-Curves relating mass of solute and solvent in the two phases to total mass of solute added.Concentration of ethyl acetate 0.35% wiv; volume of mobile phase 100 cm3; Em mass of ethyl acetate in the mobile phase; Es mass of ethyl acetate on silica gel; Sm mass of solute in the mobile phase; Ss mass of solute on silica gel. (a)Anisole k' 2.4 mass of silica gel 10.04g; (b)nitrobenzene k' 4.7 mass of silica gel 10.28 g. SUMMARY The strong hydrogen-bonding characteristics of the silanol groups on silica permit multilayers to be formed with solvents that readily hydrogen bond. Silica gel can contain three layers of adsorbed water; the first layer appears to be water strongly hydrogen-bonded to the silanol groups and is not entirely removed until temperatures in excess of 600 "Care reached.The outer two layers are hydrogen-bonded to the first layer of water and to themselves and are more easily removed by heating to 150 "C or washing with anhydrous solvents. Silica gel used in chromatography is usually activated at 200 "Cor by solvent washing and contains the first layer of strongly bound water. Activated silica in contact with a non-polar solvent adsorbs a single layer of the solvent on the surface and when the system is used in chromatographic separa- tions the solutes displace the layer of non-polar solvent and interact directly with the hydrated silanol groups. In contact with low concentrations of polar solvent such as 0.5% ethyl acetate in heptane activated silica is covered with a monolayer of ethyl acetate. If the concentration of polar solvent is increased a bilayer is formed.SOLUTE-SOLVENT INTERACTIONS ON SILICA GEL When low concentrations of polar solvent are used in a chromatographic system so-lutes associate directly with the primary layer of ethyl acetate but do not displace it. It has been shown experimentally that solutes do not displace the first layer of polar solvent until the polarity of the solute is similar to the solvent. It can be seen that there will be chromatographic conditions where the surface of the silica gel will have nearly constant interactive properties over a significant range of 250 so0 750 1000 1250 L500 mass of solute added /mg FIG.14.-Curves relating mass of solute and solvent in the two phases to total mass of solute added.Concentration of ethyl acetate 0.35 % w/v; volume of mobile phase 100 cm3; Em mass of ethyl acetate in the mobile phase; Es mass of ethyl acetate on silica gel; Sm mass of solute in mobile phase; Ss mass of solute on silica gel. (a) m-dimethoxybenzene k‘ 10.5 mass of silica gel 10.23 g; (b) benzyl acetate k’ 27.0 mass of silica gel 10.17 g. solvent concentrations. For example for a polar solvent contained at a concentra- tion between 3 and 15% w/v in a non-polar solvent such as heptane the surface will be completely covered by a monolayer of polar solvent. Further even at 15% w/v of polar solvent only about 10% of the second layer of weakly held polar solvent will be formed and thus the interaction of a solute with the surface will be with the primary layer of polar solvent.Under such circumstances changes in retention resulting from changes in solvent composition will reflect changes in solute interactions with the mobile phase and not with the surface of the stationary phase. Thus such a system can be used for examining solute interactions with the mobile phase. R. P. W. SCOTT SOLUTE-SOLVENT INTERACTIONS The distribution of a solute between two phases results from the balance of the forces between the solute molecules and the molecules of each phase. These forces can for example be ionic polar and dispersive. It is therefore possible to define the distribution coefficient (K)of a solute between two phases as the ratio of the magnitude of the total forces on the solute in phase I to the magnitude of the total forces acting on the solute in phase IT.27 Thus for a series of ndifferent types of interactions K,the distribution coefficisnt of a solute between phase S and M can be defined as where 9 is a constant which will incorporate the probability of position of contact and will be decided by the size and geometry of the molecules concerned Fis the magi- tude of the respective force between the solute molecule and the phase molecule P is the probability of molecular interaction and f(T) will incorporate the thermal energy of the molecule at the time of contact.f(T)will also include another thermal prob- ability factor that will determine whether the potential energy of the associated mole- cules due to intermolecular forces is greater or less than the kinetic energy of the solute molecule and will thus decide whether association takes place or not.Further if the separations are carried out at constant temperature,f(T) will be constant and can be incorporated in q,and if only polar and dispersive interactions are considered where the subscripts p and D denote polar and dispersive interactions respectively. Now the probability of interaction of a solute with one of the phases will be pro- portional to the concentration of the interacting parts in each of the respective phases. Thus where cpand cDare the concentrations of polar groups and dispersive groups in the respective phase. INTERACTIONS IN GAS CHROMATOGRAPHY In gas chromatography there are no significant interactions in the gas phase and thus the value of K will be directly related to interactions in the stationary phase only.Thus the equation that describes the distribution coefficient will take the fol- lowing form K = ~2’pcp+ VDFDCD. (4) This equation is similar to that used by Laub and P~rnell~~ who proposed the fol- lowing equation to describe the distribution coefficient of a solute between a gas and a binary solvent mixture KR = KR(‘4)VA + G(S)vs (5) SOLUTE-SOLVENT INTERACTIONS ON SILICA GEL where KR is the solute distribution coefficient for the mixture KicA) and K& are the corresponding distribution coefficients of the solute in the pure phases A and S respectively and VAand Vsare the volume fractions of phases A and S respectively. It is seen that eqn (4) and (5) are very similar but whereas eqn (4) separates the components of the distribution coefficient into contributions from different types of molecular interactions the equation by Laub and Purnell involves the overall inter- action of each-of the two phases.Results obtained by Littlewood30 reported by Laub and Purnell 29 demonstrated the validity of eqn (5) for the distribution coefficient of a series of alcohols chromatographed on a mixture of squalane and dodecanol as the stationary phase. However as an increase in dodecanol content increased the hydroxyl content and thus the polarity of the stationary phase proportionally whereas the dispersive interactions remained sensibly constant and of considerably smaller magnitude (the distribution coefficient of the solutes in dodecanol was much higher than in pure squalane) the results were also in agreement with eqn (4).Furthermore the results included measurements of six different mixtures ranging from pure dode- canol to pure squalane. Eqn (5) was also confirmed by results obtained by Purnell and coworkers31 for the distribution coefficient of a series of alkanes measured on stationary phases consisting of mixtures of n-C18H38 and n-C36H74 hydrocarbons. However as only dispersive forces were involved these results also confirm the appli- cability of eqn (5). Further results by Purnell and coworkers32 employing mixtures of squalane and dinonyl phthalate as stationary phase to examine a wide range of solutes did not provide a precise linear relationship as suggested by both eqn (4) and (5).However although an increase in dinonyl phthalate would result in a propor- tional increase in the extent of polar interactions the complementary reduction in squalane content would not result in a proportional decrease in dispersive interaction due to the increased contributions of the dispersive nonyl chain in the dinonyl phthalate. Thus Cp and CD for polar and dispersive interactions would not be represented by the concentrations of the two stationary phases and eqn (4) would not be expected to be appropriate for such a solvent mixture. INTERACTIONS IN LIQUID-SOLID CHROMATOGRAPHY In liquid-solid chromatography the corrected retention volume (V’)of a solvent can be taken as the product of the distribution coefficient (K)and the surface area of the stationary phase As and thus from eqn (2) an equation for the reciprocal of the retention volume 1/ V’ can deduced where M and S refer to the mobile phase and stationary phase respectively.Now if different concentrations of polar or semi-polar solvents in a dispersion medium such as heptane are employed then provided the concentration of polar sol- vent is kept between 3 and 15% w/v as previously discussed the activity of the silica gel will be constant due to the formation of the primary layer and thus the denomi- nator in eqn (2) becomes constant and l/V’ = A + Bc (7) where A and B are constants. Scott and examined the relationship predicted by eqn (7) employing a series of different solvents and a given solute and a series of different solutes with a given solvent.The results obtained for the former series of experiments are shown in R. P. W. SCOTT fig. 15 as curves relating the reciprocal of the corrected retention volume of phenyl methyl alcohol against the concentration of each polar solvent in the mobile phase. The validity of eqn (7) was established for both series of experiments by obtaining linear plots for 1/V’ against concentration of polar solvent such as those shown in fig. 15. The slope of the curves relating l/V’ against polar solvent concentration 05 04 0.3 I n r( L 3 02. v 01 0 1 lb 15 20 concentration of polar solvent in n-heptane (% w/v) FIG.15.-Graphs relating the reciprocal of the corrected retention volume of phenyl methyl carbinol to the composition of the mobile phase containing different polar solvents in n-heptane.Column 25 cm x 4.6mm; column packing Partisil 10. ((I) Isopropanol; (6) n-butanol; (c) n-pentanol; (d)dioxan; (e)tetrahydrofuran; (f)methyl acetate; (g)ethyl acetate; (h) butyl acetate. would from eqn (6) be related to Fp,the polar interaction between the solute and solvent. The authors showed a linear relationship between the logarithm of the slopes of the curves and the polarizability per cm3 of the individual solutes or solvents. An example of a set of curves demonstrating this relationship is shown in fig. 16 where the polarizability per cm3 of each solute is calculated from the following equation E-1 polarizability per cm3 = -& -+ 2 and E is the dielectric constant of the respective solute or solvent.The relationship between the interactive forces as described by the slope of the curves in fig. 15 is empirical and the correlation was carried out in order to try to relate the polar interactions with some appropriate rational electrical property of the molecules concerned. The linear curves relating I/ V’ to solvent concentration demonstrate that the concentration of the polar solvent appears to control the probability of interaction. This assumes that in the liquid/solid chromatographic system the properties of the surface of the silica coated with the monolayer of polar solvent are indeed constant and changing the solvent concentration in the mobile phase is solely responsible for changes in distribution coefficient and thus the retention.From the work described 66 SOLUTE-SOLVENT INTERACTIONS ON SILICA GEL previously this is a rational assumption but needs experimental verification or else the conclusions concerning the effect of concentration on the probability of interaction are not confirmed. Confirmation was obtained by determining the distribution coefficients of a series of solutes between water and n-heptane + heptyl acetate mixture; both phases were completely immiscible. The interactions in the hydrocarbon phase were modified by addition of the heptyl acetate. Heptyl acetate was chosen as it is also insoluble in water and had similar dispersive characteristics as heptane itself.Thus on the addi- tion of heptyl acetate the ester group introduces polarity into the solvent mixture and -3 .o OCFI u c c . I bL methyl acetate v a Y -M n-pent on o I isopropanol -1.0 I I I I I 0.5 0.6 0.7 0.0 0.9 solvent polarizability /~m-~ FIG.16.-Graph of log d(l/V')/dc against solvent polarizability per unit volume for phenyl methyl carbinol eluted from silica gel employing different solvents. thus the probability of polar interaction will be controlled by the acetate concentra- tion. The dispersive interactions however will remain sensibly constant as the dis- persive nature of heptyl acetate is close to that of heptane. The distribution coeffi- cients of ethyl acetate tetrahydrofuran and n-pentanol were measured at different concentrations of n-heptyl acetate in the methane.Known quantities of the indivi- dual solutes were added to the n-heptane + heptyl acetate mixture together with a reference substance n-decane and the concentration of the solute was determined by gas chromatography before and after equilibrating with water at 25 "C. As neither the n-heptyl acetate or the n-heptane were soluble in water the interactions of the solute in the water remained constant whereas the polar interactions in the n-heptane increased proportionally with the concentration of n-heptyl acetate. In fig. 17 curves relating the distribution coefficient of the solutes with respect to the n-heptane solvent I mixture are plotted against the concentration of n-heptyl acetate in the heptane.It is seen that as predicted by eqn (3) and (4) a linear relationship is obtained. The curves shown in fig. 17 are complementary to those shown in fig. 15 validating the assumption that interactions on the surface of silica gel were constant. It should be emphasized that the results in fig. 17 also give strong support both for the equation of Purnell and Laub [eqn (4)] as well as eqn (3) put forward by Scott and Kucera. Un-fortunately the slopes of the curves given in fig. 17 cannot be plotted against the polarizability of the solute as the polar interactions are between the solute and water in one phase and the solute and n-heptyl acetate in the other. Thus there is no R. P. W. SCOTT comparable relationship to that given in fig.16. In fig. 15 the distribution results from polar interactions with pure ethyl acetate on the silica surface and ethyl acetate in dilute solution in n-heptane i.e. the same polar interactions are involved in both phases thus the interactions can be related to the polarizability of the solute. 0' I I I I I 1 10 20 30 40 50 60 ?O 8'0 concentration of heptyl acetate in heptane /g FIG. 17.-Graph relating distribution coefficient of solutes between n-heptane + heptyl acetate mixtures and water against solvent composition. (a)Ethyl acetate; (b)tetrahydrofuran; (c) n-pentyl alcohol. CONCLUSIONS In liquid-solid chromatography employing silica gel as the stationary phase the interacting surface is complex and can consist of layers of molecules that may be water a solvent or a mixture.Solutes distributing between the mobile phase and the silica surface rarely interact with the silanol groups if at all but with adsorbed layers of water or solvent molecules. Because of the multilayer formation of solvent on the surface of silica gel conditions can be chosen where the surface consists largely of a monolayer of solvent molecules which will have constant interactive properties. Thus under these conditions the interactions on the stationary phase can be maintained constant and by varying the composition of the mobile phase the solute interactions with the mobile phase can be examined. It has been shown that the concentration of the polar solvent in the mobile phase conditions the probabilites of 68 SOLVENT-SOLUTE INTERACTIONS ON SXLICA GEL interaction.Thus a linear relationship is obtained between the distribution coefficient and solvent composition. The effect'of solvent composition on the control of the probability of molecular interaction has also been supported by examining the results from the distribution of solutes between completely immiscible liquids such as water and n-heptane + n-heptyl acetate mixtures. There are strong indications that the polar interactions involved in solute distribution are exponentially related to the polarizability per cm3 of the solute. However this can only be demonstrated where the polar solvent is present in both phases and therefore has so far only been sub- stantiated in liquid-solid chromatography.Tables 1-4 and fig. 1-8 and 13-16 are reproduced courtesy of the Journal of Chromatography. Fig. 9-12 are reproduced courtesy of the Journal of Chromato-graphic Science. T. Graham Encyclopedia of Chemical Technology (Wiley Interscience New York 1954) vol. 12 p. 1861. F. E. Bartell and Y. Fu J. Phys. Chem. 1929,33,676. C. Okkerse Ph. D. Thesis (Delft University of Technology The Netherlands 1961). K. S. W. Sing and J. D. Madeley J. Appl. Chem. 1953,3,549. A. G. Foster and J. M. Thorp The Structure and Properties of Porous Materials (Butterworth's Scientific Publications London 1958) p. 229. I. E. Neimark I. B. Slinyakova and M. A. Piontkovskaya KolloidZh. 1956,18 61. I. E. Neimark and I. B. Slinyakova Kolloid Zh. 1956 18 219. a I. E. Neimark R.Yu. Sheinfain N. S. Kruglikova and 0. P. Stas Kolloid Zh. 1963 25 73. I. E. Neimark R. Yu. Sheinfain N. S. Kruglikova and 0. P. Stas Kolloid Zh. 1964,26 595. lo J. H. de Boer and J. M. Vleeskens Kon. Ned. Akad. Wet. Proc. 1957 B60,45 54,234. l1 J. H. de Boer and J. M. Vleeskens Kon. Ned. Akad. Wet. Proc. 1958 B61,2,85. l2 K. R. Lange J. Colloid. Sci. 1965 20 231. l3 G. J. Young and T. P. Bursh J. Colloid Sci. 1960 15 361. l4 J. J. Fripiat and J. Uytterhoeven J. Phys. Chem. 1962,66 800. J. Fraissard I. Solomon R. Caillat J. Elston and B. Imelik J. Chim. Phys. 1963 60 676. l6 R.P. W. Scott and S. Traiman J. Chromatogr. 1980 196 193. l7 J. H. Anderson Jr. and K. A. Wickersheim Surf. Sci. 1964 2 252. la S. A. Mitchel Chem. Ind. 1966 23 924. l9 The Physical and Chemical Aspects of Adsorbents and Catalysts ed.B. G. Linsen (Academic Press London and New York 1970). 2o R. P. W. Scott and P. Kucera J. Chromatogr. 1978 149 93. 21 R. P. W. Scott and P. Kucera J. Chromatogr. 1979 171 37. 22 J. M. Vleeskens Ph.D. Thesis (Delft University of Technology The Netherlands 1959). 23 R. P. W. Scott and P. Kucera J. Chromatogr. Sci. 1975 13 337. 24 J. Uytterhoeven E. Hellinckx and J. J. Fripiat Silicates Znd. 1963 28 241. 25 J. A. Hockey Chem. Ind. 1965 2 57. 26 S. Glasstone Textbook ofPhysical Chemistry (Van Nostrand New York 2nd edn 1946) p. 736. 27 R. P. W. Scott and P. Kucera J. Chromatogr. 1975 112 425. 28 R. P. W. Scott J. Chromatogr. 1976 122 35. 29 R. J. Laub and J. H. Purnell J. Am. Chem. Soc. 1976,98 30.30 A. B. Littlewood and F. W. Willmot Anal. Chem. 1966 38 1031. 31 R. J. Laub D. E. Martire and J. H. Purnell J. Chem. Soc. Faraday Trans. 2 1978 74 213. 32 M. W. P. Harbison R. J. Laub D. E. Martire J. H. Purnell and P. S. Williams J. Phys. Chem. 1979 83 1262.

