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.