Concurrent solution and adsorption phenomena in chromatography. I

John R. Conder, David C. Locke, and John H. Purnell. J. Phys. Chem. , 1969, 73 (3), pp 700–708. DOI: 10.1021/j100723a035. Publication Date: March 19...
0 downloads 0 Views 1MB Size
Download PDF
J. R. CONDER,D. C. LOCKE,AKD J. H. PURNELL

700

Concurrent Solution and Adsorption Phenomena in Chromatography.

I. General Considerations by J. R. Conder,’&D. C. Locke,lb and J. H. Purnell Department of Chemistry, University College of Swansea, Swansea, Glamrgan, Great Britain

(Received August 29, 2968)

The dependence of the several sources of solute retention in chromatography-bulk liquid partition, liquid interfacial adsorption, and solid support adsorption-on solvent volume and on surface areas is considered for a variety of practical situations. The relative magnitudes of the individual contributions may then be qualitatively predicted over a wide range of stationary phase loadings, for the situations of (a) constant sample size and (b) constant concentration of solute in the mobile phase. The individual contributions are finally summed to show qualitatively the various forms of variation of the net retention volume with liquid loading which may be met with in practice. The results are presented in graphical form. Certain features are found in these curves which are useful for diagnosis of interfacial adsorption effects in chromatography. The predicted curves are in formal agreement with published experimental plots. Means for the recognition of and correction for interfacial adsorption effects in experimental data are suggested, For chromatographic systems in which all three mechanisms occur, only bulk liquid partition coefficients can be determined unequivocally by chromatography alone. The range of application of chromatography to the measurement of physicochemical data is considerably broadened by these conclusions. The quantitative interpretation of chromatographic data is often complicated by the occurrence of solute adsorption at one or more of the interfaces present in the system. Solid supports for the stationary phase have long been known to interact with some types of solutes and preventative measures are widely taken. Interactions at the other interfaces, on the other hand, have received little theoretical and no practical consideration. Martin2 was the first to demonstrate that solute adsorption on the surface of the bulk liquid (Gibbs adsorption) could markedly affect experimental retention volumes in some gas-liquid partition chromatographic (glpc) systems. This finding was reproand directly subduced and extended by Pecsok, et stantiated by the static measurements of Martire, Pecsok, and P ~ r n e l l . ~I n? ~each case, a “polar” stationary phase was used. The suggestion was originally made28 that an appreciable Gibbs adsorption effect would be observed only with such solvents. Later,5*6 this statement was generalized to include not only Polar solvents, but any system in which the solute activity has lately .f’ exceeded about 5’ shown this to be a sufficient condition, but has further demonstrated that liquid surface excess effects may also be observed with certain polar solute-polar solvent systems in which the solute activity coefficients are only around units. Pecsok and Gumps have recently measured, by a static method, the relevant solution and surface tension data for a variety of polar solutes in the nonpolar liquid, SqUalane, and concluded that a considerable retention volume contribution from Gibbs adsorption should be found in the corresponding glpc system, I n direct conflict with this, the study of the same systemsby Urone and Parchere appeared to reveal only the occurrence of T h e Journal of Phvaical Chemistry

substantial adsorption on the surface of the solid support. Thus the general basis of the hypothesis of liquid surface adsorption in glpc is to some extent challenged and, certainly, doubt is cast on occurrence of the phenomenon with nonpolar solvents. It is the purpose of this paper to define as quantitatively as possible all sources of solute retention in chromatography and thus to test the practice of detecting and measuring Iiquid surface adsorption by chromatographic means. I n so doing we are able to offer a reconciliation of the conflicting data and to propose better procedures for the chromatographic measurement of surface phenomena. While all quantitative investigahions have so far been restricted to glpc systems, the results of this paper are equally applicable to liquid-liquid chromatographic (llc) systems, where the phenomena described have also been tentatively detected.1° (I)(a) Department of Chemical Engineering, University College of Swansea. (b) Department of Chemistry, Queens College of the City University of New York, Flushing, N. Y. 11367. (2)(a) R. L. Martin, Anal. Chem., 33, 347 (1961); (b) R.L.Martin, ibid., 35, 116 (1963). (3) R.L. Pecsok, A. de Yllana, and A. Abdul-Karim, ibid., 36, 542 (1964). (4) D. E. Martire, R. L. Pecsok, and J. H. Purnell, Nature, 203, 1279 (1964). (5) D. E. Martire, R. L. Pecsok, and J. H. Purnell, Trans. Faraday fk61,2496 (1965). (6) D.E. Martire and L Z Pollara in “Advances in Chromatography,,, vel, J. c, Gidd;ng; and R.A, Keller, Ed,, Marcel Dekker, New York, N. Y . ,1966,p 335. (7) D.E. Martire, Anal. Chem., 38, 244 (1966). (8) R. L. Pecsok and B. H. Gump, J. Phys. Chem., 71,2202 (1967). (9) P. Urone and J. F. Parcher, Anal. Chem., 38, 270 (1966). (10) D. C. Locke in “Advances in Chromatography,” vel. 7, J. c. Giddings and R. A. Keller, Ed., Marcel Dekker, New York, N. Y., 1968,