ISSN:0301-5696    

DOI:10.1039/FS9801500049

出版商:RSC    

年代:1980

数据来源: RSC

摘要:

Solute-Solvent Interactions on the Surface of Reverse Phases Interactive Characteristics of some Short-chain Aliphatic Moderators having Different Functional Groups BY R. P. W. SCOTT* Chemical Research Department Hoffman-La Roche Inc. Nutley New Jersey 071 10 U.S.A. AND C. F. SIMP SON^ School of Molecular Sciences University of Sussex Brighton BNl 9HR Received 27th August 1980 The desorption-adsorption coefficients and distribution coefficients of a series of aliphatic alcohols carboxylic acids and aldehydes between water and ODs2 reverse phase have been determined using a chromatographic procedure. The moderator concentrations over which the measurements were made were kept at sufficiently low levels so that moderator/moderator interactions in the mobile phase and moderator interactions with adsorbed moderator were kept minimal.It is shown that the desorption-adsorption coefficients decrease and the distribution coefficients between water and the reverse phase increase exponentially with the carbon number of the moderator confirming the system can be described by the Martin equation. As a result substances having chains of five carbon atoms or more could produce complete coverage of the surface when present at a level of only a few percent in the mobile phase. Thus the surface of the reverse phase can be strongly modi- fied without significantly affecting the solvent characteristics of the mobile phase. It was shown that dispersive interactions between the reverse phase and the hydrocarbon chain of the moderator were the same for each moderator and from the slope of the log K against carbon-number curves it was also shown that the logarithmic increment for each methylene group in the alkyl chain was the same for all moderators and independent of the functional group.The difference in the distribution co-efficients between different moderators having alkyl chains of the same length depends solely on the dispersive interactions between the functional groups with the reverse phase and the functional group with any residual silanol groups that may be present. The method used can also determine the effec- tive chromatographic surface area of a given reverse-phase column. It is suggested that the technique could also be used to determine the effective surface area of different types of bonded phases give a measure of the residual polarity of the stationary phase and the magnitude of the interactive capacity of the hydrocarbon portion.Previous work examined the association of aliphatic ion-exchange materials such as the alkyl sulphonates with the surface of a reverse phase and demonstrated that they were adsorbed as a monolayer exhibiting Langmuir-type adsorption isotherms. In the same publication the adsorption of solvents as a monolayer on the surface of the bonded phase was also demonstrated. As ion-exchange interactions can be intro- duced on the surface of the reverse phase by adsorption of a suitable ion exchanger it would be reasonable to assume that other types of interactions could also be intro- duced by the adsorption of long-chain aliphatic alcohols acids or esters.For the system to be chromatographically effective the interacting agent would have to be held sufficiently strongly to the surface of the bonded phase to provide stability and the extent to which it was held would depend upon the chain length of the aliphatic portion. It was therefore of interest to determine the desorption-adsorption co-* Present address Perkin-Elmer Inc. Norwalk Connecticut U.S.A. t Present address Chelsea College University of London Manresa Road London SW3 6LX. 70 SOLUTE-SOLVENT INTERACTIONS ON THE SURFACE OF REVERSE PHASES efficients together with the distribution coefficients of a series of aliphatic moderators of different chain length with a given reverse phase.From such data the effect of chain length on the desorption-adsorption coefficients of the adsorbed solvents could be determined and the concentration required for the solvent completely to cover the surface could be calculated. It is likely that the interactions of the alkyl chain of the moderator with the sta- tionary phase would be dispersive in nature and it would also be likely that this inter- action would be independent of the functional group. The contribution of the func- tional group however to the distribution coefficient and thus retention would vary from functional group to functional group. For this reason the relative dispersive interactions of the hydrocarbon chain of a series of alkyl modifiers of different chain length having different functional groups was investigated.THEORETICAL The relationship between the desorption-adsorption coefficient of a substance which exhibits a Langmuir-type adsorption isotherm with the corrected retention volume of that substance has already been discussed.2 However an alternative algebraic procedure needs to be developed to permit other factors of the chromato- graphic system to be included in the expression relating the corrected retention volume of the moderating solvent with the moderator concentration; for example the distri- bution coefficient of the moderator in pure water and the effective chromatographic surface area of a given column. f 1 squarecm -+ Consider 1 cm2 of surface carrying an adsorbed layer of moderator at a concentra- tion of C gcm-2 in contact with a liquid containing C g of moderator per cm3 of the solvent.Let the molecular weight of the moderator be A4 and the area covered by the moderator molecule when adsorbed on the surface be S. Area of surface exposed = 1 -CS NS where N is Avogadro’s number. (1) M The number of molecules N, leaving the surface will be proportional to the con- centration of adsorbed molecules. i.e. Nl =/?Cswhere /? is a constant at a given temperature. The number of molecules striking and adhering to the exposed surface will be proportional to the concentration of the moderator in the mobile phase. i.e. N2= u(l -C,NS/M)C where u is another constant for the same temperature. Under equilibrium conditions Nl = N2 or PC,= u(l -C,NS/M)C,.Thus aC -uC,NSC,/M = PC and Cs(P+ aNSC,/M) = uC,. or R. P. W. SCOTT AND C. F. SIMPSON where K is the net effective distribution coefficient of the moderator between the sta- tionary phase and the solution of the moderator in water where g = p/a = the desorption-adsorption coefficient of the moderator. Now Y’ = Kv where V’ is the corrected retention volume of the moderator when chromatographed on a reverse phase where the moderator is present at a concentration of C in water and v is the total chromatographically available surface area of the moderator in the column. Thus from eqn (3) Thus if 1/ Y’,is plotted against C and the intercept and slope are ,u and w respectively and v = g/p. (6) It is seen from eqn (5) that the desorption-adsorption coefficient can be calculated from the slope and intercept of a l/Y’ curve plotted against moderator concentration provided the surface area of the moderator molecule is known.Furthermore the effective chromatographic surface area of the column can be calculated from the ratio of the desorption-adsorption coefficient to the intercept w as shown in eqn (6). A value for the effective chromatographic surface area could be extremely useful in comparing two reverse-phase columns and this could be achieved by simply examining the same water/moderator system on each column and determining the slopes and the intercepts of the curves as indicated above. Rearranging eqn (2),the equation for the adsorption isotherm can be obtained Eqn (7) describes the well-known Langmuir isotherm curve which can be calculated from the experimentally determined desorption-adsorption coefficients molecular weight of the moderator the surface area of the moderator molecule and Avogadro’s number N.From this equation the concentration of moderator necessary to com-pletely cover a given fraction of the surface could also be calculated. It should also be noted from eqn (4) that at infinite dilution (i.e. C = 0) Thus as V’ = Kv the distribution coefficient of the moderator is given by K = l/g where K is now the distribution coefficient of the moderator between the stationary phase and pure water. 72 SOLUTE-SOLVENT INTERACTIONS ON THE SURFACE OF REVERSE PHASES EXPERIMENTAL The apparatus used has been previously described3 and consists of a Waters 6000 M pump a thermostatted column fitted with a Valco automatic injection valve having an injec- tion volume of 2mm3 a Waters refractometer detector model 401 arid an appropriate recor- der.Water was qirculated through the column jacket and through the detector jacket from a thermostat that was maintained at 25.75 0.05 "C. The supply from the pump was first passed through a 3 ft coil submerged in the thermostat prior to entering the Valco valve in order to preheat the mobile phase to the correct temperature. The outlet from the detector was connected directly to a 10or 25 cm3 burette the choice depending on the retention volume of the substances being measured. A flow rate of 1 cm3 min-I was used; a burette reading was taken on injection and a reading again taken at the peak maximum.The burette was read to within 0.02 cm3; duplicate runs were carried out and the average of the two readings taken providing the duplicate did not differ by more than 0.05 cm3. If a greater difference was noted further replicate runs were made. The reverse phase employed was ODs-2 manufactured by Whatman Inc. Clifton N.J. and was packed in a 25 cm long column 4.6 mm i.d. This type of bonded phase was the so called " bulk " or polymeric reverse phase which tends to give somewhat higher retention than the alternative " brush " type4 and as previously shown does not give anomolous retention characteristics at high water concentra- tions as the " brush " type phases do.5 The mobile phases used were made up by weighing a known mass of moderator into a 100 cm3 standard flask and diluting to 100 cm3 with water.Moderator concentrations were limited to a maximum of 4% wjv to ensure that changes in interactions in the mobile phase were maintained at a minimum and thus changes in retention volume resulted from changes in the surface characteristics of the stationary phase only. The pump was purged with each new mobile phase and 50 cm3 of the mobile phase pumped through the column to ensure equilibrium before measurements were taken The concentration of the sample was adjusted so that the minimum mass of moderator in- jected was used to provide a peak 1/3 full-scale deflection at the maximum detector sensitivity. Dead volumes were measured for each mobile phase composition by determining the reten- tion volume of sodium chloride contained as a 0.1%wiv solution in water.Three aliphatic series were investigated alcohols (methyl ethyl propyl and butyl) carboxylic acids (formic acetic and propionic) and aldehydes (acetaldehyde and propionalde- hyde). For the C1and C2members of each series concentrations of up to 2% of moderator in water were employed. However the C3 members of the series tended to be relatively insoluble leading to nioderator/moderator interactions in the mobile phase at much lower concentrations. For this reason the retentions of these respective moderators were measured over concentration ranges of up to a maximum of 0.5 or 1% w/v. The effective area of each moderator molecule was obtained by the method described by Amoore.6 Molecular models were made to scale and placed on photographic paper and exposed together with a square rep- resenting (on the same scale) 9 A.z The square and the outlines of the molecules were cut out and weighed.This procedure was carried out for the three possible axial positions of each molecule which represented the three extreme possibilities of the area that a molecule could present to the chromatographic surface of the bonded phase on contact. The results are shown in table 1. In the first three columns the surface areas that were measured in the three different positions are given and the fourth column is an average of the three. The average values were used for calculating the different chromatographic adsorption para- meters given in the appropriate equations.The results for methanol ethanol propanol and butanol are given in tables 2 3 4 and 5 those for formic acetic and propionic acid are given in tables 6,7 and 8 respectively whereas the results for acetaldehyde and propionaldehyde are given in tables 9 and 10. The results given in these tables for the alcohols acids and aldehydes shown as curves relating 1/V' against moderator concentrations are given in fig. 1,2and 3 respectively. It is seen that ex- cellent straight lines are obtained with correlation coefficients close to unity. At this point it would be of interest to comment on the change in the measured dead volume shown in tables 2-9 inclusive. The change in dead volume with moderator con- centration and between the different moderators is not novel.It has been discussed by R. P. W. SCOTT AND C. F. SIMPSON 73 TABLE 1.-sURFACE AREA OF MODERATOR MOLECULES IN A’ X -Y phase X -2 phase Y -2 phase position 1 of position 2 position 3 mean moderator minimum area formic acid 18.5 18.4 22.5 19.8 acetic acid 20.3 22.5 29.0 23.9 propionic acid 26.9 26.6 35.9 29.8 formaldehyde 12.0 16.0 16.3 14.8 acetaldehyde 18.0 22.0 22.1 20.7 propionaldehyde 20.9 28.6 29.2 26.2 methanol 12.8 18.5 15.1 15.5 ethanol 21.o 25.1 23.4 23.2 n-pro pano 26.7 31.6 30.9 29.7 n- butanol 27.1 39.4 38.4 35.0 TABLE2.-RETENTION-VOLUME DATA FOR METHANOL IN METHANOL+ WATER SOLUTIONS ~~~~~~ solvent retention dead adjusted retention composition/g cm-3 volume VR/Crn3 volume vm/cm3 volume VR/cm3 1/ V’R /~m-~ ~~~~~~ ~~ 0.0000 4.08 2.05 2.02 0.495 0.0025 3.95 2.05 1.90 0.526 0.0050 3.83 2.05 1.78 0.562 0.0100 3 -64 2.05 1.59 0.629 0.0150 3.47 2.05 1.42 0.704 0.0200 3.36 2.05 1.31 0.763 Index of determination for curve fit to the function y = A + BX 0.999 A = 0.495 B = 13.62 TABLE 3.-RETENTION-VOLUME DATA FOR ETHANOL IN ETHANOL f WATER SOLUTIONS adjusted solvent retention dead retention composition volume volume volume 1/ V’R /g CM-~ V&m3 V,,,/cm3 V&m3 /cM-~ O.oo00 8.4 2.05 6.35 0.158 0.0025 7.15 2.05 5.10 0.196 0.0050 6.25 2.05 4.20 0.238 0.0100 5.20 2.03 3.17 0.3 15 0.0150 4.545 2.03 2.52 0.398 0.0200 4.10 2.03 2.07 0.483 Index of determination for curve fit to the function y = A + SX,1.000 A = 0.156 B = 16.23 74 SOLUTE-SOLVENT INTERACTIONS ON THE SURFACE OF REVERSE PHASES TABLE 4.-RETENTION-VOLUME solvent retention /g cm-3composition V&m3 volume 0.00000 25.2 0.00336 12.92 0.00648 9.30 0.00964 7.35 0.01280 6.34 0.0200 5.OO DATA FOR n-PROPANOL IN n-PROPANOL adjusted dead retention volume volume Vm/cm3 V’&m3 2.1 23.10 2.06 10.86 f WATER SOLUTIONS l/VR /~m-~ 0.0433 0.0921 0.138 0.189 0.232 0.337 2.06 2.07 2.03 2.03 7.24 5.28 4.31 2.97 Index of determination for curve fit to the function y =1 A + BX,0.999 A = 0.0423 B = 14.80 TABLE5.-RETENTION-VOLUME DATA FOR n-BUTANOL IN n-BUTANOL -/- WATER SOLUTIONS adjusted solvent retention dead retention /g CM-~ composition VR/cm3 volume Vm/cm3 volume V’,/cm3 volume /~rn-~ 1/ V’R 0.0000 101.25 2.02 99.23 0.0101 0.0025 23.10 1.95 21.15 0.0473 0.0050 14.15 1.78 12.37 0.0808 0.0100 8.OO 1.76 6.24 0.160 0.0150 6.10 1.72 4.38 0.228 Index of determination for curve fit to the function y = A + BX 0.999 A = 0.0102 B = 14.63 TABLE 6.