Ion exchange in water-alcohol mixtures

J. inorg, nucI.Chem..1971, Vol. 33, pp. 3131to 3139. PergamonPress. Printed in Greal Britain

ION

EXCHANGE

IN

WATER-ALCOHOL

MIXTURES

R. D. S H E R R I L , M A R V I N D. S M I T H and J. L. P A U L E Y D e p a r t m e n t of Chemistry, K a n s a s State College of Pittsburg, Pittsburg. Kansas 66762

(First received 13 October 1970: in revisedjbrm 22 December 1970) Abstract- Ion e x c h a n g e of a series of quaternary a m m o n i u m salts against cesium on BioRad A G 50 W-XI was carried out in m e t h a n o l - w a t e r and 2 - p r o p a n o l - w a t e r s y s t e m s at 25°C. Alkali metal ion e x c h a n g e against cesium in 2 - p r o p a n o l - w a t e r s y s t e m s were carried out for comparison. F o r the quaternary a m m o n i u m ion e x c h a n g e s a reversal of the usual order of selectivity based on ion size was noted at high water c o n t e n t which m a y be due to expulsion of the large ions by the solvent phase. At lower water contents the normal order of e x c h a n g e was noted and the log of the selectivity coeffÉcients appeared to vary nearly linearly with the reciprocal of the dielectric constant of the solvent media. For the alkali metal exchanges, no similar inversion of normal order was noted but the logs of selectivity coefficients were not generally a linear function of I/D. Swelling data were obtained for all s y s t e m s but no direct relationship between swelling and selectivity could be determined. Solvent distribution data indicated, generally, little selective solvation of the resin phase.

INTRODUCTION

IN EARLIERpapers [ 1,2] cation exchange in various aqueous-organic media have been reported in an attempt to deduce principles determining ion selectivity. From these and other studies [3-6] it appears that selectivity coefficients can be related to dielectric constant if certain rather arbitrary assumptions are made regarding effects of solvent change on ion solvation. In order to eliminate, or at least minimize, solvation effects, exchange of a series of quaternary ammonium ions for cesium in isopropanol-water and methanol-water systems have been studied. For comparison, data for alkali metal ion exchange for cesium in 2propanol-water systems have also been investigated. Low cross linked exchangers were used to minimize any screening effects and to facilitate attainment of equilibrium in regions of low water content where resin swelling may be limited. EXPERIMENTAL

Materials. Solutions were prepared using reagent grade solvents and ion free water, Salts were all reagent grade and were dried u n d e r v a c u u m before use. T h e radioactive ~'~4Cs and 22Na were obtained carrier free in the chloride form. T h e cation e x c h a n g e r used was BioRad A G 50W-XI (50-100 mesh) obtained in the hydrogen form. T h e resins were converted to the appropriate salt form with the hydroxide or carbonate of the desired salt as convenient. Resin capacities were determined by adding e x c e s s base and back titrating the excess. Capacities were calculated as m-equiv, dry g o f the resin in the salt form. T h e cesium and sodium form resins were spiked with the carrier free '34Cs or 22Na as 1. A. G h o d s t i n a t , J. L. Pauley, T e h - H s u a n C h e m and M. Quirk, J. phys. Chem. 70, 521 (1966). 2. J. k. Pauley, D. D. Vietti, C. C. Ou Yang, D. A. W o o d and R. D. Sherrill. Analyt. Chem. 41, 2047 (1969). 3. F . G . Fessler and H. A. Strobel,J. phys. Chem. 67, 2562 (1963). 4. R. G a b l e and H. Strobel,J. phys. Chem. 60, 513 (1956). 5. Y. U. 1. l g n a t o v and N. A. lzamilov, Z.fiz. Khim. 39, 2482 ( 19651. 6. G. E i s e n m a n , Biophys. J. 2. 259. (1962). 3131

