Solvent Composition Effects in Thin-Layer Chromatography Systems of the Type Silica Gel-Electron Donor Solvent Edward Soczewinski Department of Inorganic and Analytical Chemistry, Medical Academy, Lublin, Poland THEsuitable distribution of zones in both partition and adsorption chromatography is usually obtained by the application of mixed solvents. Solvent composition effects in partition chromatography have been discussed ( I , 2). In the case of weak molecular interactions in the mixed phase, the RM values, as well as log retention times (3), should be approximately linear against the volume composition of the mixed phase. When, however, stable molecular complexes are formed between the solute and one of the solvent component of the mixed phase, the RM (or log t R ) can be linear depending upon the log volume composition of the mixed phase ( 1 , 2, 4-6). Reference ( 6 ) includes a review of papers on complexation equilibria in gas-liquid chromatography. Solvent composition effects in adsorption chromatography have been investigated by OScik (7) who applied the thermodynamic theory of solutions to explain the effect of the composition of the developing solvent on chromatographic parameters, and to relate the R.w values in complex systemswith those obtained in the respective simple solvent systems (8). The elution power of numerous solvents has been quantitatively characterized by Snyder (9-11), who carried out extensive investigations to give a theoretical basis of adsorption chromatography, including the effect of molecular structure and solvent composition on the chromatographic behavior of organic solutes. In the present communication a simple approach will be applied to characterize solvent composition effects in certain types of adsorption systems. The theoretical considerations are analogous to those employed in the treatment of partition systems involving the formation of solvation complexes (2, 4-6).
solutes the arrangement proposed by Ewell, Harrison, and Berg (14) for solvents. The role of H-bonding in adsorption is discussed by Snyder ( I I ) , Pimentel and McClellan (15), and Giles (16). Assuming that pairs of molecules interact by the formation of H-bonds, we will have the following schematic diagram of interactions involved in adsorption equilibrium:
2-2-s-s
\/
. . . . A , .. . , . . where Z is the solute, S, the solvent, and A, the active site on the surface of the adsorbent-e.g., a hydroxyl group on the surface of silica gel (17). Strong interactions of the H-bond type, leading to the formation of definite molecular complexes, can be characterized by equilibrium constants derived from the Law of Mass Action. Let us consider first the simple and yet frequent case when the 22,SS, and SZ interactions are relatively weak and can be neglected. Such a situation exists, for instance, when both the solute and the solvent are electron donors (class B after Pimentel and McClellan-e.g., pyridine and acetone, respectively), strongly adsorbed by an adsorbent of the silica gel type (class AB caused by the presence of hydroxyl groups on the surface). Let us assume that acetone is diluted with an inert solvent-e.g., cyclohexane, class N-to reduce the eluent strength of the mobile phase. The interactions are now simplified as follows: pyridine ( Z )
acetone (S)
+ cyclohexane ( N )
THEORETICAL CONSIDERATIONS Adsorption from solution is a result of competition between the solute and the solvent for the active sites on the adsorbent surface. The molecular interactions involved are often of the hydrogen bond type [n- or .ir-complexes, (12)]. Accordingly, Kiselev (13) applied to the classification of adsorbents and (1) E. Soczewidski, Adcan. in Chromatogr., 5 , 3 (1968). (2) E. Soczewidski and G. Matysik, J . Chromatogr., 32,458 (1968). ( 3 ) A. Waksmundzki, E. Soczewidski, and Z . Suprynowicz, Collect. Czech. Chem. Commcin., 27, 2003 (1962). (4) A. B. Littlewood and F. W. Willmott, ANAL.CHEM.,38, 1031 (1966). (5) J. H. Purnell, “Gas Chromatography 1966,” A. B. Littlewood, Ed., Institute of Petroleum, London, 1967, p 3. (6) B. L. Karger, ANAL.CHEM.,39, 24A (July 1967). (7) J. Okik, Przem. Chem., 44, 129 (1965). (8) J. OScik, Bull. Acad. Pol. Sci., Ser. Sci. Chim., 14, 879 (1966). (9) L. R. Snyder, J . Chromatogr., 8 , 178 (1962). (10) Ibid., 16, 55 (1964). (11) L. R. Snyder, “Principles of Adsorption Chromatography,’’ Dekker, New York, N. Y., 1968. (12) L. J. Andrews and R. M. Keefer, “Molecular Complexes in Organic Chemistry,” Holden-Day, San Francisco, Calif., 1964. (13) A. V. Kiselev and Ya. I. Yashin, “Gas-Adsorption Chromatography,” (in Russian), Nauka, Moscow, U.S.S.R., 1967.