-RETENTION-VOLUME DATA FOR FORMIC ACID IN FORMIC ACID 4-WATER SOLUTIONS adjusted solvent retention dead retention composition /g cm-3 volume V&m3 volume Vm/cm3 volume V’RICM3 1/ V’R /~m-~ 0.00000 3.05 1.90 1.15 0.869 0.00229 3.24 2.15 1.09 0.917 0.00458 3.15 2.08 1.07 0.935 0.00916 3.05 2.08 0.97 1.031 0.01 37 3.OO 2.08 0.92 1.087 0.0200 2.96 2.08 0.88 1.136 Index of determination for the curve fit to the function y = A + SX 0.986 A = 0.877 B = 13.56 R.P. W. SCOTT AND C. F. SIMPSON TABLE 7.-RETENTION-VOLUME DATA FOR ACETIC ACID IN ACETIC ACID f WATER SOLUTIONS solvent retention dead adjusted retention /g CM-~ composition V&m3 volume Vm/cm3 volume V’R/Crn3 volume /cmd3 1/ V’R 0.0000 6.00 1.95 4.05 0.247 0.0025 5.45 1.95 3.50 0.286 0.0050 4.98 1.95 3.03 0.330 0.0100 4.45 1.95 2.50 0.400 0.01 50 4.08 1.95 2.13 0.469 0.0200 3.80 1.95 1.85 0.541 ~~~~~ Index of determination for the curve fit to the function y = A + BX 0.999 A = 0.251 B = 14.60 TABLE DATA FOR PROPIONIC ACID IN PROPIONIC ACID + WATER 8.-RETENTION-VOLUME SOLUTIONS adjusted solvent retention dead retention composition volume volume volume 1/ V‘R /g cm-3 V&m3 Vm/cm3 v’R/cm3 /~rn-~ 0.0000 17.15 1.95 15.20 0.0658 0.0025 11.47 1.90 9.57 0.1045 0.0050 8.85 1.87 6.98 0.143 0.0075 7.50 1.90 5.60 0.179 0.0100 6.42 1.85 4.57 0.219 Index of determination for the curve fit to the function y = A 4-SX 1.000 A = 0.0656 B = 15.26 TABLE 9.-RETENTION-VOLUME DATA FOR ACETALDEHYDE IN ACETALDEHYDE + WATER SOLUTIONS adjusted solvent retention dead retention composition volume volume volume / V’R /g ~rn-~ V&m3 Vm/cm3 vycm3 /~m-~ 0.0025 5.72 1.97 3.75 0.267 0.0050 5.15 1.97 3.18 0.314 0.01 00 4.44 1.98 2.46 0.406 0.01 50 4.02 1.96 2.06 0.485 0.0200 3.75 1.95 1.80 0.555 Index of determination for the curve fit to the function y = A + BX 0.996 A = 0.232 B = 16.53 76 SOLUTE-SOLVENT INTERACTIONS ON THE SURFACE OF REVERSE PHASES Shoenmaker et aZ.,’ by Knox and Jurand’ and also by McCormick and Karger who have examined the phenomena in detail.The retention volume (VR) of a substance can be given by the following equation VR = Vi + K~vp+ &A where Vi is the interstitial volume V is the pore volume A is the surface area of the sta- tionary phase Kl is the distribution between the mobile phase and the stationary phase in the TABLE 1~.-RETENTION-VOLUME DATA FOR PROPIONALDEHYDE IN PROPIONALDEHYDE + WATER SOLUTIONS adjusted solvent retention dead retention /g cm-3 composition vR/cm3 volume V,,,/cm3 volume V’&m3 volume /~rn-~ 1/ V’R ~~~ 0.001 25 13.30 1.85 11.45 0.087 0.002 50 11.10 1.86 9.24 0.108 0.003 75 9.60 1.85 7.75 0.129 0.005 00 8.50 1.80 6.70 0.149 Index of determination for the curve fit to the function y = A + SX,1.0oO A = 0.0665 B = 16.56 pores and K2is the overall distribution coefficient between the surface and the mobile phase.If a solute is chosen such that K2 = 0 then VR = Vi + Klvp and if the liquid in the pores is the same as the mobile phase then Kl =r 1 and VR = Vi f vp = VO where Vo is the classically defined dead volume. In experiments given in this paper the adsorption of the moderator onto the surface which exists inside the pores can cause both the volume of the pore to change and also the solvent composition of the liquid in the pores so that Kl may not necessarily equal 1 at all times.Because a salt is used as a dead-volume marker however it is likely that K, if not unity is very close to unity. However the pore volume will change significantly and this change will be the greater where the moderator is strongly adsorbed and this is shown for propionic acid propionaldehyde and butanol in tables 5,s and 10. The change in dead volume will be the least for the least strongly adsorbed molecules for example methanol as shown in table 2 where the dead volume does not change at all and for acetaldehyde which also shows very little change. DISCUSSION OF RESULTS The linear relationship between 1/ V’ (the reciprocal of the corrected retention volume) and the moderator concentration shown in fig.1-3 confirm that the moderators are being adsorbed onto the surface of the reverse phase according to a Langmuir-type adsorption isotherm. The correlation coefficient of the curves relating mobile phase composition and the reciprocal of corrected retention volume together with the constants A and B which are the intercept and slope of the curve respectively are shown for all three series of moderators in table 11. The value of NS/M was calculated using the data in table 1 Avogadro’s number and the molecular weight of the alcohol. It is seen that although the mean molecular TABLE OF CHROMATOGRAPHIC DATA 11.-SUMMARY effective surface distribution intercept adsorption-desorption area of column coefficient NS/M IU slope coefficient 0 K /lo-' cm2 /~m-~ lg-' 1104 g Im2 cm-l formic acid 2.60 0.877 14.79 15.4 176 0.649 acetic acid 2.40 0.251 14.60 41.3 165 2.42 propionic acid 2.43 0.0656 15.26 10.4 I59 9.57 acetaldehyde 2.84 0.232 16.53 39.8 172 2.5 1 propionaldehyde 2.73 0.0665 16.56 10.96 165 9.122 methanol 2.90 0.495 13.62 105 21 3 0.949 ethanol 3.04 0.156 16.23 29.2 187 3.42 propanol 2.98 0.0423 14.80 8.52 201 11.7 n-butanol 2.95 0.0 102 14.63 2.05 202 48.6 78 SOLUTE-SOLVENT INTERACTIONS ON THE SURFACE OF REVERSE PHASES I 0 0.02 concentration of moderator 1g cm-3 FIG.1.-Plot of the reciprocal of the corrected retention volume of four aliphatic alcohols and their concentration in the mobile phase. (a) MeOH I.D. 0.999; slope 13.62; intercept 0.495.(b) EtOH I.D. 1.O00; slope 16.23; intercept 0.156. (c) PrOH I.D. 0.999; slope 14.80; intercept 0 0423. (d) BuOH I.D. 0.999; slope 14.63; intercept 0.0102. 0.8 n I E -2 0.6 9 b . H W 0.4 0.2 I I 0 05 1.0 15 2.0 concentration of moderator (acid) (% wlv) FIG.2.-Plot of l/V for three aliphatic acids against moderator concentration. (a)Formic acid I.D. 0.986; slope 14.79; intercept 0.877. (b)Acetic acid I.D. 0.996; slope 14.60; intercept 0.251. (c)Propionic acid I.D. 1.O00; slope 15.26; intercept 0.0656. R. P. W. SCOTT AND C. F. SIMPSON areas differ considerably between each moderator the value of NS/Mfor a given series is very similar for each member. This means that the mean surface area increases linearly with the molecular weight which could be expected; thus the surface area per gram for each respective moderator remains approximately constant.The de- sorption-adsorption coefficients were calculated using eqn (5). It is seen that the desorption-adsorption coefficients vary from a value of 105 x lo4 for methanol to 2.05 x lo4 for n-butanol for the alcohol series. Eqn (8) permits the calculation of the 0.6 1 0 6-5 1.0 I I .5 I 2.0 concentration of moderator (aldehyde) (% w/v) FIG.3.-Plot of 1/V for two aliphatic aldehydes against moderator concentration. (a)Acetaldehyde I.D. 0.996; slope 16.53; intercept 0.232. (b)Propionaldehyde I.D. 0.998; slope 14.98; intercept 0.017. distribution coefficient of the solvent at infinite dilution and is given by the reciprocal of "g " and shown in the last column of table 11.From the values of "g " given in table 11 and by use of eqn (7) the adsorption isotherms for the four alcohols were calculated and are shown in fig. 4. It is seen that for n-butanol and n-propanol over 95% of the surface is covered with the alcohol at a concentration of ca. 7% w/v. However at 7% w/v of ethanol only 86% of the surface is covered and with methanol at 7% w/v only ca. 67% is covered. It is fairly obvious that the adhesion of the alcohol layer to the hydrocarbon chain of the reverse phase increases rapidly with carbon chain length. In fig. 5 the distribution coefficient (K)of each member of each series is plotted on a logarithmic scale against its respective carbon number.It is seen that the curves for the homologous series of acids and alcohols give excellent straight lines having indices of determination close to unity confirming the compliance of the system with the Martin equation.l0 These curves are also in agreement with those obtained by Colin and Guiochon l1 and Berendsen.12 These authors however showed a linear relation- ships between log k' and carbon number which of course includes the surface area of the support and is thus not solely related to the interactions of the solvent with the two phases. The slopes and intercepts for the two linear curves relating log K against carbon number are shown in table 12. Included in table 12 is the slope of the line 80 SOLUTE-SOLVENT INTERACTIONS ON THE SURFACE OF REVERSE PHASES 41 complete coverage of surface by monolayer EtOH c , I 1 I I 0 0.02 0.04 0.06 0.08 0.1 concentrationof alcohol in mobile phaselg cm-3 FIG.4.-Adsorption isotherms for the C1 to C4alcohols on ODs2 reverse phase.; 3 carbon number FIG.5.-Plot of log K against carbon number for a homologous series of moderators having different functional groups. (a)Alcohols I.D. 0.999; slope 1.304; intercept 0.251 x (6)Aldehydes I.D. 1.0oO; slope 1.290; intercept 0.691 x (c) Acids I.D. 1.000; slope 1.345; intercept 0.643 x R. P. W. SCOTT AND C. F. SIMPSON joining the points for acetaldehyde and propionaldehyde together with the intercept found by the projection of the line to the log K axis.It is seen from table 12 that the slope of the log K against carbon number curves for all three series are numerically very similar clearly demonstrating that the contribu- tion to log K of each methylene group is independent of the terminal functional group of the series which again would be expected from a system that complied with the Martin equation. Thus the difference in retention volumes between an alcohol aldehyde or acid having the same number of carbon atoms in their aliphatic chain is TABLE 12.-sUMMARY OF DISTRIBUTION DATA slope intercept intercept of log K residual value for K moderators per carbon number due to functional groups alcohol 1.30 0.251 aldehydes 1.29 0.691 acids 1.35 0.643 mean 1.31 standard deviation 0.032 determined solely by the nature of the functional group.Algebraically the relation- ship between the distribution coefficient of an aliphatic substance between a reverse phase and water can be given by the following equation logK= A + 1.31 n (9) or K = Be's3'" when 1.31 is the mean slope from table 8 n is the number of carbon atoms in the side chain A is a constant depending on the functional group and A = log B. The relationship given in eqn (9) will be temperature dependent and also will vary with the nature of the stationary phase. If a specific type of stationary phase is em- ployed such as the bulk-modified materials used in this paper then the equation will hold for other stationary phases providing they have the same carbon content. If the carbon content changes this will also modify the constants in eqn (9).The functional group/stationary phase interactions will be largely dispersive in nature but due to the non-ideality of practical reverse phases polar interactions with any urireacted silanol groups will also be included. It is therefore not possible to relate the dispersive interactions of the functional group solely to the intercepts given in table 12 and consequently they are not related directly with the constant B in eqn (9). It is also interesting to note from table 11 that the surface area available to the different moderators although similar is not identical and it would appear that the alcohol was exposed to a somewhat greater area of surface than the acids and alde- hydes. Combining eqn (5) and (6) it is seen that V = NS[yM (10) and thus the calculations of the surface area p depend on the accuracy by which the surface area of the molecules S is determined and thus hinges on surface area measurement techniques suggested by Am~ore.~ It is likely that although Amoore's method gives approximately correct values for molecules errors are possible parti- cularly when different functional groups are involved.However as the calculation of the distribution coefficient depends on these area values and excellent correlation is 82 SOLUTE-SOLVENT INTERACTIONS ON THE SURFACE OF REVERSE PHASES demonstrated by the results given in fig. 5 with the Martin equation then the areas given in table 11 must also be valid as they are derived from the same basic equation.CONCLUSIONS It has been shown that adsorption isotherms for a number of aliphatic moderators on a reverse phase can be determined chromatographically. It has also been shown that the desorption-adsorption coefficient decreases exponentially with the carbon number of the moderator. Conversely the distribution coefficient of each molecule increases exponentially with the carbon number. It is clear that when using an ali-phatic modifier having a chain length of 4 or 5 carbon atoms the surface of a bonded phase could be completely covered with a monolayer and by choosing appropriately active groups the chromatographic characteristics of the surface could be changed. Providing sufficiently long carbon-chain species are used the modification can be accomplished at very low concentrations thus minimizing moderator interactions in the mobile phase.The technique described also permits the effective chromatographic surface area of a column to be determined and it has been shown that approximately 200 m2 of surface is chromatographically available from the ODs-2 column examined employing an alcohol as the surface probe. However ODs-2 reverse phase is poly- meric in nature and it may well be that a " brush " type reverse phase could have a significantly different effective surface area indeed the effective chromatographic surface area per gram could be one parameter by which a reverse phase could be characterized. It has been shown that dispersive interactions between the reverse phase and the hydrocarbon chains are similar for all the three series and independent of the func- tional groups.The distribution coefficient (K)of an aliphatic acid alcohol or alde- hyde between water and the reverse phase examined can be described by the following simple equation K = Be1.31" when B is a constant and is characteristic cf the functional group and nis the number of carbon atoms (methylene group) in the side chains. It follows that by using K for one member of a given aliphatic series the value of K for any other member of that series can be calculated. One of the authors (C.F.S.) would like to thank Whatman Inc. Clifton N.J. for support and Hoffman-La Roche Inc. for kindly providing laboratory facilities. Both authors wish to thank Ms.C. Lancaster and Mr. T. Garrison for help with the experimental work. R. P. W. Scott and P. Kucera J. Chronratogr. 1979 179 51. R. P. W. Scott and P. Kucera J. Chromatogr. 1978 149 93. R. P. W. Scott and P. Kucera J. Chromatogr. 1975 112 425. * I. Halasz and I. Sebastian Angew. Chem. Int. Ed. Engl. 1969 8 453. R. P. W. Scott and C. F. Simpson J. Chrornatogr. 1980 197 11. J. E. Amoore Ann. N.Y. Acacl. Sci. 1964 116 457. 'P. J. Shoemakers H. A. H. Billiet R. Tijssen and L. De Galan J. Clzronzatogr.,1978 149 519. J. H. Knox and J. Jurand J. Chromatogr. 1978 149 297. R. M. McCormick and B. L. Karger Anal. Chem. 1980 in press. lo A. J. P. Martin Biochem. SOC.Symp. 1949 3 4. H. Colin and G. Guiochon J. Chromatogr. Sci. 1980 18 54. l2 G. E.Berendsen Doctorate Thesis (Technische Hogeschool Delft University Press).