CONCURRENT SOLUTION AND ADSORPTION PHENOMENA IN CHROMATOGRAPHY

701

Theory

relation 6, eq 5 can be written in the form

I. Sources of Solute Retention in Glpc and Llc. Partitioning of solutes between the mobile phase and the liquid stationary phase is the predominant mechanism of solute retention in most, but not all, practical glpc and llc systems. Perhaps more often than is generally recognized, the solute may also be retained in the column by adsorption (i) at the gas-liquid interface (or liquid-liquid interface in llc) and/or (ii) as a result of the presence of the solid support. Except as discussed below, these retention mechanisms operate essentially independently. Consequently, the number of moles of solute held stationary within a length dl of the column of total length I, in equilibrium with a concentration c in the gas phase, is ndl/l where

T“N = KfobsVL where Klobs,the experimentally observed partition efficient calculated from the experimental VN and is given by K’obs = ( ~ / V L (dn/dc) )

n=

qLVL

+

QIA

+q s ~ s

=

CQ ~ $ ~

(1)

i

Here QL, QI, and QS are the solute concentrations (mole cc-I, mole em-’, and mole em-l) in the bulk liquid, gasliquid (or eluent-liquid) interface and support-adsorbed phase, respectively; VI,is the total volume of liquid phase in the column; AI and AS are the active surface areas of the liquid and support, respectively; and $ i represents VL, A I , or As. For each of these retention mechanisms, the contribution to the net retention volume of an eluted zone whose concentration in the gas phase (assumed ideal) is c is givenllJ2by the equation VN,f

=

(1

- h/O)Kt“?h

(2)

where

Ki’ = dq,/dc

(3)

,j is the usual James-Martin gas compressibility factor

and yo is the mole fraction of solute, corresponding to c, at the column outlet. Ki’is the gradient of the distribution isotherm for retention mechanism i and is to be distinguished from the distribution coefficient, given by Ka = qt/c (4) I n the limit of infinite dilution, K i f = Ki. From eq 1, 2, and 3, the total retention volume, as observed experimentally, is given by

VN = C V N , ~ 5

(1 - jyo)dn/dc

(5)

We propose to consider only conventional elution chromatography in which solute concentrations are very small (sample size -0-1 pmol). Here it is a good approximation to set jyo 0, the plot will have a nonzero intercept, KI, and a positive slope, KL. A plot of VN/VL against AI/VL gives the same information but the roles of K I and KL are reversed. These procedures give no useful information if solid support effects occur and cannot be applied at ,all unless A I is known as a function of VL. A preferable procedure, which avoids both these shortcomings, is to plot VN/VL against ~ / V L . According to eq 11, this allows KL to be determined whether A I is known or not. If A I is known, both K I and KSAScan be obtained from the plot, as described by Conder.17 If, in addition, the solutions involved are not truly at infinite dilution, Kfobsand thus VN become dependent on concentration. I n this case, experiments in which the solvent/support ratio is varied can be carried out in either of two ways: (a) maintaining constant sample size so that concentration varies, or (b) adjusting sample size to maintain constant concentration. Expeiimental mode (a) is exemplified in the work of Urone and P a r ~ h e r who , ~ varied liquid loading while using a fixed amount of sample. Analytical treatment of the results in terms of eq 8 gives no useful information in this case since there are too many variables, viz. both the distribution coefficients and the area/volume ratios. Experimental mode (b), however, is much more useful since it eliminates variation of the distribution coefficients and permits their evaluation, as described previou~ly.~~ I n conducting experiments in either modes (a) or (b), the possibility is evident that variations in the relative contributions of two or more of the effects can yield some compensation among them, so that retention behavior may appear simpler than is actually the case. It is important, therefore, to examine in detail what the concentration dependences of the individual terms in eq 8 might be. Each individual retention mechanism will be considered in turn, and finally the contributions will be summed t o predict complex retention volume relationships which might be met in practice. The Journal o/ Physical Chemistry

III. Effects of Constant Sample Size. A . Bulk Solution. The three most common types of partition isotherms encountered in chromatographic systems are depicted in Figures 1A(i), (ii) , and (iii). By definition,

Figure 1. A, Bulk liquid partition isotherms: (i) anti-Langmuir; (ii) linear; and (iii) Langmuir. B, Variation of K L with liquid loading for (i) anti-Langmuir, (ii) linear, and (iii) Langmuir partition isotherms. C, Dependence of KLVLon VL for (i) anti-Langmuir; (ii) linear; and (iii) langmuir partition isotherms.