3132

R.D. SHERRIL, M. D. SMITH and J. L. PAULEY

appropriate (Activity approximately 10eCPM/g). The salt form resins were dried under vacuum at 100-120°C before use. This drying did not cause measurable changes in exchange capacity for the resins studied. Equilibrium systems. Weighed samples of approximately lg of the dry spiked exchanger in the Cs form were equilibrated with 30 ml of an approximately 0.1N solution of the bromide of the quaternary ammonium ions or the chloride of the alkali metal ions to be exchanged. When equilibrium had been reached, the activity of aliquots of the solution was measured to determine the amount of cesium removed from the resin. Attainment of equilibrium was assumed when further statistically significant changes in activity of the solvent phase could not be detected. Equilibrium was attained rapidly for all exchanges up to about 50% alcohol content of the solvent but only slowly if at all at very high alcohol content of the solvent. For this reason, selectivity coefficients are not reported for all systems at high alcohol contents. The resin was eluted with HC1 after equilibrium had been attained to determine the activity remaining on the resin to assure that an activity balance was maintained. The exchange of Cs for 22Na was determined for the 2-propanol-water system to check the reversibility of the exchange. The Cs-Cs* exchange was included to give an indication of the reliability of the technique. The average value of the selectivity coefficient, neglecting the point at 75% 2-propanol, is 1.04 suggesting a small regular positive error of unknown origin. No attempt was made to rationalize data for other exchanges on this basis however. Selectivities. Selectivities calculated corresponded to the exchange A+ + Cs*R = A R + Cs *+, where A was an alkali metal or quaternary ammonium ion. Selectivity coefficients were calculated according to: •

*+

(AR, m-equiv./dry g of salt form resin) (Cs , cpm/ml.) K~8= (Cs*R, cpm/dry g of salt form resin) (,4+, m-equiv./ml.)

The normalities of the salt solutions, A÷, were calculated by difference from the original normalities and the concentration of ls4Cs*+ following exchange. Solvent distribution. Solvent distribution between the resin and solution phases was obtained by equilibrating the appropriate salt form of the resin with a slight excess of the particular solvent system above that needed to swell the resin and noting any changes in refractive index of the solvent. This was checked in some cases by equilibrating the resin with the solvent mixture, blotting off the excess solvent and distilling over the solvent retained in the resin phase• The composition was then determined from its refractive index. Swelling. A weighed amount of the dry resin in the appropriate salt form was equilibrated with the solvent mixture in a stoppered vessel. The swollen resin was then filtered, patted dry of excess solvent and weighed. Swelling was calculated in terms of grams of solvent per milliequivalent of dry resin. RESULTS AND DISCUSSION V a r i a t i o n o f s e l e c t i v i t y coefficients w i t h s o l v e n t c o m p o s i t i o n g e n e r a l l y d o e s n o t a p p e a r to s h o w a n y s i m p l e c o r r e l a t i o n with a n y s o l v e n t p r o p e r t y [ i - 3 ] . S o m e i n v e s t i g a t o r s [ 5 ] h a v e r e p o r t e d a l i n e a r r e l a t i o n s h i p b e t w e e n the log o f s e l e c t i v i t y coefficients a n d the r e c i p r o c a l o f d i e l e c t r i c c o n s t a n t as w o u l d b e p r e d i c t e d b y a c o u l o m b i c i n t e r a c t i o n m o d e l [6, 7] b u t the r e s u l t s s h o w n in Fig. 1 are m o r e c o m m o n l y c h a r a c t e r i s t i c . A s has b e e n s u g g e s t e d , t h e s e r e s u l t s c a n b e r a t i o n a l i z e d with the c o u l o m b i c m o d e l if c e r t a i n , n o t u n r e a s o n a b l e , a s s u m p t i o n s are m a d e r e g a r d i n g c h a n g e s in s o l v a t e d i o n i c radii with c h a n g e s in s o l v e n t c o m p o s i t i o n [ 1,2]. F o r q u a t e r n a r y a m m o n i u m salts e x c h a n g i n g w i t h c e s i u m , s e l e c t i v i t y coeffici e n t v a r i a t i o n s with c h a n g e in s o l v e n t c o m p o s i t i o n b e h a v e s o m e w h a t differently as s h o w n in Figs. 2 a n d 3. T h e d a t a suggests a r e l a t i o n s h i p b e t w e e n In K a n d l I D b u t the r e l a t i o n s h i p 7. J. L. Pauley,J.Am. chem. Soc. 76, 1422 (1954).