ISSN:0301-5696    

DOI:10.1039/FS9801500069

出版商:RSC    

年代:1980

数据来源: RSC

摘要:

Mixed-solvent Theory for Liquid Chromatography MCCANN,HOWARD AND C. ANTHONY BY MERANEY PURNELL WELLINGTON Department of Chemistry University College of Swansea Singleton Park Swansea SA2 8PP Received 28th July 1980 The competitive- and solution-interaction models are further developed. Certain assumptions previously made in the theory are shown to be either unacceptable or unnecessary. It is shown that both the Langmuir and diachoric-solution models lead to an equation relating inverse retention volume in a simple manner with volume-fraction composition of the mixed solvent. Results are presented for elution of several solutes from carbon tetrachloride + diethyl ether mixtures over the whole range q= 0-1. This provides the first data covering the whole concentration range.Although the same general equation describes both theoretical approaches it is shown how they may be distinguished in the light of experiment. It is concluded that all data so far published fall into a group in which over the approximate range q = 0.1-1,there is no competitive adsorption of the solvent components and the diachoric model applies. The importance of extending studies over the range q = 0-1 is eniphasised as also is the need to work with solvent pairs capable of competitive adsorption over a wider range of q. The overwhelming majority of liquid-chromatographic (1.c.) analyses are conducted with a multi-component mobile phase. In view of the explosive growth in the use of 1.c. techniques it has become a matter of importance to analysts that some system should be developed whereby the choice of mixed solvents is simplified.The only current system available is that developed by Snyder' and in the absence of any alter- native its use at least as a qualitative guide is expanding. Snyder adopts a highly approximated theoretical approach ; the following represent the assumptions under- lying the basis others being necessary to the full elaboration homogeneity of the adsorbent; infinite dilution of the solute; ideality of the mixed liquid phase; equality of the molar volume of any species in the mobile and in (on) the adsorbed layer ; equality of all molar volumes of all species in the system; full coverage of the surface (monolayer) at all times; retention of solute when eluted by a mixed solvent is the sum of individual contributions due to competition for adsorbent by solute with the individual solvent components.Since separation is seen as resulting from the contest for surface sites with the solvent playing the role of an inert mobile phase as in principle does the gas in gas chromato- graphy this approach has come to be called the competitive model. Snyder then writes as the basic equation for partition of solute S between solvent (A + B) and adsorbent K(AB)s = xgK(A)~+ x&B)s (1) where xa represents a mole fraction in the surface layer and K's represent stoichio- MIXED-SOLVENT THEORY FOR LIQUID CHROMATOGRAPHY metric partition coefficients for elution by solvents A B and A + B respectively. Here K is defined by e.g.C representing mol/adsorbent weight in the surface layer and Cs representing mol/ volume in the liquid phase. Thus K(A)s has the dimensions of volume/weight so that according to convention the retention volume of S eluted by pure A would be v(A)s = &A)s wa (3) where W is the weight of adsorbent in the column. Several theoretical groups 2-7 have sought either to provide a rationalisation of Snyder’s method or to modify it or even to supplant it. With only one exception’ however they have retained most of the foregoing assumptions but for reasons of their own have chosen to work not with stoichiometric partition coefficients but with alternatives defined in terms of mole fraction e.g. with pure A K‘(A)s = (x?/x~)A* (4) It is a simple matter to show that a corresponding pair of K and K’ for a pure eluent are related via e.g.where V is the monolayer volume of adsorbed A. Since all molar volumes are assumed to be identical the monolayer volume is constant for all compositions of (A + B) mixtures hence it is legitimate subject to this substantial restriction to replace all K by K’ in eqn (1). Thus K‘(AB)s = xOAK’(A)~ + ~S’(ms-(6) It is immediately apparent that this equation is of no direct practical value since the several xu cannot be determined. However by introducing the equilibria setting rearranging and substituting via KBA = &A/.& yields the basic equation from which most recent theoretical development 43 has sprung. It is in fact unnecessary to assume equality of molar volumes to justify use of eqn (7).As has been pointed out V(AB)sis the sum of the individual retentions due to A and B. Thus V(AB)~= KA)SWA + &B)sWB (8) where WAand WBare respectively the weights of adsorbent covered by A and by B. Evidently WA+ WB= Wa and = K’(A)sVX + K’(B)~V~ V~AB)~ = K‘(A)snZrA + K‘(B),n;pB (9) M. MCCANN H. PURNELL AND C. A. WELLINGTON where Vz and V are the volumes of ng and ng moles of A and B in the surface layer and FAand vBare their molar volumes. Now V(AB)s = &AB)sWa = Kf(AB)s(VA + YB) (n + ng)l(nA+ nB)* Thus and The introduction of the molar volume of the eluting mixture VAB depends upon the assumption that any excess volume of mixing is trivial a situation so common as to be almost general.We thus see that eqn (1) and (6) are correct only when rA= vB and are subject to very considerable arithmetic error since even quite common mixed solvents may have molar volumes differing by factors of two or more. Further we see that a more general expression is available although VAB is of course composi- tion-dependent. Eqn (10) may now be manipulated to eliminate x terms exactly as previously des- cribed and leads without assumption to eqn (7). Thus whilst eqn (1) and (6) are totally limited to the case of FA= rB,eqn (7) appears to have a wider validity. This rather surprising result prompts us to look for an explanation. One is found by substituting back via their definitions for the various K'. This yields whereas by definition In the foregoing "ng is the full monolayer number of moles of A deposited from pure A containing nA@ moles and similarly for €3 while (n + n:) is the total number of moles in the monolayer deposited from A + B.These two equations become equivalent when (n mn;lnAo)= ng and (nB ,ng/nBo> = n:. Since the surface layers of A and B are assumed to be independent i.e. non-interacting clearly in each region n = ,,,ng and ng = ,,,ng. Thus implicit in the model is the requirement nA = nAo and nB = nBo which in turn means that the solvent components are to be regarded as effectively immiscible. Such behaviour has been reported by us8 for a number of gas-liquid chromatography systems and termed diachoric. The model discussed above is thus equivalent to that of elution from a column containing W of adsorbent by solvent A of volume VAfollowed by a subsequent elution from a column containing WB of adsorbent by volume VBof B.THE LANGMUIR MODEL It is commonly stated that the foregoing model is equivalent to Langmuir adsorp- tion of A B and solute S. Indeed in the special case that rA= vB= Psit is possible to derive eqn (7) as we show below; however the result is not general. MIXED-SOLVENT THEORY FOR LIQUID CHROMATOGRAPHY The standard equation for competitive Langmuir adsorption yields for the frac- tional surface coverage (OS) by S Full monolayer coverage can only occur when ZK C + 1 and since in addition KsCs is small we have Each K in eqn (12) represents a ratio of the forward and backward rate constants in the relevant adsorptive equilibrium and should not be confused with partition coeffi- cients and equilibrium constants such as were defined earlier.Now 6s =nga,f WaAa where a is the molar area of S and A is the area per unit weight of adsorbent. But n;/Wa= Cg and we may set u,/Aa = a. Then (acf/cS) = aK(AB)~= [(KA/KS)cA +(KB/KS)cBl-l-We may set CA= qAvA and CB= vBvBwhere p represents a volume fraction pro- vided that we again assume that any excess volume of mixing is trivial. Thus It is a standard result of the Langmuir approach that e.g. thus (aKA/ FA&) = (as/Aa) (cSlcAFA) (eA/oS)' Since S is at infinite dilution CAcorresponds to the pure liquid value uiz.CAo= FA-' hence (Cs/CAVA)= Cs. Further (a8AlAaes) =asniaAiAaasn and at a full monolayer nga =AA and AAIA = Wa.Thus (aKA/rAK,)= CsWafn = C,/C; = K(*)s-l. We thus find VA &4B)s K(A>s This result is more attractive than is eqn (7) for a variety of reasons e.g. (a) the interaction on the surface is competitive; (b) there is no restriction with regard to relative molecular sizes; (c) there is no explicit assumption of additivity of retention; (d) it is couched in terms of the practically measurable quantity K rather than the indeterminable quantity K' and so can be tested. M. MCCANN H. PURWELL AND C. A. WELLINGTON It shares with the earlier model only the assumptions of surface homogeneity and infinite dilution of S both reasonable as a starting point.From the viewpoint of the practising chromatographer the latter need never prove a restriction whilst clearly the former can be modified in the light of practical experience. Although no assump- tion of sdvent ideality is made the solute activity is by implication independent of solvent composition. THE DIACHORIC MODEL In this model which is a development of our g.1.c. findings we make only the initial assuniptions of infinite dilution of S homogeneity of the adsorbent and estab- lishment of equilibrium between phases. We set up first the three-phase equilibrium for the A B and S system. I We now introduce one further assumption based upon our observation that with many mixed liquid solvents the liquid/gas partition coefficients for solute Sare given by the diachoric equation8 so a result identical with that derived viathe Langmuir approach.It is thus not obvious how one would distinguish between the two models but it is at least possible to test them not only generally but in particular since both KIg and KSgcan be determined experimentally for any system. The diachoric model has affinities with that proposed by Scott and K~cera,~ MIXED-SOLVENT THEORY FOR LIQUID CHROMATOGRAPHY which has been labelled the solvent-interaction m~del~-~ and seen as an alternative to the competitive model although Scott and Kucera did not take this view. Since the dead volume corrected retention volume is given by V = KW, i.e. Provided the excess volume of mixing is trivial qAcan be replaced by C and eqn (21) then becomes Scott and Kucera’s empirical relation.Despite the fact that the general form of the various equations presented is reason- ably widely known the literature contains no single example of a study carried out over the whole range p = 0-1. Scott and Kucera7 have provided by far the most comprehensive data yet their work relates only to variations of q~of 0 to ca. 0.3 with the single exception of elution of desoxycorticosterone alcohol by isopropanol + n-heptane mixtures covering 80 volume percent. Furthermore all published data relate to the high-retention-volume region where the greatest probability of failure of the relatively primitive models discussed here is to be expected. Even so it is clearly established that in virtually every instance there is some range of linearity of a plot of inverse retention volume (or capacity factor) against 9.Not surprisingly in the light of the foregoing discussion few examples of any linearity of inverse retention with mole fraction have been identified and log/log plots have been more widely used in this context. It appears to us that detailed theoretical development prior to acquisition of com-prehensive data is likely to be counter-productive. In consequence we present here data for a few typical systems for which data covering the whole solvent composition range have been acquired. EXPERIMENTAL The experimental unit comprised an Altex model 1OOA pump/model 20 microprocessor coupled with a 20 mm3 Rheodyne (7010) injector a Pye-Unicam 1.c.-U.V.detector and Hewlett-Packard 3380A integrator/timer. The columns used were commercial 25 cm x 4.6 mm i.d containing Hypersil(5p). The solvents diethyl ether and carbon tetrachloride werz of the best quality available and were stored over sodium. Both the columns and solvent reservoir were immersed in a water thermostat held at 25 “C. Solutes for injection were highly diluted in the solvent used for the particular elution since this was found to be essential for accurate and reproducible measurement of retention volume. Flow rates were continuously measured during elutions; this again was found to be essential since the short-term stability of the pump was no better than &0.5%. On this account also solute samples were injected repetitively at fixed time intervals usually in groups of ten.Flows measured at room tem- perature were corrected to 25 “C using tabulated values of the coefficient of cubical expan- sion. Each final data point represents an average of between thirty and fifty elutions per solute at each solvent composition. RESULTS Fig. 1 illustrates a plot of the data according to eqn (21) for phenol elution. Shown on this plot too is a series of error bars that define the range of values of inverse retention volume that would result from a spread of -&0.5% in the total retention volume i.e. that inclusive of dead volume at various values of inverse retention MCCANN H. PURNELL AND C. A. WELLINGTON I 'I I I I 0.5 1.1 volume fraction diethyl ether FIG.1.-Plot of inverse retention volume of phenol against volume fraction of diethyi ether in carbon tetrachloride (35 "C).Column 25 cm x 4.5 mm i.d. packed with Hypersil (5,~). volume. It is clear in the light of the associated error bars that there is no alternative to drawing a continuous straight line from 0 = 0 to 1. Fig. 2 shows corresponding plots for elution of nitroethane (A) and nitromethane (B). The error bars of fig. 1 are again relevant and it is again certain that between 8 = 0.1 and 1 the data are linear but curve sharply downward between 8 =0.1 and 0. Finally in fig. 3 are shown the results for elution of 3-phenyl-propan-1-01. In volume fraction diethyl ether FIG.2.-Plot of inverse retention volume of nitroethane (A) and nitromethane (B) against volume fraction of diethyl ether in carbon tetrachloride (25 "C).Column as in fig. 1. MIXED-SOLVENT THEORY FOR LIQUID CHROMATOGRAPHY this case the data are linear from 0 = 0 to ca. 0.8 and then curve so sharply that the retention volume is virtually constant to 8 = 1. The partition coefficient of nitromethane between the solvent pairs diethylether + water and carbon tetrachloride + water was approximately determined. The ratio which gives the value of the partition coefficient between diethyl ether and carbon tetrachloride was 6.8. 0 0.5 1.o volume fraction diethyl ether FIG.3.-Plot of inverse retention volume of 3-phenyl-propan-1-01 against volume fraction of diethyl ether in carbon tetrachloride (25 "C). Column as in fig. 1. DISCUSSION The zero intercepts at 0 = 0 shown in fig.1 and 3 are consistent with our total inability to elute these solutes with pure carbon tetrachloride in any reasonable time although they could subsequently be eluted by adding some ether to the eluent stream. They are however very soluble in carbon tetrachloride and so the effectively infinite retention must indicate essentially irreversible adsorption in the presence of carbon tetrachloride. Since the retention of phenol with pure diethyl ether is ex- tremely small while the solubility is considerable phenol clearly competes very weakly with the ether for adsorption sites. We can thus conclude that over the greater part of the solvent composition range the adsorbent surface is entirely ether coated and that in consequence the variation of retention is dominated by the change of solvent composition since the adsorbent surface is essentially a constant.According to the diachoric model [eqn (18)] the ratio of the intercepts at 9 = 0 and 1 of the extra- polated straight lines should correspond to the partition coefficient ratio for the sol- vent system diethylether + carbon tetrachloride. The value derivable from fig. 2 for the intercept ratio for nitromethane is ca. 6.0. In view of the substantial uncer- tainty in the extrapolation this is not unreasonable agreement with the value deter- mined by us. The data are therefore consistent with a model in which the ether provides the adsorbed monolayer over the greater part of the solvent composition range thus pro- viding a fixed fraction of surface for solute adsorption the extent of which is then determined by its activity in solution which in the present instance if no others is described by eqn (17).Only over a very short range in the region of 100% carbon tetrachloride is there competitive adsorption between the solvent components. Even here though there will be solvent effects and so the simple Langmuir (competitive) model would not necessarily apply. It seems clear that for such systems as these the M. MCCANN H. PURNELL AND C. A. WELLINGTON results for which are very similar to others so far p~blished,~*~-' the competitive model so far as it is currently developed is unlikely to be an adequate basis for general theoretical development.The foregoing view is entirely consistent with that expressed by Scott and Kucera,' who have found no exception to limited linearity of inverse retention with molarity of solvent. Further they have shown that in the low-q region the curvature is des-cribable in terms of Langmuir-like behaviour in a number of instances. However the situation is not entirely clear since in many instances the linear part of their data is correlated with a negative intercept at q = 0. This is clearly impossible and im- plies an upward curvature in this region. If this is true the solute is displaying anti- Langmuir behaviour which implies solute niulti-layer formation as distinct from the solvent multi-layering invoked by Scott and Kucera" in the case of certain other systems.The data of Slaats et al can all be interpreted in the same way since for each of their six systems the data provide very lengthy linear regions of the plots of inverse retention against q with in one effective linearity to q = 0 and in the other five a very rapid fall downward below q = 0.1. Interestingly for all six the value of inverse retention at q -=0 is essentially zero. We thus see that to all intents and purposes all data for normal-phase elution from silica published to date point in the same direction and indicate with some certainty that for these systems at least the competitive model is of limited validity. The semi- quantitative success of the Snyder system is presumably a consequence of the fact that the form of governing equation is the same as that for the solvent model.It must be noted that the view expressed above relates to solvent systems in which the relative adsorption of the solvent components is very disparate. For instance it is a simple matter to show via eqn (12) that for competitive adsorption to become not- able only below q = 0.1 there must be a factor of 100 difference in the Langmuir constants KA and KB. All systems so far studied seem more or less to fulfil this requirement as the adsorption isotherms of Scott and KuceralO show. It would clearly be of great interest now to study more widely solvent systems of comparable KA and KB,when more complex behaviour might be anticipated. Finally it is worth- while to point out that future studies must extend over the whole composition range since only then can all the information necessary to theoretical testing be determined.We thank the S.R.C. for a maintenance grant to M. M. L. R. Snyder PrincQles of Adsorption Chromatography (Marcel Dekker New York 1968); Anal. Chem. 1974 46 1384. J. Oscik Przem. Chem. 1965 44,129; J. Oscik and J. K. Rozylo Chroniatographia 1971 4 516; J. Oscik and G. Chojnacka J. Chromatogr. 1973 93 167. P. Jandera and J. Churacek. J. Chromatogr. 1974 9 207. E. Soczewinski and W. Golkiewicz Chromatographia 1971,4 501 ; 1973,6 269; E. Soczewin-ski J. Chromatogr. 1977 130 23. M. Jaroniec J. K. Rozylo and B. Oscik-Mendyk J. Chromatogr. 1979 179,237; M. Jaroniec B. Klepacka and J. Narkiewicz J. Chromatogr. 1979,170 299; M. Jaroniec J. K. Rozylo and W.Golkiewicz J. Chromatogr. 1979 178 27. E. H. Slaats J. C. Kraak W. J. T. Brugman and H. Poppe J. Chromatogr. 1978 149 255. R. P. W. Scott and P. Kucera J. Chrornatogr. 1978 149 93; 1979 171 37. J. H. Purnell and J. M. Vargas de Andrade J. Am. Chem. SOC.,1975,97,3585; R. J. Laub and J. H. Purnell J. Am. Chem. SOC.,1975. 98 30. R. P. W. Scott J. Chromatogr. 1976 122,35. lo R. P. W. Scott and P. Kucera J. Chromatogr. 1979 171 37.