KLis equal to ~ L / Gand is calculable from the isotherm by construction of chords from the origin to any point on the curve.” The corresponding variations of KL with VL and of KLVL with VL are, respectively, shown in Figures 1B and 1C. I n each case, (a) represents an anti-Langmuir isotherm, (b) a linear isotherm, and (c) a Langmuir isotherm. As V L is increased at constant sample size, the liquid phase becomes more dilute in solute and this can be represented as moving along the isotherm to a different point. Since VL increases linearly, and proportionally more rapidly than KL changes, the variation of KLVL with VL is as shown in Figures 1C. I n glpc, case (a) is by far the most common; cases (b) and (c) are observed only in the presence of very strong negative deviations from Raoult’s law, such as might be caused by complexing, hydrogen bonding, or other form of association. Insofar as peak shape is determined by the partitioning process, case (a) leads to peaks with trailing edges steeper than their leading edges (skew ratio, 7 >l).11p1881Q B. Liquid Surface Adsorption. The relationship between the liquid surface partition coefficient, KI, and the Gibbs adsorption isotherm has been reviewed re(17) J. R . Conder, J. Chromatogr., 39, 273 (1969).

(18) G. F. Freeguard and R. Stock in “Gas Chromatography 1962,” M. van Swaay, Ed., Butterworth and Go. Ltd., London, 1962, p 102.

(19) A. J. B. Cruiokshank and D. H. Everett, J. Chromatogr., 11, 289 (1963).

CONCURRENT SOLUTION AND ADSORPTION PHENOMENA IN CHROMATOGRAPHY ~ e n t l y . ~ The J ~ surface excess concentration, I‘z(l), which is proportional t o the variation of solution surface tension with solute mole fraction, (dyldxz), is by definition related to KI according to

K~ = r2(1)/c (13) Figure 2A(i) presents a typical y-x2 plot; the initial rapid decrease in y usually occurs over a quite narrow range of xz. For example,8 in the methanol-squalane system at 30°, y decreases from 27 dyn cm-1 at xz = 0 t o its minimum value of 22 dyn cm-l at x2 = 0.02. I n Figure 2A(ii) is shown the resulting variation of KI with x2 and, in Figure 2A(iii), that of K I with VL.

Figure 2. A, (i) Change in solution surface tension, y, with solute concentration, 2 2 ; (ii) variation of K I with solute concentration, 2 2 ; (iii) change in K I with liquid loading, VL, a t constant sample size. B, Liquid surface area, AI, as a function of liquid loading, VL, for (i) support wetted by stationary phase and (ii) support not wetted by stationary phase. In (i), vertical broken line: corresponds to VL of formation of an ideal uniform monomolecular liquid film. I n (ii), upper and lower curves: expected extremes of anticipated AI-VL plots. C, Variation of KIAI with VL for (i) wetted and (ii) nonwetted solid supports, a t constant sample size. Vertical broken line: point of monolayer formation.

The liquid surface adsorption isotherm is Langmuir in form ( K I decreasing with increasing concentration), which produces skew ratios, 7 < 1. Since the curvature of this isotherm can be more pronounced than that of the bulk partition isotherm, the effect on peak shape can be greater. Consequently, tailed peaks can be anticipated when sufficient liquid interfacial adsorption occurs, unless solutes are introduced in quantities small enough to produce solutions at infinite dilution. C. Liquid Xurface Area. Martinza and Pecsok, et aZ.,a have presented experimental plots of liquid surface area vs. VL. Martire, et a1.,6 have independently

703

confirmed the form of these plots as presented in Figure 2B(i). The broken vertical lines in the figures indicate the point of monolayer formation for an ideally uniform liquid film. Such a film would reach monolayer solvent/support ratio in the vicinity of 0.1 wt %. (For clarity, the VL scales are expanded at low VL to the left of the broken lines.) Experimental techniques used for measurement of AI have been either the continuous flow method of Nelsen and Eggertsen20 (used by or a modified BET technique using nitrogen as the a d ~ o r b a t e . ~I n either case, what is in fact being measured is a gross area (liquid surface plus any exposed solid surface), since all exposed surfaces are equally accessible to adsorbable vapors. It is also noteworthy that the cross section for adsorption of nitrogen differs substantially from that for most gc solutes. Martire, et u Z . , ~ J determined AI by first determining in a static system the K I value for cyclohexane in p,p’-thiodipropionitrile. Using glpc-measured K I A I values2,afor the same system, they then calculated A I values and thus constructed a “corrected” AI-VL plot. The value of AI extrapolated to 0.1% solvent loading (Le., the approximate monolayer point for a uniform film), ~2 m2 g-1, is quite close t o that found for uncoated Chromosorb using a BET-organic vapor technique.Z1 This is to be expected if the liquid is distributed upon the solid in a monolayer, and, insofar as such extrapolation is valid, provides some evidence for the smooth distribution of this stationary phase on this s ~ p p o r t . ~At , ~high V L values, AI asymptotically approaches a limiting value. Clearly, this limiting area is that of the support less the area of its narrow pores and channels. These two area limits are analogous to the comparison of the true length of a coastline (VL = 0) with that of the 3 mile limit (VL >> 0) which ignores estuaries, inlets, etc. One feasible independent method for estimating AI is applicable only to low surface energy solid supports such as Teflon, The total surface could be measured by a BET-organic vapor technique. Subsequent examination of the same wetted packing by electron microscopy should allow calculation of the average size and geometrical distribution of liquid droplets on the surface. The total surface area could then be apportioned to the exposed solid surface and the liquid surface areas. There is no reason to believe that the form of the AI-VL plots for the systems previously studied2-6 apply to all chromatographic solid support-solvent combinations. Smooth liquid distributions (such as described by Figure 2B(i) can be achieved only if the liquid wets the solid support surface. The consequence of nonwetting of the support upon the resulting AI is (20) F. M. Nelsen and F. T. Eggertsen, Anal. Chem., 30, 1387 (1958). (21) R.H.Perrett and J. H. Purnell, J. Chromatogr., 7,455 (1962). Volume 75,Number S March 1960