Ion exchange in water-alcohol mixtures

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Fig. 2. Natural logarithm of selectivity coefficients vs. reciprocal of dielectric constant for quaternary ammonium ion exchange for Cs in 2-propanol-water systems at 25°C. NH4 Me4N -au~N_(~ Kc~ - O , • Kc~ - A , •K ~ET4N s - ~ , • gc~ ,~'r,S D,. g nc~ •

3134

R . D . SHERRIL, M. D. SMITH and J. L. P A U L E Y

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changes with solvent composition. At both high and low water contents, the variation of In K with lID is reasonably linear but with different slopes in each region. Below a dielectric constant of about 40-45 (lID = 20-22 × 10-3) the "normal" order of exchange is noted and the slopes appear to be as anticipated by the coulombic model in that exchange coefficients for ions differing most widely in solvated radii change the most rapidly with change in dielectric constant. Thus it would appear that, in the region of low water content, results might be reasonably explained on the basis of coulombic interactions between ions. A comparison of the two solvent systems however suggests that this explanation is not completely satisfactory since slopes in the low dielectric constant region are significantly different in the two solvent systems. It is not likely that the ions involved show any significant coulombic solvation in either system thus it would not appear that changes in slope can be reasonably rationalized on the basis of differences in relative solvated ionic radii in the two systems. As shown in Fig. 4, this effect of changes in the nature of the solvent system on the rate of change of In K with 1/D is not limited to the quaternary ammonium exchanges. Limited data, not shown in the figure, for the Li-Cs exchange in methanol-water systems indicated the slope is larger than for the ethanol-water systems. The results shown in Figs. 2 and 3 for both solvent systems in regions of high water content are similarly not in agreement with a simple coulombic model. This model would predict that the smaller ion should be preferred by the exchanger. In the case of the quaternary ammonium-cesium exchanges in water, this is not the case; in fact, there is an almost complete reversal of the expected

Ion exchange in water-alcohol mixtures

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order, the order being n-Bu4N + > n-Pr4N + > Cs + > Et4N + > Me4 N+ > N H + ~ -. This effect has been noted [8, 9] for weak acid exchangers and explained as being due to London forces acting between the resin and the large organic quaternary ammonium ions. An alternative explanation is, however, possible. The quaternary ammonium halides in water have been investigated by a number of authors [9-20] with the general conclusion that at least three factors may be important in accounting for their behavior; water structure enforcement effects of large organic ions, water structure enforced ion pairing and, at higher concentrations, micelle formation. The relative importance of the first two of these factors is apparently a function of the anion as reflected by the fact that for the chlorides, the activity coefficients at low concentrations increase in the order MeaN + < Et4N + < Pr4N + < Bu4 N+ while for bromides and iodides the order is reversed[14]. Comparison of the apparent molal enthalpies and entropies of the several halides does not show any inversion of order with change of anion suggesting that water structure enforcement is the major effect[15]. Free energies are small compared to enthalpies or 8. 9. 10. I 1. 12. 13. 14. [ 5. 16. 17. 18. 19. 20.