I
0
Assuming that the activity coefficients are constant [see, however, reference (Is)],the two adsorption equilibria are characterized by: the adsorption constant of the solute, KAz
=
XAZ/XAXZ (1)
the adsorption constant of the solvent, KAs
=
XAS/XAXS ( 2 )
where X denotes mole fraction (the components of the system are: A, AS, AZ, S, 2, and N). Deviations from the theoretically expected behavior, caused by variations of the activity (14) R. H. Ewell, J. M. Harrison, and L. Berg, Ind. Eng. Chem., 36, 871 (1944). (15) G. C. Pimentel and A. f,. McClellan, “The Hydrogen Bond,” Freeman, San Francisco, Calif., 1960. (16) C. H. Giles and I. A. Easton, Adcan. in Chromatogr., 3, 67 (1966). (17) C. J. 0. R. Morris and P. Morris, “Separation Methods in Biochemistry,” Interscience, New York, N. Y.,1963. (18) D. H. Everett, Trans. Faraday SOC.,61, 2478 (1965). VOL. 41, NO. 1, JANUARY 1969
179
coefficients with the composition of the developing solvent, can be expected to be parallel for solutes of related molecular structure. It is assumed here that the constants K A Z and KAS characterize all hydroxyl groups on the surface available to the molecules-i.e., that the groups are very similar in properties (or have a certain average H-bonding property). This is a simplification in view of specific electron shifts, geometry of the surface hindering the adsorption of larger molecules, etc. It is seen from the equations that single point adsorption is assumed. The last assumption seems to limit the solutes to substances possessing only one functional group; however, it can be presumed that the presence of the electron donor solvent prevents most functional groups of the solute, except the strongest ones, from adsorption by silica so that single point adsorption also occurs for many polyfunctional solutes. This presumption has been confirmed by L. R. Snyder in his letter to the author of the present communication. A fragment of the letter is quoted here in view of the clear quantitative characterization of the adsorption mechanism in systems under discussion: “It is worthwhile to consider the net adsorption energy of various sample groups as a function of solvent strength. For a group i which is not delocalized (by the presence of another more strongly adsorbing group k in the same molecule), its net adsorption energy is given as [eot- atto],where at is the effective size of the group i and €0 is the solvent strength. For some critical solvent strength ac, we see that the net adsorption energy of group i can become equal to zero; namely, ec = Qto/a,. For solvent strengths in excess of ec, the net adsorption energy of group i is actually negative. This means that group i is then not adsorbed, for its adsorption is not thermodynamically favored. Let us examine the values of eo for various groups i. From data given in reference (//) (Chapters 8 and 10) we can calculate these values in a straightforward manner: Silica Group i Ar-SCH3 Ar-OCH3 Ar-N02 AI-SCH3 Ar-C02CH3 Ar-COCH3 Ar-COOH
Q0t
ai
1.29 1.83 2.77 2.94 3.45 4.69 6.1
3.2 4.6 7.5 7.4 8.1 9.2 8.3
Alumina EC
0.40 0.40 0.37 0.40 0.43 0.51 0.73
EC
1.6 1.6 2.1 1 .o 1.5 2.5 4.5
“For all but the strongest groups i (largest Q i o values), we see that these groups will not be adsorbed on silica when the solvent strength exceeds a value of about 0.4. This means that such groups should not be adsorbed on silica from solvents as strong as acetone (eo = 0.47) or dioxane (ao = 0.49). If we also take into account the weakly adsorbing groupsi.e., carbon atoms-which link strong groups in a sample molecule, as well as the delocalization of strong groups, it now becomes quite reasonable to postulate for strong solvents and silica ds adsorbent that most samples are adsorbed by a single attachment to the adsorbent surface. However for alumina as adsorbent, the calculated tc values are in all cases greater than €0 for the strongest known solvent (methanol, €0 = 0.95); therefore group desorption and single-site attachment for polyfunctional sample molecules is not generally expected on alumina (but note case of phenols on alumina [101).” The ratio XAs/XA, in view of the dynamic character of adsorption, is also equal to the ratio of the time an average 180
ANALYTICAL CHEMISTRY
hydroxyl group is in the solvated form and the time it is in the free form ( t A s / t A ) . It follows from Equation 2 that the adsorption of solute Z c a n change this ratio by shifting the equilibrium. However, for very low concentrations of the solute, the shift can be expected to be insignificant. The next assumption is that the molecules of the solute interact only with those hydroxyl groups which are free (not H-bonded) at the time of the contact. The mole fraction of free hydroxyl groups is X A
=
XAS/XSKAS
(3)
It can be assumed that, in view of low concentration of the solute and strong A-S interactions, X A S>> X A , X A Z(except for dilute solutions of the active solvent). On the other hand, taking into account Equations 1 and 3, the partition coefficient is equal to K
=
XAZ/XZ = K A Z X A= KAzXAs/KAsXs
(4)
Because the term Xz denotes the concentration of the migrliting molecules of the solute, and X A Zthe concentration of molecules immobilized by adsorption, the partition coefficient determines the R, value of the solute: R, = l/(l K). Accordingly,