704 pictured in Figure 2B(ii). Instead of a sharp rise up to the monolayer point followed by a gradual tailing off,the liquid can now only form isolated droplets which at small VL have only a small total surface area. As more liquid is added, the combined droplet area increases until sufficient solvent is present to cause the drops t o coalesce. A I must then level off or fall off, again to the same limiting values as found for the wetted support. AI-VL plots of the type shown in Figure 2B(i) are expected for most solvent-support systems. Figure 2B(ii) should better describe the cases of solvents on Teflon supports and perhaps strongly polar liquids on silanized firebrick. D. Contribution of Liquid Surface Adsorption to Retention. The contribution of liquid surface adsorption to retention is given by the product K I A I . For each of the expected AI-VL relationships, KIAI is plotted as a function of V L in Figures 2C(i) and 2C(ii). The variations of KI and of A I with VL are opposed and the relative sizes of the variation determine the overall form of K I A I . For the unwetted support, comparison of Figures 2C(i) and 1C immediately shows that no compensation, in terms of slopes of opposite sign, is possible between K I A I and KLVL,so that the net retention volume must increase with VL. However, in the case of a wetted support, if A I decreases with VL faster than KI increases, the product can decrease with VL. This behavior would allow compensation, but only if the relative contribution of KIAIto VN was significant compared with that of KLVL. Evidently, an observed lack of dependence of VN on VL by no means indicates the absence of liquid surface adsorption. This point assumes critical importance since it has hitherto been axiomatic that the converse is true. E. Solid Support Adsorption. It is generally supposed that adsorption induced by the support takes place only at the uncoated solid surface, but account should be taken also of the possibility of adsorption at the liquid-solid interface where solute and solvent compete for the support surface.22 The existence of the latter has been conclusively demonstrated recently by Urone, et al.,23 for the system acetone-tri-o-tolyl phosphate-Chromosorb P or W. If the surface is completely covered, all apparent support effects must clearly stem from this type of adsorption. On the other hand, at less than monolayer coverage, or if the solvent does not wet the support, adsorption can occur at both the liquid-solid and exposed interfaces. Both types are taken into account here, as well as the effect of wetting and nonwetting of the support. If the support is wetted, the total area available for adsorption varies with VL as shown in Figure 3A(i). The upper curve relates t o adsorption at both types of location, and the lower to adsorption on exposed solid only. These two types of behavior are characteristic of nonpolar and polar stationary phases, respectively, The Journal of Physical Chemistry

J. R. CONDER,D. C. LOCKE,AND J. H. PURIVELL

Figure 3. A, (i) Change in area, As, of solid support accessible to polar solute, with VI,. Upper and lower curves: nonpolar and very polar stationary phases, respectively. Vertical broken line: point of monolayer formation. (ii) Solute adsorption coefficient on solid support, K s , as a function of solute concentration in the stationary phase, E, for Langmuir adsorption isotherm. (iii) Dependence of Ks of a polar solute on VL, a t constant sample size. Upper and lower curves: stationary phases which are, respectively, nonpolar and more polar than the solute. T’ertical broken line: liquid monolayer point. B, Variation of (i) KsAs and (ii) K s A s / V Lwith VL, for a polar solute, at constant sample size. Upper and lower curves: stationary phases which are nonpolar and more polar than the solute, respectively. C, Variation of (i) KsAs and (ii) K s A s / V L ,on nonwetted support, for constant sample size.