T. R. E. Kressman and J. A. Kitchener, J. chem. Soc. 1208 (1949). G. E. Boyd and Q. U. Larson,J.Am. chem. Soc. 89, 6038 (1967). J. Steigman and M. D. Monica, J. phys. Chem. 74, 516, (1970). H. S. Frank and Wen-Yang Wen. Discuss. Faraday Soc. 133 (1957). W. Riemann III,J. Chem. Educ. 38,338 (1961). N.S. Frank and A. S. Quist, J. chem. Phys. 36,604 (1961). S. Lindenbaum and G. Boyd, J. phys. Chem. 68.911 (1964). S. Lindenbaum, J. phys. Chem. 70.814 (1966). R. M. Diamond,J. phys. Chem. 67,2513 (1963). R. A. Horne and R. P. Young, J. phys. Chem. 72, 1763 (1968). M. B. Reynolds and C. A. Kraus, J. Am. chem. Soc. 70, 1709 (1948). S. Lindenbaum, J. phys. Chem. 74, 3027, (1970). S. Lindenbaum, L. Leifer, G. E. Boyd and J. W. Chase, J. Phys. Chem. 74, 761 (1970).

3136

R . D . SHERRIL, M. D. SMITH and J. L. P A U L E Y

entropies and thus it is possible that while structure enforced ion pairing, or at least anion dependence of this effect, may not contribute to any significant degree to molal entropies or enthalpies, these relatively small contributions or differences in contribution may result in a significant dependence on the anion of the relative order of molal free energies or activity coefficients with respect to cation size. The order of exchange of the quaternary ammonium ions at high water content noted in this investigation would suggest that water structure enforced ion pairing, or the dependence of this effect on cation size, is the most significant effect in determining relative selectivity coefficients or free energies. If it is assumed that the relatively large resin sulfonate anions are more effective than bromide ions in promoting structure enforced ion pairing, then the observed order is as would be expected [16], that is, ion pairing with the resin anion and thus enhanced selectivity would be expected to decrease with decreasing cation size as observed. It would be of interest to compare the data presented here for relative selectivities for the quaternary ammonium bromides with data for the chlorides and iodides. It would be expected from this argument that differences in selectivity would be reinforced for the chlorides since activity coefficients for the chlorides increase with increasing ion size further favoring the preference of the resin for the larger cation, while the reverse effect would be expected for the iodides due to the increased significance of structure enforced ion pairing of these salts in the solution phase. If micelle formation is significant at higher salt concentrations [14, 17], this should be reflected in a significant dependence of selectivities and their relative order on co-ion concentration in the solution phase since micelle formation should decrease the activity coefficients of the quaternary ammonium ions in the solution phase with a consequent decrease in the apparent preference of the resin for the larger ions. The change in relative order of selectivities to the "normal" order as the alcohol content of the solvent phase is increased, is also in agreement with this explanation. Solvent structure enforced ion pairing is only possible in a strongly structured solvent such as water. Thus, as alcohol is substituted for water in the solution phase, it would be expected that the dependence of activities upon cation size should revert to the normal order [18]. This may also account for the observed decrease in selectivity coefficient at a given dielectric constant in going from the 2-propanol-water system to the methanol-water system. At the same dielectric constant there would be a much larger volume or weight percent of the more polar methanol in water than 2-propanol thus disrupting the water structure to a greater degree. This would decrease the significance of the solvent structure enforced pairing which was presumably responsible for the noted inversion of normal order of selectivities in water. A somewhat different approach has been suggested by Lindenbaum[19] and others [20]. They emphasize the importance of the solvent but do not require ion pairing due to non-coulombic interactions. They suggest that changes in sequence of the magnitudes of osmotic coefficients with changing size of the anion may be qualitatively accounted for on the basis of the relative effect of the cation and anion on the degree of hydrogen bonding of the solvent. This would imply that differences in the relative order of selectivity for polystyrene sulfonic acid resins and polymethacrylic acid resins for the tetraalkylammonium ions