ISSN:0301-5696    

DOI:10.1039/FS9801500083

出版商:RSC    

年代:1980

数据来源: RSC

摘要:

An Analysis based on Established Theory of Mixed-solvent Behaviour in Gas-Liquid Chromatography BYPETERF. TILEY School of Chemistry University of Bath Bath BA2 7AY Received 27th June 1980 Seven published sets of results on gas-liquid chromatographic mixed-solvent systems yield data on nearly two hundred ternary liquid systems and these were analysed for conformity with the Scatchard-Hildebrand-Flory-Huggins model of liquid mixtures. Whilst the expected quadratic (In KR,q) relation applies in most cases consistent values of solvent-solvent interaction parameters are not obtained when specific solute-solvent attractions exist. The use of alkane solutes appears to give constant and meaningful values of these parameters even when specific solvent-solvent interac- tion occurs.A computed simulation of solute-additive complexing in a solvent medium shows a linear (KR,c) relation at relatively low additive concentration but indicates that any calculation of formation constants by this method must involve very considerable quantitative uncertainty. Two recent books lv2 have reviewed the application of gas-liquid chromatography (g.1.c.) to the determination of thermodynamic data including the precautions neces- sary to ensure that the experimental measurements of retention volumes and partition coefficients may be directly related to the equilibrium between liquid and gas phases. Under such circumstances the measurement of partition coefficients in a binary mixed-solvent phase yields information about the ternary liquid system solute (1) + solvent (2) + solvent (3).The fact that the g.1.c. measurement is made with the solute approximating very closely to a state of infinite dilution gives a considerable simplification of any theoretical equations and consequently the solute can be used as a probe to explore the solvent-solvent interactions. The advantage of this approach is that once a given set of mixed-solvent columns have been prepared results can be rapidly obtained for a variety of probe molecules thus providing a severe test of any model of ternary systems. That ternary and multicomponent-fluid phase equilibria are of vital interest in chemical engineering is self-evident and the stimulus from this source has probably provoked as much theoretical work in this field as any other.Recent aspects are reported in ref. (3) wherein a plea can be found4 for greater academic attention to multicomponent mixtures of polar compounds. A relatively new theoretical model the Uniquac eq~ation,~ which effectively comprehends many previous well-known equations has met with considerable success in modelling the excess free energy and hence activity coefficients in liquid mixtures. The same approach has been extended to the calculation of activity coefficients from group contributions the Unifac method,6 although it is still controversial whether this approach is satisfactory for specific molecular association. Probably the most rigorous theories of liquid mixtures involving large non-polar molecules are those of Flory and co-workers and of Janini and Martire [e.g.see ref. (I) chap. 51. The latter theory has been applied quantitatively* and with some suc- cess to g.1.c. systems comprising n-alkane solutes in mixed n-alkane solvents. Exten-sion of the treatment to systems involving polar components with or without molecular MIXED-SOLVENT BEHAVIOUR complexing is not at present possible. However current thermodynamic theories of specific interactions in non-electrolytes have been recently re~iewed.~ In short workers in the g.1.c. field have no lack of theory for correlation and inter- pretation of their results. It is not the intention of this paper to investigate the precise application of the more sophisticated models to g.1.c. mixed-solvent systems but rather to explore the extent to which one of the long-established models may provide a useful approximation to such systems.The model is that of the " regular solution " and in considering it the remarks of one of its co-originators'O should be borne in mind namely " The best advice . . . is to use any model in so far as it helps but not to believe that any moderately simple model corresponds very closely to any real mixture ". In view of the varying definitions extant at the time Hildebrandl' at a Faraday Discussion in 1953 defined a regular solution as " one in which thermal agitation is sufficient to give practically complete randomness ". He pointed out that the concept of random mixing was embodied in the original Flory-Huggins expression for the entropy of mixing of molecules of very unequal size but would not be applicable to solutions in which there are specific " chemical " hydrogen-bonding or orientational effects.It is noteworthy that although the solubility-parameter interpretation of molecular interactions has often been associated with the regular-solution model the solubility-parameter concept is in no way an essential feature of the hypothesis of random mixing. Using the regular-solution model with the Flory-Huggins combinatorial term included results in the following eq~ation'~,'~ for g.1.c. mixed-solvent partition co- efficients KRC2,) KR(3)are the partition coefficients in the pure solvents KR(2.3) is that in the mixed solvent of volume fraction q2of component (2). V is the molal volume of the solute and ~23 is a parameter supposedly characteristic of the solvent-solvent interaction.Provided all partition-coefficient values have been measured at precisely the same temperature the application of eqn (1) requires no solute data other than molal volume. The testing of eqn (1) against experimental results involves three criteria which are in increasing order of severity (a) a quadratic dependency of In KR(2.3) on q2 (b) an independence of the value of ~23 on the solute and (c) the significance of the g.1.c. value of ~23 in relation to any non-g.1.c. phenomenon. Various published data sets of KR values in mixed-solvent systems are examined below in order to test the applicability of eqn (1). The complete antithesis of the regular solution is the micropartitioning model of a liquid mixture proposed by Purnell and LaubI4 and elaborated at length by Laub and Pecsok.2 On this model a homogeneous liquid mixture even at temperatures far removed from critical solution departs so far from random mixing that it may be treated as an assembly of microscopically immiscible groups of like molecules.The evidence for this theory is that a number of mixed-solvent g.1.c. systems show an approximately linear relation between KR(2,3) and q2as in eqn (2) Both MartireI5 and TileyI3 have shown that more conventional models can lead to such behaviour under given circumstances and until further theoretical developments have been explored it seems better to treat this as a purely empirical phenomenon " the mixed-solvent linear approximation ".It should be noted that there have been many P. F. TILEY serious theoretical attempts to treat the problem of non-random mixing two relatively recent ones being the Wilson16 and Uniquac' equations. COMPUTED RESULTS Eqn (1) was fitted to the systems show below. With the exception of data set (0) under each data set are listed the mixed solvents the temperature and the number of compositions studied and for each solute is given the computed value of the solvent- solvent interaction parameter ~~~/mol The curve-fitting procedure was based dm-3. on minimising the square of the residuals of In KR which is equivalent to assuming an equal percentage error on all KRvalues for a given solute. The percentage standard deviation of KRis reported as an average for all solutes in a given solvent system.For any given solute this was calculated as 0 100{~[(KR exptl. -~t calc.)/~Rexptl.l2/fi) 'a n DATA SET (0) In order to assess the constancy or otherwise of the computed ~23 values for a given solvent system an attempt was made to examine the effect of ran- dom error in the KR measurements. Sets of (KR p) values were generated with pre-set values of the parameters of eqn (1) using seven p values equally spaced. Using a statistical algorithm pseudo-random errors at a pre-set level of standard deviation were built into the KR values. Ten such data sets were generated for each level of standard deviation and all sets were then fitted to eqn (1) to recover the ~23 values.The stan- dard deviation of the ~23 values was calculated which was found to be independent of the values of the equation parameters and dependent only on the percentage error in KRand in particular on the range of compositions chosen. It would of course be dependent on the number of composition points but this effect was not investigated since about seven composition points i.e. five in addition to the pure solvents have been used by many workers. TABLEEFFECT OF RANDOM ERROR IN KRON THE ~23 VALUES OBTAINED FROM EQN (1) composition range dx23) p values pre-set (%) calc./mol dm-3 0.0-1.o 2 0.73 0.0-1.o 1 0.51 0.0-1 .o 0.5 0.21 0.0-0.5 0.5 0.95 0.0-0.25 0.5 3.9 The results are shown in table 1 where it is immediately apparent that significant values of ~23 can only be obtained by studies over the full composition range of p = 0.0-1.O.This point has already been noted by Parcher and Westlake,17 and therefore published g.1.c. results which only cover a restricted composition range have been omitted from this study. DATA SET (1)18 Squalane + dinonylphthalate at 30 "C using 12 composition points* for the following solutes pentane 2.29; hexane 2.36; heptane 2.39; octane 2.47; * The KRvalues are not published but the authors themselves report the fitting of eqn (1) to their results and the x23 values above were calculated from their reported values of x(=)~[=VZxz3with com-ponent (2) being dinonylphthalate]. MIXED-SOLVENT BEHAVIOUR cyclohexane 2.43 ; methylcyclohexane 2.46; benzene 3.70; toluene 3.41.Average a = 0.4". DATA SET (2)12 Dinonylphthalate + trinitrotoluene at 82 "C using 7 composition points for the following solutes octane 8.3; nonane 8.3; decane 8.3 ; methylcyclo-hexane 8.3; benzene 9.0; toluene 7.8 ; ethylbenzene 6.4; isopropylbenezene 6.6; o-xylene 8.2; m-xylene 8.5; p-xylene 7.5. Average a = 1.2:4. DATA SET (3)19 Squalane + dodecyl laurate at 60 "cusing 7 composition points for the following solutes cyclohexane 0.10; heptane 0.15; methylcyclohexane 0.21 ; ethylcyclohexane 0.17 ; cis-2-heptene 0.19 ; trans-2-heptene 0.19 ; trichlorethene 1.02; tetrachlorethene 0.42 ; benzene 0.52 ; toluene 0.48 ; xylene 0.55; fluoro-benzene 0.64; chlorobenzene 0.94; methoxybenzene 0.78 ; benzotrifluoride 1.06; octaflurotoluene 2.32.Average o = 0.3%. DATA SET (4)20 Squalane + dibutyl tetrachlorophthalate at 80.3 "C using 8 composi-tion points for the following solutes (T = thiophene) cyclopentene 0.1 ; cyclopen-tadiene 2.1 ; benzene 3.4; diethyl ether 0.03; divinyl ether 2.1 ; furan 4.4; methyl- furan 2.8; T 4.6; 2-methylT 2.5; 3-methylT 3.3; 2,5-dimethylT 3.0; 2-ethylT 25; 2-chloroT 4.3; 2,5-dichloroT 2.9; 2-bromoT 4.9; pyrrole 16.2; l-methyl- pyrrole 6.6. Average a (omitting last two) = 0.50//0. DATA SET (5)21 Tetradecane + tributyl phosphate at 25 "C using 10 composition points for the following solutes pentane 7.3; hexane 6.5; cyclohexane 7.5; ben-zene 10.3 ; carbon tetrachloride 8.6; chloroform 50.3; dichloromethane 42.9 ; dichloroethane 3 1.9 ; trichloroethane 13.1.Average o (first five only) = 3.0%. DATA SET (6)28 Two mixed-solvent systems were studied namely squalane + dode-canol and squalane + lauronitrile at temperatures 40-80 "Cusing 6 composition points for 27 aliphatic solutes. Apart from pentene hexane and heptene all the soluest were polar comprising alcohols ketones acetates halides cyanides and nitroalkanes. The fit of eqn (1) was generally poor (a = 3-10%) and the ~23 values varied widely frcm 2 to 40 mol dm-3. The lowest ~23 values for both systems were obtained with the non-polar solutes. DATA SET (7)23 Using 5-9 composition points the systems shown below were studied at 25 "C (except System 1 at 43 OC) System 1 phenol -/-aniline. System 2 m-cresol + benzylamine. System 3 m-cresol -+ benzyl alcohol.System 4 m-cresol -k acetophenone. System 5 m-cresol + benzonitrile. System 6 benzylamine + aniline. System 7 nitrobenzene + aniline. System 8 decalin + benzyl alcohol. System 9 acetophenone + tetrabromethane. System 10 acetophenone + dichlor-acetic acid. System 11 N-methyl pyrollidone + dimethyl sulphoxide. System 12 benzonitrile + dimethyl sulphoxide. Seven solutes were used as shown in table 2. Note that these published data are of y" values of the solute which were converted to equivalent KR data for use in eqn (1) on the basis that KR(2,3) = a/y" V(2.3) where a is a constant for a given solute which disappears in the derivation of eqn (1). DISCUSSION QUADRATIC (In K,,Cp) RELATION The fit of eqn (1) appears to be within experimental error for most results data set (6) being the exception.The linear (KR,p) relation is certainly not applicable to data sets (l) (2) (5) and (7) but gives a considerably better fit for data set (4) (with a remarkably low standard deviation of 0.1 %) a marginally worse fit for data set (3) and there are a few instances in data set (6) which seem to show a linear relation. It is interesting that the results of Eon et al. in data set (4) were on their model of com- P. F. TILEY 97 plexing shown to give a linear relation between (V;2,3) &(2,3)) and x2,where V(2,3) is the molar volume of the mixed phase and x2 the mole fraction of additive. Noting that for zero excess volume V(2,3)= x2V2+ (1 -x2)V3and that q2 = X?V~/V(~,~) it can readily be shown that the linear relation of Eon et al.is mathematically identical with eqn (2)of Purnell and Laub. In other words a linear (KR,y)relation can equally well be claimed to support a theory of complexing as to support a micro-partitioning theory. Two further points must be made. Firstly a g.1.c. column prepared from a TABLE 2.