on adsorptive solid supports. The variation in the apparent K S associated with either As curve is shown in Figure 3A(iii). The initial rapid fall in KS is due t o blocking of pores and other active sites by solvent, and is most marked for polar solvents. Beyond the point of monolayer coverage a region of negative curvature reveals the influence of the strongly curved isotherm, which is expected t o be most frequently of the Langmuir type, as indicated by the KL-E plot shown in Figure 3A(ii). The sharp minima arising in the Ks-VL curves at the point of monolayer formation are notable. The L VL resulting dependences of KsAs and of K ~ A s / V on are shown in Figures 3B(i) and (ii). A remarkable feature which emerges for nonpolar stationary phases is that if the solute is sufficiently polar, a strong hump may develop in such plots. This effect is unique to nonpolar liquids which wet the support and is a useful diagnostic feature. The alternative situation, in which the solvent does not wet the support, is summarized by the plots of KsAs and KsAs/VL against VI, in Figures 3C(i) and (ii). (22) J. C. Giddings, Anal. Chem., 35, 440 (1963). (23) P. Urone, Y. Takahashi, and G. H. Kennedy, ibid., 40, 1130 (1968).

CONCURRENT SOLUTION AND ADSORPTION PHENOMENA IN CHROMATOGRAPHY I n this case, there can be no marked changes associated with a point of monolayer formation. No hump is observed and behavior is similar for polar and nonpolar solvents. Since nonwetted supports (e.g,, Teflon) are likely to be only very weakly adsorptive toward most solutes, the contribution of KSASto the net retention volume may in this case be in~ignificant.~~ F. Combined Contribution to Retentwn. Different systems may display gross retention effects which reflect the combined contributions of several or all of the physical processes discussed. A number of possibilities arise which we consider in turn. Suppose, first, that the contribution of KsAs to VN can be neglected. We then have three possible situations: (a) KLVL>> KIAI; (b) KIAI >> KLVL; and (c) KLVLN KIAI. Cases (a) and (b) need not be discussed since the dependence of VN upon VL, etc., is as shown earlier in Figures 1C and 2C, respectively.

705

urement and calculation, however, is that of VN/VL against ~ / V L shown , in Figure 4B(iii). According to eq 9, the intercept on the ordinate axis gives KL at infinite dilution, so that plots for different sample sizes all extrapolate to the same point. The positive curvature of the plot is determined mainly by AI, but is reduced somewhat by the variation of KI with concentration. The latter factor also causes the plot for a small sample size to lie above that for a larger sample size. Variation of KL with concentration has a much smaller influence on the curvature of these plots, since the liquid surface absorption isotherm is of greater curvature than the bulk liquid partition isotherm. The second situation of relevance is where KLVL‘v KIAI ‘v &As. Figures 4C are derived from compounding the earlier plots for wetted supports. The upper and lower curves refer t o the extreme cases of solvent polarity, Le., nonpolar and strongly polar, respectively. Again, when solute is adsorbed at the nonpolar liquid-solid interface, a hump is observed in the plots, as in Figures 4C(i) and (ii), but is less pronounced than when KsAs predominates; if KsAs is sufficiently small compared with KLVLand KIAI, this hump may not be observed. Finally, when KIAI >> (KLVL KIAI), we again have behavior which is distinctive enough to be useful diagnostically (Figures 3B). An example of such behavior is to be found in the work of Urone and P a r ~ h e r . ~ These authors represented their data in the form of retention volume per gram of column packing against percentage loading of solvent on the support. This method of presentation, first adopted by Martin,2ais less basic than a plot of VN against VL, since the weight of solid support is included among the variables. I n consequence, the data can be readily interpreted only if the packing density is independent of liquid loading. Since this assumption was apparently met for the packings used by Urone and Parcher, their data show exactly the same behavior as that of the upper curves in Figure 3B. IV. Efects of Constant Mobile Phase Concentration. Constant mobile phase concentration can be achieved either by operation at infinite dilution or by one of several available finite concentration techniques, described e l ~ e w h e r e . 1 ~ ~ 1 ~ ~ 1 ~ If C is maintained constant while VL is varied, then since a point has been fixed on each of the three relevant distribution isotherms, each of the gi, and consequently Ka’, of eq 9 must be constant. Only VL, AI, and As can vary. It is then a straightforward matter to construct the appropriate plots. Since KL is constant, the variations of KL and KLVLwith VL are the same as that shown in Figures lB(ii) and IC(ii), respectively. The KIAIplot differs from Figures 2C in that the curve follows the &VI, plot because K I is also constant.

+

Figure 4. A, Variation of net retention volume, VN, with VI, for the case where KLVLN KIAI >> &As, for (i) wetted and (ii) nonwetted solid supports, a t constant sample size. Vertical broken line: corresponds to VI, a t which monolayer formation is complete. B, Variation of VN/VL with VI, for (i) wetted and (ii) nonwetted solid supports, and (iii) variation of VN/VI, with 1/VL, all a t constant sample size. Upper and lower curves in (iii): smaller and larger sample sizes, respectively. C, Net retention volume of a polar solute for &,VI, ‘v &AI = &AS, a t constant sample size, for wetted support. Upper curve: nonpolar stationary phase; lower curve: stationary phase more polar than the solute. Vertical broken line: point of monolayer formation.