Ion exchange in water-alcohol mixtures

313 7

must be related to differences in the ability of the resin anions to influence solvent structure. Since, however, it would appear that solvent structure in the resin phase should be somewhat limited by steric requirements, it would appear difficult to predict how the resin anion should affect solvent structure. This approach, like structure enforced ion pairing, does, however, emphasize the importance of including entropy changes of the solvent due to changes in solvent structure in any satisfactory theory which will account for ion exchange selectivities. The nature of the interface between the resin and solution phases may also be significant in determining selectivities. It is at least striking that the order of selectivity at high water content parallels the surface activity of the quaternary ammonium ions as reflected by the Hofmeister series. As the alcohol content of the solvent increases, the interface between the solvent phase and the resin phase becomes less pronounced and by Traube's rule the preferential absorption of the larger ions at the interface should become less pronounced leading to the observed "normal" order of selectivity as the water content of the solvent phase is decreased. It may be, however, that these interfacial effects are reflections of the general effects noted before and do not represent additional unique factors. Resin swelling behaviour with changes in solvent composition as shown in Figs. 5-7 is significantly different for the resin in the quaternary ammonium ion form compared to simple inorganic ion forms, the decrease in swelling with increased alcohol content being much less marked. The change in the relative degree of swelling of the Cs + and NH4 + form resins with respect to the quaternary ammonium ion resins and the marked decrease in swelling of all inorganic ion form resins as the alcohol content of the solvent phase is increased suggests that the solvation of these ions may be related to changes in selectivity coefficient

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3138

R . D . SHERRIL, M. D. SMITH and J. L, PAULEY

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Ion exchange in water-alcohol mixtures

3139

but that this relation is not a simple one. For the quaternary ammonium ion form resins, Coulombic solvation effects are unlikely to be significant[17] suggesting that swelling in this case may be primarily a matter of the expansion of the resin pores by these large ions. This would permit relatively free access by solvent molecules even in regions of low dielectric constant where the formation of ion pairs would tend to decrease electrostatic effects leading to the uptake of polar solvents. Swelling for these resins appears to be less a function of dielectric constant of the solvent than weight or volume composition. In comparing solvent uptake for these resins in the water-methanol and water-2-propanol systems at approximately the same weight concentration of alcohol, they are nearly the same but slightly larger for the methanol-water systems. This may reflect the smaller size of the methanol molecule and thus its greater ability to penetrate the resin pores extended by the large quaternary ammonium ions attached to the resin phase. This model for swelling may also account for selective solvation effects. No selective solvation was noted for any of the alkali metal ion form resins in agreement with other work [ 1,2, 21] but a slightly higher affinity for water was noted for the n-Pr4N ÷ and n-Bu4N + form resins at higher 2-propanol concentrations. This could again be accounted for by the somewhat greater ability of the relatively small water molecules to penetrate the distended resin pores compared to the larger alcohol molecules. There seems to be no apparent correlation between ion selectivity and resin swelling or resin solvation for exchanges involving the quaternary ammonium ions. CONCLUSIONS

From this and other investigations of ion exchange in mixed solvent systems, it is apparent that relative ion selectivities are markedly dependent upon the nature of the solvent system. This provides another dimension for consideration in application of ion exchange to analytical separations. The reversal of order and marked changes in selectivity with changing solvent composition noted for the quaternary ammonium ion systems could lead to useful analytical applications. It is, however, apparent from this and other investigations that current models for ion exchange are not adequate for the quantitative prediction of effects due to changes in the solvent systems. Dielectric constant and ion solvation effects are certainly significant but it would appear from the effects of changing solvents on the quaternary ion exchangers that any successful attempt to develop a comprehensive model for predicting ion exchange behaviour must give more serious consideration to solvent-solvent interactions as well as probably solvent-resin interactions and solvent structure changes. Resin swelling effects do appear to be related to selectivity in some manner but again the swelling behaviour of the quaternary ammonium resins with changes in solvent composition suggests that steric effects as well as electrostatic or ion solvation effects must be considered. Acknowledgements--Taken in part from dissertations presented to the graduate school of Kansas State College of Pittsburg in partial fulfillment of requirements for the Master of Science Degree. Presented in part to the Division of Physical Chemistry of the ACS Midwest Regional Meeting, Kansas City, Mo. (Nov. 1969). 21. D. Recihenberg and W. WalI,J. chem. Soc. London 3364 (1956).