-cOMPUTED x23 VALUES FROM DATA SET (7) system 1 2 3 4 5 6 pentane -4.2 -5.6 -3.0 -1.8 hexane -4.5 -18.5 -4.9 -2.2 -1.4 1.1 -heptane -3.8 -4.0 -2.0 -1.4 cyclohexane -3.9 -18.3 -4.6 -2.5 -1.9 0.8 methyl butene -5.2 -25.8 --2.2 0.0 isoprene -4.9 -23.3 --2.0 -1.1 benzene -2.9 -18.6 -1.6 -2.4 -0.6 -2.7 o(%) 2.2 2.0 3.5 2.5 3.1 1.7 system 7 8 9 10 11 12 pentane 3.1 I 0.2 -8.6 hexane 3.1 13.7 0.0 -3.9 10.4 7.4 hep tane 2.5 -0.0 --6.8 cyclohexane 2.3 12.6 0.1 -4.7 11.1 9.2 methyl butene 3.2 11.3 -0.07 -6.3 9.6 5.3 isoprene 2.0 10.4 -1.7 -6.9 9.8 4.5 benzene 0.6 9.4 -2.8 -6.6 11.8 5.2 o(%) 1.5 3.7 1.8 2.6 2.6 2.2 mechanical mixture of two separately coated packings (" mixed-bed '' column) would certainly give a linear (KR q) relation so that the partioning behaviour of such a column should be predictable from eqn (2).Provided both phases exist as liquids at the column working temperature then such a column could well be preferable for any analytical purposes. The only minor disadvantage is that a mixed-bed column would be thermodynamically unstable in that gas-phase diffusion would eventually lead to a homogeneous mixed-solvent phase with depending on the system changing partition- ing characteristics.The second point is that if certain solutes particularly the alkanes (see below) give approximately equal KRvalues in the two pure solvents then eqn (1) should approxi- mate very closely to eqn (2) since a ~23 value close to zero would be expected.I3 This is clearly the case with data set (3). CONSTANCY OF X23 VALUES The second test of the model used in eqn (1) is that the ~23 values for a given solvent system should be independent of the solute. Even allowing for the effect of random experimental error as simulated in the results shown in table 1 it is obvious that the MIXED-SOLVENT BEHAVIOUR criterion is not met for any of the data sets. In view of the fact that eqn (1) is based on the hypothesis of random mixing this is not surprising since all the solvent systems include at least one polar constituent and many of the solutes used are themselves polar.Specific attractions are to be expected whether these be dipole association or charge-transfer complexing and indeed much of the work reported is claimed to be studies of complex formation. However when no specific solute-solvent interaction would be expected as with the alkane solutes a study of the results shows a reasonable constancy of ~23 (it is regretted that some workers report no results for any alkanes). The very careful work of data set (l) involving twelve composition points shows remarkably con- sistent ~23 values for the alkanes as also do the results in data sets (2) and (3).In the case of data set (7) where a high accuracy was not claimed (1-3 %) and possible experi- mental error in ~23 of around $11 mol dm-3 would be anticipated the results for the alkanes show reasonable constancy. The only other non-polar solutes studied have been either the alkenes or the aromatic hydrocarbons. Data set (3) seems to show the alkenes yielding the same ~23 values as the alkanes but data sets (l) (2) (3) and to some extent (7) show that benzene and its homologues show a divergence. This is readily explicable on the basis of specific interaction between the delocalised electrons of the benzene ring and the polar group on the solvent molecule. A further point arises as to whether for a given system ~23 is independent of the composition of the mixed-solvent phase.Perry and Tiley l2concluded that even with the alkanes ~23 showed a linear dependence on composition which effectively implied a cubic (In KR,9)relation. In quantifying the excess free energy of binary liquid systems many workers have proposed that two parameters (at least) are required in addition to the combinatorial term or its equivalent. For exact theoretical descrip- tion this may well be true but it must be recognised that once additional adjustable parameters are included in any fitting of experimental results the molecular signifi- cance of the values so obtained may in fact become much more nebulous. In parti- cular there arises in the curve-fitting process a possible correlation between any pair of parameters so that no unique set of values exists.z4 For that reason this paper does not present any investigation of the composition-dependence (if any) of ~23.SIGNIFICANCE OF G.L.C. X23 VALUES The careful and painstaking vacuum microbalance studies of vapour-liquid equilibrium carried out by Ashworth and Hookerz5 effectively validate the g.1.c. determination of ~23 for the squalane + dinonylphthalate system. For the alkanes the former give a mean value of 2.69 mol dm-3 compared with 2.41 mol dm-3 for data set (1) and for benzene values of 3.80 and 3.70 mol dm-3 respectively. Since the microbalance studies were made at finite solute concentration in the ternary systems this seems a very encouraging result. The conclusion that g.1.c. studies of alkane solutes yield a consistent ~23 value for a given solvent system leads to the question of the thermodynamic significance of ~23 in describing the excess free energy of the binary mixture of solvents in particular with respect to binary phase equilibria.The use of probe solutes to explore interactions in polymer systemsz6 is based on the anticipation that g.1.c. studies could be legitimately interpreted for such purposes. The very extensive studies shown in data set (7) should provide a test of this hypothesis since the volatility of the solvents used allows a com-parison with orthodox studies of binary vapour-liquid equilibrium. Unfortunately only one of the solvent systems appears to have been studied at a P. F. TILEY temperature anywhere near 25 "C.Holtzlander and Riggle2' report isothermal vapour-liquid studies for the binary aniline + nitrobenzene system at 57 "C from which the binary ym values may be extrapolated as 1.24 and 1.19 respectively. These yield a mean value of ~23 = 2.0 mol dm-3 at 57 "Ccompared with the g.1.c. mean value for the alkanes of 2.7 mol dm-3 at 25 "C as shown for System 7 in table 2. On simple theory,12~23 would be expected to be inversely proportional to temperature so this shows reasonable agreement between the two values. The highest positive mean value for the alkanes in table 2 is 13.2 mol dm-3 which occurs for System 8 decalin + benzyl alcohol. Theory2* shows that a critical solu- tion temperature should occur when Taking V2 (decalin) = 0.157 dm3 and V3 (benzyl alcohol) = 0.104 dm3 this gives ~23 = 15.8 mol dm3 and the two liquids should be completely miscible at 25 "C,which they are.However some simple laboratory experiments with laboratory-grade re- agents showed critical unmixing at around 2 "C. Again allowing for an inverse temperature dependence of ~23 and for considerable uncertainty in the g.1.c. value there seems some measure of agreement here. For the first five systems of table 2 negative ~23 values are computed indicative of specific attractions existing in these solvent pairs. This would be expected from the chemical nature of the components where acid-base (or acceptor-donor) interaction must occur in each case. Systems 2-5 show the interaction of m-cresol with benzyl- amine benzyl alcohol acetophenone and benzonitrile respectively with ~23 values of -18.4 -4.8 -2.4 and -1.6 mol dm-3.The relative strength of these acceptor- donor interactions as measured by g.1.c. show good qualitative agreement with the '' solvent donicities " (donor power) evaluated by G~tmann~~ which give benzyl- amine 9 benzyl alcohol > acetophenone > benzonitrile. The fact that ~23for phenol + aniline (-4.1) is much less negative than for m-cresol + benzylamine (-18.4) is also explicable on this basis. It seems possible that even for these systems where solvent-solvent association must certainly predominate the use of eqn (1) to interpret the g.1.c. results for alkane solutes yields ~23 values which are at least qualita- tively meaningful with respect to the solvent-solvent interactions.Whilst the results of data set (7) provide a useful test of the proposed model it is not in fact a complete test because all components solutes and solvents are of similar size. Hence the Flory-Huggins combinatorial term plays a very small part and the omission of this term giving the original Scatchard-Hildebrand theory would in most cases have little effect on the calculated ~23 values. MOLECULAR COMPLEXING A model postulating the existence of a 1 :1 complex between solute (1) and additive (2) in solvent (3) has given rise to the well-known eauation where Kc/dm3 mol-1 is the formation constant of the complex and C2/mol dm-3 the concentration of additive. Leaving aside the question of the thermodynamic formu- lation of the equilibrium constant Kc,eqn (3) still remains an approximation.The non-ideal behaviour of the binary solute (1)-solvent (3) interaction is describable in terms of non-specific interactions using an activity coefficient y&. However the derivation of eqn (3) assumes that any perturbation of this non-ideality produced by additive (2) can be ascribed entirely to specific interactions conforming to the law of MIXED-SOLVENT BEHAVIOUR chemical equilibrium. The use of alkanes as solutes where complexing is highly improbable has shown that this assumption is not tenable in many cases. Martire30 has published theoretical studies of the approximate nature of eqn (3) and has proposed a better formulation thus where a is a parameter reflecting the " physical " non-ideality perturbation of the ternary system and the variation in molal volume produced by the additive.At low concentrations Martire suggested that a should be roughly constant so that a linear (KR c) relation results but for a given solvent system a will be solute-dependent. Hence Kc values derived from eqn (3) may be in error by uncertain amounts. Using the model and the system parameters of Perry and Tiley,12 a simulated test of eqn (4) was carried out and an estimate made of the magnitude of a for the tri- nitroluene (TNT) + dinonylphthalate (DNP) system wj th benzene and its homo- logues as solutes. In this case hydrocarbon-TNT complexes were postulated on the basis of spectroscopic evidence. Values of KR were computed at TNT concentrations 0-0.5 mol dmV3 using the thermodynamically exact equation l2 K is the thermodynamic formation constant based on pure-component standard states and subscript 4 refers to the complex.Singly-subscripted y values refer to the mixed-solvent phase and were calculated from regular solution theory as describedl2 and the necessary parameters were assigned values taken from ref. (12). The com- puted KRvalues were then fitted to eqn (3) to obtain values of the " apparent '' K in each case. These were compared with the " true " K values calculated from the assigned values of K and from infinite dilution y values for the components in the solvent medium thus,12 Values of cx were obtained as the differences between apparent and true K values and are shown in table 3.The use of this theoretical model with the parameters from ref. (12) produces good TABLE 3.-vALUES OF o! COMPUTED USING THE PARAMETERS OF THE TNT $-DNP SYSTEM12 apparent K true K a 0.088 0.035 0.043 0.081 0.050 0.037 0.041 0.023 0.018 0.001 0.026 -0.025 0.097 0.051 0.046 0.072 0.048 0.024 0.076 0.062 0.014 linear (KR c) relations in accordance with eqn (4) with correlation coefficients all in excess of 0.995 but as can be seen with these low K values the " correcting term " a is of similar magnitude to K itself and varies from solute to solute. As a result even the relative order of the apparent K values is not the same as the true order. It is P. F. TILEY 101 noteworthy that the error resulting from the use of eqn (3) is much greater than that produced solely by the use of an equilibrium constant in terms of concentrations rather than activities.If eqn (6) is used to calculate K at finite concentrations of additive the variation of K is only ca. 10% at C2 = 0.5 mol dm-3. Being based on the parameters found for the TNT + DNP system the results of table 3 cannot obviously be generalised. Owing to the complexity of the expressions for activity coefficients it is not possible mathematically to reduce eqn (5) to any simple variant of eqn (3) such as eqn (4) and indeed Ma~tire~~ derived cqn (4) from a purely empirical polynomial expansion. Further computational studies would be necessary to examine the effect of different values of the parameters on the value of a.Indications are that a can be either smaller or very much larger than the values of table 3. The Martire-RiedlS1 approach to g.1.c. studies of complexing seems less prone to inherent error provided one accepts the basis on which it is founded. This method depends on using a reference solvent as nearly as possible analogous to the complexing solvent with respect to molal volume and polarisability thereby equalising the corn- binatorial term and the dispersion forces contribution to molecular interactions. Any difference in partitioning behaviour after discounting these effects is then ascribed to specific complex formation describable by the law of chemical equilibrium. On the other hand Perry and Tiley12 attempted a quantitative estimate of the non-specific interactions by use of a semi-empirical solubility parameter treatment and any residual effect was then ascribed to complexing.Both approaches have some .logical justifica- tion but it is only fair to indicate that other workers in the g.1.c. field have not used any model of chemical equilibrium in order to quantify the effects of specific inter- actions. For example Karger et aZ.32used an expanded solubility parameter treat- ment which was claimed to include terms for orientation and induction forces as well as for donor-acceptor interaction. Meyer and Bai~cchi~~ used g.1.c. measurements to determine the enthalpy and entropy changes of specific interactions without pro- ceeding via calculation of association constants.Provided the limitations of each approach are appreciated then the choice of model may well be determined by the object of the investigation. CONCLUSION As stated previously,12 g.1.c. studies in the context of this paper are thermodynamic studies of phase equilibrium and the seeking of a unique interpretation in terms of molecular interactions is akin to debating how many angels can dance on the point of a pin. The pragmatic approach of Scatchard,lo quoted above has much to commend it bearing in mind that pragmatism is not to be equated with pure empiricism. In framing models of mixed-solvent g.1.c. systems it would seem wise to take account of the considerable body of established work on liquid mixtures; and today it is a truism to say that the use of the computer facilitates studies of the quantitative impli- cations of theoretical models.The analysis presented in this paper makes only trivial demands on computer resources but reveals the limitations both of regular solution (Scatchard-Hildebrand-Flory-Huggins) theory and of g.1.c. studies of molecular complexes. I am indebted to Dr. A. J. Ashworth and Mr. D. M. Hooker for helpful discussions and to Mr. N. P. Morgan for some assistance with the computing. J. R. Conder and C. L. Young Physicocheniical Measurement by Gas Chromatography (Wiley Chichester 1979) chap. 5 and 6. 102 MIXED-SOLVENT BEHAVIOUR R. J. Laub and R. L. Pecsok Physicochemical Applications of Gas Chromatography (Wiley Chichester 1977) chap. 5 and 6. Phase Equilibria and Fluid Properties in the Chemical Industry ed.T. S. Storvick and S.I. Sand- Ier ACS Symp. Ser. no. 60 (American Chemical Society Washington D.C. 1977). J. H. Prausnitz Phase Equilibria and Fluid Properties in the Chemical Industry ed. T. S. Storvick and S. I. Sandler ACS Symp. Ser. no. 60 (American Chemical Society Washington D.C. 1977) p. 59. D. S. Abrams and J. H. Prausnitz AZChE J. 1975 21 116. A. Fredenslund J. Gmehling and P. Ramussen Vapour-Liquid Equilibria using Unifac(Elsevier Amsterdam 1977). T. F. Anderson and J. M. Prausnitz Ind. Eng. Chem. Process Res. Dev. 1978 17 552. * R. J. Laub D. E. Martire and J. H. Purnell J. Chem. SOC. Faraday Trans. 2 1978,74 213. A. G. Williamson Chemical Thermodynamics ed. M. L. McGlashan (Specialist Periodical Report Chemical Society London 1978) vol.2 chap. 6. lo G. Scatchard Chem. Rev. 1949 44,7. l1 J. H. Hildebrand Discuss. Faraday Sac. 1953 15 9. l2 R. W. Perry and P. F. Tiley J. Chem. SOC. Faraday Trans. I 1978 74 1655. l3 P. F. Tiley J. Chromatogr. 1979 179 247. l4 R. J. Laub and J. H. Purnell J. Am. Chem. SOC. 1976,98 30. l5 D. E. Martire Anal. Chem. 1976 48 398. l6 G. M. Wilson J. Am. Chem. SOC. 1964 86 135. l7 J. F. Parcher and T. N. Westlake J. Phys. Chem. 1977 81 307. M. W. P. Harbison R. J. Laub D. E. Martire J. H. Purnell and P. S. Williams,J. Phys. Chem. 1979 83 1262. l9 L. Mathiasson and R. Jonsson J. Chromatogr. 1974 101 339. 2o C. Eon C. Pommier and G. Guichon J. Phys. Chem. 1971 75 2632. 21 M. M.Kopecni Z. E. Ilic and S. K. Milonjic J. Chromatogr. Sci. 1979 17 253. 22 A. B. Littlewood and F. W. Willmott Anal. Chem. 1966 38 1031. 23 P. Vernier C. Raimbault and H. Renon J. Chim. Phys. Biol. 1969 66 960. 24 J. H. Prausnitz Phase Equilibria and Fluid Properties in the Chemical Industry ed. T. S. Storvick and S. I. Sandler ACS Symp. Ser. no. 60 (American Chemical Society Washington D.C. 1977) p. 43. 25 A. J. Ashworth and D. M. Hooker J. Chromatogr. 1979 174 307. 26 D. D. Deshpande D. Patterson H. P. Schreiber and C. S. Su Macromolecules 1974 7 530. 27 G. W. Holtzlander and J. W. Riggle AZChE J. 1955 1 312. 28 T. L. Hill Introduction to Statistical Thermodynamics (Addison-Wesley London 1960) p. 409. 29 V. Gutmann Electrochim. Acta 1976 21 661. 30 D. E. Martire Anal. Chem. 1973 46 1712. 31 D. E. Martire and P. Riedl J. Phys. Chem. 1968 72 3478. 32 B. L. Karger L. R. Snyder and C. Eon Anal. Chem. 1978 50 2126. 33 E. F. Meyer and F. A. Baiocchi J. Am. Chem. SOC. 1977 99,6206.