Case (c), however, is of considerable interest. VN is the sum of separate bulk and surface liquid contributions, and is plotted in Figures 4A(i) and (ii), for wetted and nonwetted supports, respectively. The derivative plots of VN/VL against VL are drawn in Figures 4B(i) and (ii), corresponding to Figures 4A(i) and (ii), respectively. The most important plot for purposes of meas-

(24)

J. J. Kirkland, Anal.

Cham., 35, 2003 (1963). Volume 79,NumbeT 9 March 1980

J. R. CONDER, D. C. LOCKE,AND J. H. PURNELL

706 Likewise, plots of KsAs against VL follow those of Figure 3A(i). Two compound cases are considered for the condition of constant sample concentration. I n the first, when (KLVL KIAI >> KsAs, VN will vary with VL according t o Figure 5A(i). In this figure, differences in

+

A

Figure 5. A, Net retention volume a t constant solute concentration in the mobile phase, for the case where KLVL N KIAI >> KsAs. I n (i) and (ii): upper curves: wetted support; lower curves: nonwetted support; broken curves: KIAI > KLVL; solid curves: KLVL> KIAI; vertical broken line: point of monolayer formation. Upper curves in (iii) smaller and larger sample sizes; curves do not intersect at VL = a. B, Net retention volume at, constant solute concentration in the mobile phase, where KLVLN KIAI &As. (i) Variation of VN with VL. Upper curve: nonpolar stationary phase which wets the support; middle curve: polar stationary phase which wets the support; lower curve: nonpolar stationary phase which does not wet the support. Vertical broken line: point of monolayer formation. (ii) Variation of VN/VLwith VL. Upper curve: nonpolar stationary phase which wets the support; curve for polar, wetting stationary phase is similar. Lower curve: nonpolar stationary phase which does not wet the support. Vertical broken line: point of monolayer formation.

relative importance of KLVLand KIAI are taken into account; the pair of broken curves apply to systems in which liquid surface adsorption predominates over bulk partition, and the pair of full curves to the reverse situation. The upper curves of each pair refer t o wetted supports, and the lower curves to nonwetted supports. It is of interest that KirklandZ4 found precisely the behavior shown in the lower solid curve of Figure 5A(i) for butanol solute on squalane coated onto Kel-F, which is not wet by the squalane. The VN/VL - VL and the VN/VL - ~ / V Lplots are formally quite similar for the cases of constant concentramtionand constant sample size, as is made clear by comparison of Figures 5A(ii) and (iii) with 4B(ii) and (iii). The main differences are twofold. First, KL, still given by the intercept on the ordinate axis, now varies with solute concentration, but to no great extent, since curvature of the bulk partition isotherm is less pronounced than that of the adsorption isotherm. Secondly, curvature in the plot for a given concentration now reflects only The Journal of Physical Chemistry

the variation of AI, since both KL and KI are constant. Thus if AI is known as a function of VL, KI, as well as KL, can be determined. Secondly, we consider the case where KLVL N KIAI 'V K&. The resulting plots of VN and of VN/VL are as illustrated in Figure 5B and the behavior is not always formally distinguishable from the situation of constant sample size (Figure 4C). The solid support adsorption-dominated situation, as may be expected, is describable by Figure 3A(i).

Discussion The results of most importance are obviously those describing overall net retention volume for each of the various combination cases considered above. Some experimental results are available from the literature for comparison. It should be pointed out, however, that Figures 4 and 5, which involve sums of contributions, can only be illustrative in that their exact form is determined by the extent to which each individual retention mechanism contributes. For any particular situation, the figures describing these individual sources of retention should be consulted before drawing conclusions from the overall retention diagrams. I n most fundamental chromatographic work, an attempt is made t o reach infinite dilution by the use of the smallest convenient detectable sample size consistent with an acceptable signal t o noise ratio. for example, used 0.02 pl (ca. 0.2 pmol) of each component over the whole range of liquid loadings studied. This sample size produces chromatographic solutions of mean solute mole fractions of the order of lop3. According to the solution tension measurements of Martire, et u Z . , ~ at these concentrations (d-yldx), and therefore I ' Z ( l ) is almost constant, so that KI will not vary with V L . This is clearly a case where infinite dilution is effectively achieved. Martinza proved that in his systems KsAs = 0, so his results should be described by a plot in the form of Figure 5A, Le., by the sum of Figures 1C and 2B(i). Some calculations based on his data are of interest here. For solution in p,p'-thiodipropionitrile at 25", the KIAI contribution for isooctane at low loadings (1.5y0w/w) is about 90% of VN, while at 25% loading, it is 48%. For cyclopentane, at 1.5% w/w of solvent, the liquid surface contribution is 7701, of the total, while at 25% w/w, it is only 6% of VN. Thus for isooctane, the VN-V, plot should be similar t o the broken portion of the upper curve in Figure 5A(i), while that for cyclopentane should be closer to the solid upper curve of the same figure. This is, in fact, what is evident in Figure 3 of Martin's paper.2a The more interesting systems t o compare are those squalane systems studied by Urone and Parehero and by Pecsok and Gump.8 Constant sample size was used in the glpc study,$ and for methanol was 10 pmol in 1 cc of helium carrier gas. For untreated firebrick,