+
RM =
log RI/(l - R,) = -log K = log K A S - log KAZ - log X A S
+ log X s
(5)
The first two terms on the right hand side of Equation 5 are constants. As stated above, for not too low values of Xs, the third term ( X A s )can be expected to vary within a narrow range. Because the solvent is in excess in comparison to the surface hydroxyl groups, then X A
+ + XZ
XAZ
+
XAS
> eoa in Snyder’s notation) and that one molecule of the solute is displaced by one molecule of the solvent. As the energy effect is determined mainly by H-bonding, the displacement of the molecules of the inert solvent N can be expected to play a minor part so that 1 : 1 displacement can be assumed even in cases when the solute and the active solvent differ (within reasonable limits) in molecular sire. Furthermore, the OH groups on the silica gel surface (distance between groups ca. 3 A) can bend and rotate so that monofunctional molecules can be accomodated on vicinal OH groups, with the adsorption exchange in the 1 :1 ratio, in spite of differences in the molecular size. The effect of the adsorbent surface area, characterized by the term log X A s ,is also apparent from Equation 5. Because the concentration of solvated surface hydroxyl groups is, under the conditions discussed above, proportional to the surface area of silica gel, it follows that RMshould be linearly dependent on log of the specific surface area of the adsorbent, in accordance with the fundamental equation of Snyder’s theory (Reference / I , Equation 8-3). The derivation of Equation 5 presented in this communication differs from Snyder’s considerations. The terms of
4t Figure 1. R.ri us. log mole fraction of active solvent. u. tetralin acetone. b. cyclohexane dioxane
+ +
400r
The solutes are: 0, o-nitroanifine; (3, m-nitroaniline; 0 , p nitroaniline. Adsorbent : silica gel for chromatography, Merck, Darmstadt. For experimental details see reference (22)
40
I
I
20
40
+
Ssds
e
Zads
+
Seoln
with equilibrium constant equal to K A Z / K A S .Equation 5 can be generalized for cases when the solute is displaced by the active solvent in other molecular ratios (see Snyder’s book). Let us consider yet a more complex case when the solvent interacts strongly also with the solute so that solute-solvent complexes are formed :
+
S Z $ SZ Ksz = XszjXsXz (6) The solvation of the solute can appreciably reduce its adsorption affinity. As HeFmanek, Schwarz, and Cekan have pointed out (19), solute-solvent interactions can even change the eluotropic sequence of solvents. The role of solvation has also been stressed by Bark and Graham (20); on the other hand, Snyder (11, 21) has found that for most solvent systems possessing low or moderate elution strength, solvation effects are of minor importance. If it is assumed that free molecules of a monofunctional solute are adsorbed by free hydroxyl groups on the surface, then
K =
Xz
XAZ Xsz Xz
+
XA Z
+ KszXsXz
XAZ -~
Xz 1
1
+ KszXs
For Ksz = 0, the simpler Equation 4 is obtained. It is seen from Equation 7 that the formation of solute-solvent complexes shifts the partition in favor of the mobile phase (decreased values of the partition coefficient K and thus increased values of R J . Therefore, solvents which interact both with the adsorbent as well as with the solute, can have higher elution strengths. In cases when KszXs >> 1 and the unity in the denominator of Equation 7 can be neglected R M = --log K
log K A S K S Z ~ KA Zlog X A S 4- 2 log X s (8) (19) S. Heimanek, V. Schwarz, and Z. cekan, Collect. Czech. Chem. Commun., 28, 2031 (1963). (20) L. S. Bark and R. J. T. Graham, J. Chromatogr., 27, 116 (1967). (21) L. R. Snyder, ibid.,18,461 (1965).
1
I
60 80 400
b.
a.
Equation 5 are expressed by mole fraction concentrations and two equilibrium constants, KAz and KAS. In Snyder’s papers (9, 10) only the overall equilibrium is considered (also in other molecular ratios, common especially for adsorption from weak solvents) Zsoln
I
and it follows that the mobility of the zone is then more strongly dependent on the concentration of the active solvent, the slope of the R.Mus. log XS line being 2. For decreasing concentration of the active solvent, the slope gradually decreases to 1 (when KszXs
1500
A
-
p 4000 4500 WAVELENGTH
6)
i
O
4
5000
Figure 1. Spectral scans of vanadyl and vanadate ions Solutions contain 172 mg of vanadium in 50 ml of 20 H2!304. Vanadium in solution a was oxidized with KMn04 while that in solution b was reduced with SOZ. Solutions scanned in 1-cm cells
EXPERIMENTAL
Apparatus. A Cary Model No. 14 recording spectrophotometer equipped with slidewire No. 1480560 was used throughout. (1) T. Moeller and J. C. Brantley, ANAL.CHEM., 22, 433, (1950). (2) C. V. Banks and D. W. Klingman, Anal. Chim. Acta., 15, 356 (1956). (3) D. C. Stewart and D. Kato, ANAL.CHEM., 30, 165 (1958).
182
ANALYTICAL CHEMISTRY
Reagents. A standard europium solution (1 mg/ml) was prepared by dissolving sufficient Euz08(Kleber Laboratories) in HCl. The solution was then standardized by gravimetric analysis (oxalate precipitation). In addition, Fisher-purified ammonium metavanadate and europium-free yttrium oxide prepared by ion exchange at these laboratories were used. Procedure. Phosphor samples (700 mg) and four standards containing 396 mg of ammonium metavanadate and