ISSN:0301-5696    

DOI:10.1039/FS9801500093

出版商:RSC    

年代:1980

数据来源: RSC

摘要:

Activity Coefficients at Infinite Dilution from Gas-Liquid Chromatography BY TREVOR M. LETCHER Department of Chemistry Rhodes University Grahamstown 6140 South Africa Received 22nd July 1980 Activity coefficients can be determined at infinite dilution with great precision using gas-liquid chromatography. The development of this technique is reviewed and recent results and treatment of results discussed. Gas-liquid chromatography (g.1.c.) is undoubtedly one of the most useful scientific inventions of the century. Its rapid development from James and Martin’s first experiment of 1952 bears testimony to its importance and usefulness.’ Since the basic process is an equilibration of a solute between two immiscible phases the chromato- graphic technique may be used to measure such physical properties as activity co- efficients second virial coefficients of gas mixtures partition coefficients adsorption and partition isotherms and complex formation constants.Other properties which can be measured with less accuracy from secondary measurements or from tempera- ture-variation studies include surface areas heats of adsorption excess enthalpies and excess entropies of solution. A number of reviews and discussions on these measurements have appeared in the literat~re.~’~~ The present work is restricted to a review of activity coefficient measurements. Activity coefficients are perhaps the most important and fundamental property in the thermodynamic study of liquid mixtures. They can be used to obtain all the other thermodynamic solution properties such as excess enthalpies.Gas-liquid elu- tion chromatography offers a rapid method of determining activity coefficients at infinite dilution. Conder and Purnell have developed a method of determining acti- vity coefficients at finite concentration^^^-^^ and it has recently been used by other worker~.l~-~~ To do this the elution technique must be supplemented by controlling the solute concentration in the carrier gas. In this type of chromatography the recor- der no longer records a peak but an integral form of the peak. This paper will be concerned with activity coefficients at infinite dilution obtained by the elution method. The nature of gas-liquid chromatography unfortunately limits the choice of liquid mixture.The solute must be rather volatile if retention times are to be reasonable and the solvent or stationary phase must be a liquid at the temperature of the experi- ment with a sufficiently low vapour pressure so as not to “ bleed ” off the column during the course of an experiment. Further limitations restrict the choice of carrier gas to those which are insoluble or nearly insoluble in the stationary phase and the range of mixtures to those in which adsorption effects of any kind are negligible. For systems which fall within these limitations accurate activity coefficients at infinite dilu- tion can be obtained which would be very difficult to determine in other ways. The restrictions on some of these properties can be relaxed but it does involve added experimentation.For example solvent bleeding can be overcome by either weighing the column before and after or by inserting a pre-column containing the 104 ACTIVITY COEFFICIENTS FROM G. L.C. volatile solvent. in any event regular checks must be made on the column specifica- tions. Adsorption effects have been dealt with by measuring retentions using columns of different solvent loadings.22 Activity coefficients at infinite dilution are important and useful properties to chemical engineers solution chemists and theoreticians. To the chemical engineer they are of interest in the design of plants that involve liquid-vapour equilibrium. To the solution chemist they are important in understanding the mixing process. Perhaps their most interesting and successful application has been in the testing of various solution theories.This is possibly due to the large number of systems that have been analysed (because the results can be obtained so rapidly using g.1.c.) and to the infinitely-dilute condition which in many cases presents the theorist with simpler equations and fewer complications. THEORETICAL THE UNREFINED THEORY The idea of gas-liquid chromatography goes back to 1941 when Martin and Synge mentioned in a paper on general ~hromatography~~ that it should be possible to use a gas as a mobile phase and a liquid as a stationary phase. They related the equilibrium partition coefficient K to retardation properties using a plate theory. Their general equation when considered in relation to gas-liquid chromatography (for zero pressure difference across column) relates the retention volume of the solute VR,to the gas hold-up volume V, and the solvent volume V3according to VR = vc + KV3.(1) (In this work the solute will be referred to as component 1 the carrier gas as com- ponent 2 and the solvent as component 3.) The idea was not taken up and it was left to Martin this time in conjunction with James,' to describe a separation using g.1.c. (They separated some volatile fatty acids by partitioning between nitrogen and a mixed liquid phase of silicone oil and stearic acid.) More important they presented the first theory specifically applicable to gas-liquid chromatography by taking into account the compressibility of the mobile phase. This involved applying a correction factor to the gas volumes of eqn (1).In terms of Everett's notation24 this correction term J; can be generalised as n (Pi/P0)"-1 Jr = -m (PJP,)"-1 where Piand Porefer to inlet and outlet pressures respectively. In 1956 Martin25 again hinted at future developments in this field and speculated that gas-liquid chromatography might be useful in studying the solution thermo- dynamics of gas and liquid phases. In the same year Porter et aZ.26related the net retention volume V, to the activity coefficient of the solute at infinite dilution YE according to where n3is the amount of liquid solvent on the column and Pothe vapour pressure of the solute at the temperature T of the experiment. The retention volume V is determined from the column outlet flow-rate U, by T.M. LETCHER where tRand tG are the retention times for the solute and an unretained gas and V is the dead-space volume or gas hold-up volume at mean column pressure PoJ;. This theory assumes that (a) idealized chromatographic conditions exist ; (b)the car- rier gas and solute vapour behave as ideal gases; (c) adsorption effects of any kind are absent and (d) the carrier gas is insoluble in the solvent. The measurements based on the above theory give activity coefficients for n-alkane systems which are within a few percent of the results obtained from the more refined theory discussed below. REFINEMENTS TO THE THEORY The next major step in the evolution of determinations of accurate activity co-efficients came in 1961 when Everett and Stoddard2' took into account the solute vapour and solute $-carrier-gas imperfections.An important outcome of this work was the possibility of obtaining the mixed virial coefficient B12. DestyZ8 applied these ideas to the determination of BI2values and used an extrapolation procedure based on the equation In VN= In VNo+ ppoJ3 (5) where V," is the extrapolated retention volume at zer9 mean column pressure where V;is the molar volume of the solute and Bllthe second virial coefficient of pure solute. p is given by where Vy is the partial molar volume of solute at infinite dilution in the stationary phase. Both of the above treatments assume that the partition function does not change significantly along the column but remains constant at the mean column pres- sure value.Everett24 attempted to avoid this assumption and developed a detailed theory of the pressure-drop effect which led to a different extrapolation procedure. The Bristol group29-31 reformulated the differential equation describing the local elu- tion rate in the column and suggested a third extrapolation procedure In VN = In VN" + ppnJ:. (8) This was tested using a numerical integration procedure and shown to be superior to the previous extrapolation techniques.28*24 Moreover this theory takes into account small imperfections in the carrier gas and is thus suitable for carrier gases such as hydrogen helium nitrogen oxygen and argon. For carrier gases which are appreciably non-ideal they proposed where b = B2,/RTand is the second virial coefficient of the carrier gas.A further refinement done by the Bristol group32 involved the solubility of the carrier gas in the stationary liquid. They showed neglecting second-order effects that the retention volume (for a pressure drop across the column of (200 kPa) is related to pressure po according to In Vk = In F'; + /3'poJ$ (10) where /3' = /?-/-3,[1 -a ;2y"1 I06 ACTIVITY COEFFICIENTS FROM G.L.C. and A is defined by the expansion of x2 (mole fraction of carrier gas in the solvent) as a series in the local carrier-gas pressure x2 = AP2 + pP2”+ . . . . (12) where p is the coefficient of the second-order pressure term. Eqn (1 1) includes the two effects resulting from the solubility of the carrier gas namely the increase in the number of moles of stationary liquid and the change in the activity coefficient due to the change in the nature of stationary liquid.The importance of deriving eqn (10) and (1 1) lies in the fact that it shows that because the quantity (2 1n yz/2x2)is virtually inaccessible the property BI2 cannot be obtained unambiguously from g.1.c. measure- ments with a solvent in which the carrier gas is appreciably soluble. The significance of the carrier-gas solubility and eqn (1 1) in particular has been discussed in terms of experimental results by the Bristol gr~up~’’~~ and by Pecsok and Wind~or.~~ THE TRUE RETENTION TIME Existing theories of g.1.c. predict a unique retention time. Experimentally how- ever a peak spread is observed so it is therefore necessary to speculate where on this peak the true retention time may be found.I have included some of the peak pro- perties that have been used to define this “ true ” retention time. The peak-initial time tI and peak-final time tF are determined from the intersection of the base-line time with the tangents to the leading edge and trailing edge respectively. The peak- tangent time tT,on the other hand is determined from the point of intersection of the tangents to the leading edge and trailing edge. The peak-half-area time is defined as the net time that divides the area under the peak into two equal parts. The peak- maximum time tMis obtained from the time of peak maximum and the peak-average retention time tIFis obtained from the mean of tI and tF.Because of the lack of knowledge concerning the detailed processes taking place in a g.1.c. column the present-day theories cannot hope to unravel the problems associ- ated with peak asymmetry. Possible causes that have been given by various wor- kers 7*35*36including eddy diffusion in packed columns non-equilibria between phases sample size surface heterogeneity and non-zero response time of the detecting system. A somewhat surprising answer to the question of true retention time was given in some of the earlier attempts to derive thermodynamic properties from g.1.c. measure- ment~.~~-~~*~~ The peak initial time tl was used the justification being that this gave the best agreement with data from static measurements.Later it was that if sufficient care was taken to achieve effective infinite dilution of the solute and if the measurements were done over a range of pressure and analysed by a theory which culminates in eqn (8) then the peak tangent gave good agreement with static measure- ments. This seems far more reasonable as it is somewhere near the top of the peak although for skew peaks this may not be true. Many worker~~~q~’ have discussed the “ first time moment ” or “ centre of gravity ” of a chromatographic peak undergoing elution. In the absence of longitudinal or eddy diffusion this property has been shown to be equal to the ideal thermodynamic retention time for zero-pressure-drop columns. More recently theories regarding the first time moment has been extended by and by Buffham4’ to include pressure- drop columns and the first time moment has been related to thermodynamic proper- ties.Buffham used the “ mean residence time ” f which is equivalent to the “ first time moment ” f = /om t ci(t)dt //om ci(t)dt T. M. LETCHER 107 where cl(t)is the response of the solute molecules i at time t after injection. He related ito thermodynamic properties and showed that it can be used to obtain results similar to those of Cruickshank et aZ.,30Stalkup and Kobaya~hi~~ and Koonce et ~l.,~~ without the restrictions previously considered necessary.45 has recently arrived at a solution to the g.1.c. situation for which the diffu- sion of solute in the liquid phase is the rate-determining step for equilibration.In this work Hicks shows that for this situation the peak-average retention volume VIF can be used to obtain thermodynamic properties. To support this he showed that the flow-rate dependence of retention volume as observed in the benzene + glycerol system22 disappears if peak-average retention volumes are used instead of peak-tangent retention volumes. Experimentally tIFcan be obtained directly from the peak tan- gents without elaborate equipment or with the problems associated with the “ first time m~rnent.”