CONCURRENT SOLUTION AND ADSORPTIONPHENOMENA IN CHROMATOGRAPHY strong solid support interaction can be anticipated and the result should appear as the upper curve in Figure 3B(i). Such a maximum is in fact found in Figure 1 of ref 9. A replot of Urone and Parcher’s data as VN/VL 2’s. VL is formally similar t o our Figure 3B(ii), showing a well-developed hump at low solvent/support ratios. The magnitude of the solid support adsorption swamps out the other retention mechanisms. If pure partition was observed, methanol should elute before acetone, since its activity coefficient-vapor pressure product is about 5 times that of acetone. The relative support adsorption coefficients for this firebrick must therefore be in the ratio of about 15/1 [Ks(methanol)/Ks(acetone) t o match the published datang Urone and Parcher’s data for tri-o-tolyl phosphate stationary phase are dominated at low liquid loadings by support adsorption also. However, as more liquid is added, interfacial adsorption and partition become dominant, resulting in a plot similar to the lower curve of Figure 4C(i) for support coated by a polar solvent. Data are also given for silanized firebri~k.~The most obvious result of the support treatment is a reduction in the observed retention volume of methanol by a factor of 20 to 40. Clearly the majority of adsorbing sites have been eliminated. According to the static measurements of Pecsok and Gump,* the infinite dilution values of K L and KI, extrapolated t o 7 5 ” , are for methanol 2.92 and 80 X cm, respectively. Assuming that Urone and Parcher’s 1 m X 4 mm i d . columns contained 10 g of packing, values of KLVLcan be calculated. Subtracting this KLVLfrom the VN values determined from a redrawn plot of the published f i g ~ r ethe , ~ remainder is (KIA1 KsAB). Using values of AI taken from Martire, et aLj6we can then calculate values of ( K I KsAs/AI) for columns containing 0.5 to 16% w/w of squalane. These values range from 34 cm at 0.5% w/w to 109 X X cm a t 16% w/w squalane. KsAs/AIcan now be calculated since values of KI as a function of the solute mole fraction in solution can be calculated from Pecsok and Gump’s data as follows. Values of the solute mole fraction in solution at the column inlet are calculated from the known sample size, sample volume, and VL, and vary from 0.06 at 0.5% w/w to 0.04 at 16% w/w. (It is interesting that over the range of liquid loadings from 0.5 to 16% w/w, the maximum solute mole fraction varies only by a factor of 1.5.) If the reasonable assumption is made that the center of the solute band undergoes dilution by a factor of about 50 in moving t o the outlet at end of the column, the mole fraction is about the outlet. At this concentration, calculation from Pecsok and Gump’s extrapolated data gives a value of KI for the methanol-squalane system of about 20 X cm for all the liquid loading used. Thus KBAs/ A I goes from about 10 X cm at 0.5% w/w to about 90 X cm a t 16% w/w. Solid support adsorption must therefore provide a very significant con-

+

+

707

tribution to the overall solute retention in this system even after careful silanization. The exact relative magnitude of the contribution cannot be deduced, since computation of precise values of KI would require more exact information than is available as to the solute concentration profile and the dependence of the solution surface tension on concentration in this system. The important conclusion is that these three contributions-partition, liquid surface, and solid surface adsorption-are all participating to roughly the same extent and add up t o an apparent linear variation of VN with VL. Because support adsorption masks liquid interfacial adsorption in the system studied by Urone and Parcher,9 no inconsistency exists between their experimental results and the known occurrence* of liquid interfacial adsorption in the system.

Conclusions The glpc systems so far chosen for study of the Gibbs effect have been largely free of complicating support effects and so have provided valuable information. If the technique is to be extended beyond the relatively limited range of suitable systems of this type-and there may well be great need for this, for example, t o allow study of complexing reactions by glpc-certain practical and theoretical criteria must clearly be met. First, we see from earlier argument that the use of columns of much less than about 5y0w/w loading is undesirable since no theory exists for the liquid surface situation in the absence of a bulk phase. It is also advantageous to use a sensitive detector which permits recording of peaks small enough to approach symmetry in profile. If asymmetrical peaks cannot be avoided, a procedure is available for dealing with the situation, but is longer and involves some loss of accuracy.l’ Treatment of data is best effected by plotting VN/VL against ~ / V Lat constant concentration which may, of course, be infinite dilution. This gives K L without need for any ancillary data, and can also yield K I and KgAs if AI is independently determined. Values of KL are given by the intercept on the ordinate axis and, since KL varies more slowly with concentration than KIAI or KsAs, the plots for different, but small, concentrations should all extrapolate t o the same point. Operation a t constant sample size, as opposed to constant concentration, does not lead to evaluation of thermodynamic parameters but is useful for one particular purpose: the appearance of a hump, such as that shown in Figures 3B(i) and (ii), in a plot of VN or VN/VL against VL,is a diagnostic test for support adsorption in the presence of a nonpolar solvent wetting the support. More generally, the diagrams previously given can be used to draw conclusions about the types of interaction involved in a given chromatographic system, operated at either constant concentration or constant sample size. Finally, because of the possibility of compensaVolume 73,Number 8 March 196.9

D. F. CADOGAN, J. R. CONDER,D. C. LOCKE,AND J. H. PURNELL

708

tion between two or more mechanisms, it is incorrect to draw conclusions about surface effects on the basis of measurements at only two different loadings. Use of a wide range of loadings is mandatory.

Acknowledgments. J. R. C. thanks the University of Wales for an I.C.I. Research Fellowship. D. C. L. thanks the National Science Foundation for a Postdoctoral Fellowship.

Concurrent Solution and Adsorption Phenomena in Chromatography. 11.

System Alcohols-Squalane

by D. F. Cadogan, J. R. Conder,l* D. C. Locke,lb and J. H. Purnell Department of Chemiatry, University College of Swansea, rSzoansea, Glamorgan, Great Britain (ReceCed Auguat 13,1968)

Gas-liquid partition chromatography has been used for the first time to determine accurate vaIues of the limiting activity coefficients of Ca-Ca alcohols in squalane over the temperature range 50-70'. A newly devised data-treatment procedure was used to circumvent the problems of liquid and solid support interfacial adsorption which occur in these systems, and which have plagued previous chromatographic investigations. The results validate the corrective procedure used, thus allowing the extension of chromatographic teohniques to the study of the thermodynamics of such systems.

I n part I,2*we considered in qualitative detail the problems of interpretation which arise when chromatographic retention arises from the simultaneous occurrence of bulk solution, liquid interface adsorption, and solid interface adsorption. Comparison of the alternative procedures of evaluating retention data corresponding to constant sampIe size or to constant sample concentration clearly showed the latter to be preferable. It was then shown that true bulk partition coefficients could always be obtained if the appropriate data treatment methods were adopted.2 This work describes an experimental glpc test of the methods devised. Alcohol-squalane systems have been chosen for study because they show complex retention characteristics in high degree. I n a previous study of such system$ the influence of surface retention was not correctly allowed for and it is of interest to determine the extent of the error introduced.

Experimental Section Columns of stainless steel tube of about 50 cm length, in. o.d., and 0.028 in. wall thickness were packed with squalane supported on carefully HMDS-silanized 100-120 mesh Sil-0-Cel. Four such columns were prepared, the percentage of squalane in the coated packing varying from 7.3 t o 32.5 by weight. Squalane was deposited on the solid support from solution in ether, The exact amount of squalane deposited was deter'/4

mined by weighing* The were checked by refluxing weighed portions of the packing with aceThe Journal of Phyaical Ch:hemislry

tone in a Soxhlet extraction apparatus until the packing was reduced to a constant weight. I n all cases the weights of squalane on each column deduced from several extractions were within 0,3% of each other and in agreement with the expected weight on the column to at worst 3% and usually much better. The percentage loading was taken as the mean of the two values so obtained. The columns were qntained in a forced air thermostat electronically controlled to within *0.01 O and with a total temperature difference between the ends of the oven of less than 0.1'. Hydrogen carrier gas, dried by passage through 5 8 molecular sieve and silica gel, was flow controlled to better than 1%, the flow being frequently checked by use of a thermostated soap bubble flow meter. Liquid sampIes were injected from a 0-1 p1 Hamilton microsyringe through a, silicone rubber septum contained in an injection block heated to 102". The solutes were injected separately and in mixtures with no noticeable effect on their retention volumes. The retention volume of the peak maximum for each solute was measured for a series of sample injection sizes ranging from less than 0.01 to 0.07 pl and found t o (1) (a) Department of Chemical Engineering, University College of Swansea. (b) Department of Chemistry, Queens College of the City University of New york, Flushing, N, Y. 11367.

(2)(a) J. R.Conder, D. C. Looke, and J. H. Purnell, J. Phw. Chem., 7 3 , 700 (1969); (b) J. R. Conder, J. Chromatogr.,39, 273 (1989). (a) A. B. Littlewood and F. W. Willmott, Anal. Chem., 38, 1031 (1966).