~~*~’ The flow independence of V, does seem to point to tlFbeing an excellent estimate of the ideal retention time although much more work is required on systems with asymmetric peaks to show its range of validity. The first moment is possibly better at least in theory because it does not require assumptions about the nature of the kinetic processes governing the equilibration between phases but it is experimentally very inconvenient.The problem of locating the true retention volume is however usually only important for solutes which have short residence times and have very asymmetric peaks. It is only then that the peak-average retention times differ significantly from the peak-maximum peak-tangent peak-half-area or mean-residence retention times. SURFACE ADSORPTION EFFECTS Surface adsorption is perhaps the most important limitation of the g.1.c. for deter- mining activity coefficients. Martin48 suggested that this adsorption at the gas-liquid interface can be related to the retention volume and proposed VN = kV3 + k A3 where k is the adsorption coefficient and A3 the area of the liquid surface.Unfor-tunately there is no way of separately determining these two terms by chromatographic experiments alone and an extrapolation procedure must be used to obtain VN.22 RESULTS AND EQUIPMENT GENERAL If the activity coefficients are not required to any great accuracy (&5% uncer-tainty) then eqn (3) will suffice provided that adsorption effects are insignificant. For such measurements a simple gas chromatograph with the column in a well-controlled water bath is suitable. The type of detection is not important so long as sample detection of 0.5 pmol is possible. The advantage of katharometer detection is that the flow meter can be placed downstream of the column.For more accurate determinations of activity coefficients it is necessary to take into account carrier-gas and solute imperfections. Assuming no adsorption of the solute on the solvent or solid support and assuming the carrier gas is not appreciably soluble in the solvent then eqn (10) is suitable The uncertainty in the activity coefficients determined in this way has been estimated ACTIVITY COEFFICIENTS FROM G.L.C. to be <0.4%. Application of eqn (10) usually infers the determination of the reten- tion volume as a function of pressure (p J;) from which the mixed second virial co- efficient BI2,and the activity coefficient yg can be obtained. Medium-high-pressure g.1.c. does require more sophisticated apparatus.It must include a high-pressure injector pressure control values special metal-glass seals and a flow meter capable of operating at pressures of 1200 kPa. A detailed breakdown of equipment design will not be given here. This has been well covered by recent review^.^*'^-^^ The activity coefficients of many hundreds of systems have been determined using this method. I will discuss some of the results especially those that have a bearing on solution thermodynamics. The results before I967 have been discussed in reviews by Young’ and Kobayashi et aL5 Many of these results were done on ill-defined sub- stances such as silicon oils and apiezon greases and will not be discussed here. Most of the systems discussed here have been reviewed by the author.I2 Condor and Young13 have also recently reviewed g.1.c.-determined activity coefficients.n-ALKANE MIXTURES AND RELATED SYSTEMS The activity coefficients for these systems measured by g.1.c. have been the most useful and successful in testing solution theories. The repetitive nature of the carbon chains make these systems ideal in this respect. Lattice theories in particular have proved very successful. Alkane systems are fortunately convenient to study because the properties such as molar volumes vapour pressures and virial coefficients have been well documented. The pioneering work for these systems was done by Kwantes and Rijnder~~~ who studied normal and branched-chain alkanes in n-octane n-decane n-hexadecane n- tetracosane and n-pentatriacontane.Measurements have also The results are consistent with static mea~urement~.~~~~~ been reported by Little~ood,’~ Martire and P01lara,~* and by Pease and Thorburns3 but these do not agree well with static measurements. The most reliable and comprehensive g.1.c. activity-coefficient measurements for n-alkane systems have been done by the Bristol group31~33~39~s4-56 using medium- high- pressure g.1.c. and taking all carrier-gas and solute imperfections into account. They have examined the C4-Cs n-a1 kane solutes in c,&2 n-alkane solvents. Generally the results indicate that the smaller the disparity in carbon number between solute and solvent the closer is the activity coefficient to unity. The measured activity co- efficients range from 0.930 for the heptane + hexadecane system at 303 K to 0.695 for heptane + dotriacontane at 348 K.Activity coefficients for many aik-1-ene + alkane systems have also been measured by this Tewari et aLS*have also measured the activity coefficients of n-alkane and n-alk- 1-ene solutes in long-chain n-hydrocarbon solvents (in this case C24 C30 and C36). Their results substantiate the general trends and results obtained by the Bristol group. Letcher and Marsicano 59 have measured the activity coefficients of other unsatur- ated Cs and c6 straight-chain hydrocarbon solutes in n-octadecane n-octadec- 1-ene n-hexadecane and n-hexadec- 1-ene solvents. The activity coefficients for these systems do not form simple trends because the various types of double bonds (ter- minal internal and conjugated) influence intermolecular interactions in different ways.Definite trends can however be seen in the interactional contribution T‘. Branched-chain alkane systems are more difficult to fit into a lattice-theory picture. Nevertheless activity coefficients for such systems have been rep~rted.~~-~~ Cycloalkane + n-alkane systems have been extensively investigated by Letche~.~O-~~ T. M. LETCHER 109 Benzene in n-alkane solvents has been studied by the Brist ol and by Let~her.~~ Recent work by Letcher on hydrocarbon + siloxane systems66 and Group IVA tetramethyl compounds + hydrocarbon^^^ have been done with the expressed pur- pose of testing theories of liquid mixtures. Many other systems have been investi- gated and been reviewed12-14 and will not be discussed here.Perhaps one of the most useful applications of this technique is in predicting finite-concentration activity co- efficients from the infinitely-dilute result. Work in this field has been done by Let- cher and Netherton,68 Bogeatzes and Tassios6’ and Hussey and Parcher.” THEORETICAL TREATMENT OF ACTIVITY COEFFICIENTS The activity coefficients at infinite dilution have been analysed in many different ways in attempts to understand the interactions of solute with solvent. The earliest efforts based on empirical relationships for homologous series of solutes interpreted the results in terms of group interactions structural effects polarity and electron donor and acceptor capacitie~.~-I~ These ideas have served as foundations for the more sophisticated theoretical interpretations.The fundamental approximation that has been used in most of the theoretical treat- ments considers the activity coefficient to be separable into two parts In yl (configuration) + In y1 (interaction). (15) The configurational and interactional contributions can be considered independent so long as the interactions are small. The configurational contribution due to the mix-ing of long-chain and short-chain molecules can be related to simple lattice proper- ties.7 A simplified version of the theory originally given independently by Flory 70 and Huggins,71 gives -In Yi (config.) = [(I -93)/Xil -k (1 -k I/r)vl3 (16) where 93 = rx3/(xl + rx3)* (17) The symbol q3refers to the volume fraction of the long-chain molecules and r to the ratio of sites occupied by the long- and short-chain molecules.For the infinitely dilute condition that applies in g.l.c. eqn (16) becomes In y (config.) = ln(1jr) + (1 -l/r). (18) The ratio r is often taken as the ratio of molar volumes of the long- and short-chain compounds. For the infinitely-dilute condition the interactional contribution is given by72 In y1 (interaction) =x (19) where x is the interaction parameter. It is in the interpretation of x that the various solution theories differ. Many theories such as Hildebrand-Scatchard solubility parameter theory pertur- bation methods congruence principle Flory Orwoll and Vrij theory and the Prigo- gine cell theory have been tried.These attempts have been reviewed by Condor and Young13 and will not be discussed here. The segment and contact-point treatment has been very successful and will be discussed briefly. The original theory was presented by Tompa73 and was later developed by McGlashan et aZ.74 Various forms of it have been very successful in treating the interactional parameter x,obtained from experimental activity coefficients at infinite ACTIVITY COEFFICIENTS FROM G.L.C. dilution and the calculated configurational contribution obtained from eqn (1 8). Applying this theory to mixtures involving only two types of segments or contact points (A and B) and to data obtained at infinite dilution the interactional contribu- tion is given by W x = rl(Ol -0,)‘ kT -where 0 and O3 are the A type segment fractions of molecules of type 1 and 3 and W is the interchange energy of the two types of segments defined in terms of hypothetical molecules containing only one kind of segment.This interchange energy should be constant at a given temperature for all mixtures containing only the two types of seg- ments. and Tewari et aLs8have been most successful in applying The Bristol gro~p~~-’~ this theory to n-alkane mixtures. Young29 has extended this theory to include three types of segments. In this case the interactional parameter x,becomes where ai Piand d1 are the fractions of segments of type A B or C of a molecule i (i = 1 or 3) and WAB,WBcand WACare the interchange energies of A and B B and C and A and C respectively and n is the number of segments on the smaller molecule.This theory has been applied by the Bristol group,57 by Tewari et aLs8and by Let- &er63,65 -68 to n-alkene + n-alkane and to benzene + n-alkane systems. Letcher and Marsicano 59 have extended this to include n-alkene + n-alkane systems. Tewari et aL7’ have also applied it to halogenoalkane solutes in alkane solvents. This theory becomes unwieldy when more than three segments are used. The slopes of the plots of x against carbon number for many systems have been obtained by LetcheP7 and give an insight into the predictive ability of this technique. Recently work done by LetcheP7 on the Group IVA tetramethyl compounds + n-alkanes (hexadecane to dotricontane) has shown that for any particular n-alkane the x value is independent of the central atom of the tetramethyl compounds.A. T. James and A. J. P. Martin Biochem. J. 1952,50 679. J. R. Conder Progress in Gas Chromatography vol. 6 of Advances in Analytical Chemistry and Instrumentation ed. J. H. Purnell (Interscience New York 1968). J. H. Purnell Endeavour 1964,23 142. D. E. Martire and L. Z. Pollara Adv. Chromatogr. 1965 1 335. R. Kobayashi P. S. Chappelear and H. A. Deans Ind. Eng. Chem. 1967,59 63. J. C. Giddings and K. L. Mallik Ind. Eng. Chem. 1967 59 19. ’C. L. Young Chromatogr. Rev. 1968 10 129. * H. W. Habgood The Solid-Gas Interface ed. E. A. Flood (Marcel Dekker New York 1967) vol 2. H. Brusset D. Depeyre and M. Fromant Chromatographia 1972,5 576. lo (a) M. A. Khan Lab.Pract. 1961 10 547 709. (b) M. A. Khan Lab. Pract. 1962 11 120 195. S. Kenworthy J. Miller and D. E. Martire J. Chem. Ed. 1963 40 541. l2 T. M. Letcher Chemical Thermodynamics ed. M. L. McGlashan (Specialist Periodical Reports The Chemical Society London 1978) vol. 2 pp. 46. l3 J. R. Conder and C. L. Young Physicochemical Measurements by Gas Chromatography (John Wiley London 1979). l4 R. J. Laub and R. L. Pecsok Physiochemical Applications of Gas Chromatography (John Wiiey New York 1978). T. M. LETCHER 111 l5 J. R. Conder and J. H. Purnell Truns. Furuduy SOC. 1968 64 1505. l6 J. R. Conder and J. H. Purnell Trans. Furaduy SOC. 1968,64 3100. l7 J. R. Conder and J. H. Purnell Trans. Furuday SOC. 1969 65 824. J. R. Conder and J.H. Purnell Truns. Furuduy SOC.,1969 65 839. l9 P. A. Sewell and R. Stock J. Chromatogr. 1970,50 10. 2o C. J. Chen and J. F. Parcher Anal. Chem. 1971 43 1738. 21 C. L. Hussey and J. F. Parcher Anal. Chem. 1973 45 926. 22 A. J. B. Cruickshank B. W. Gainey C. P. Hicks and T. M. Letcher Trans. Faruduy SOC., 1969 65 1014. 23 A. J. P. Martin and R. L. M. Synge Biochem. 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ISSN:0301-5696    

DOI:10.1039/FS9801500103

出版商:RSC    

年代:1980

数据来源: RSC