ION-EXCHANGE PROCESSES IN AQUEOUS DIMETHYLFORMAMIDE MIXTURES
heterogeneity in molecular weight, but not for the strength of the hydrodynamic interaction h. I n conclusion, in the present system near the 0 temperature, the normal stress component ull - u33 was proportional to K~ and c / M over a wide range of variables, and the component u~~- u33was negligible.
52 1
The effect of solvent power on the normal stresses will be studied in the following paper. Acknowledgment. We are indebted to Professor Tadao Kotaka for his kind advice and stimulating discussions. Thanks are also tendered to the Ministry of Education of Japan for a grant-in-aid.
Ion-Exchange Processes in Aqueous Dimethylformamide Mixtures'
by A. Ghodstinat, J. L. Pauley, Teh-hsuen Chen,2 and M. Quirk3 Department of Chemistry, Kamas State College of Pittsburg, Pittsburg, Kansas
(Received August 90,1966)
Lithium-sodium, potassium-sodium, cesium-sodium, and sodium-cesium exchange on Dowex 50W X-1 in 0, 5, 10, 25, 50, 75, and 90% dimethylformamide-water mixtures were studied. In general, the logarithm of the exchange coefficients varied with the reciprocal of the dielectric constant as would be predicted for coulombic interactions. However, a minimum was observed for the potassium-sodium and the cesium-sodium exchanges with increasing dimethylformamide concentration. The sodium-cesium exchange showed a corresponding maximum. Radioactive tracer techniques were used to determine exchange coefficients. Solvent uptake and solvent distribution data were also obtained. In general, water is preferred in the resin phase. The preference decreases with higher dimethylformamide concentrations. Total solvent uptake also decreased with increasing dimethylformamide concentration, although much less markedly for the Li resin than for the others.
Introduction Nonaqueous solvents as ion-exchange media have been investigated in recent years by several I n general, mixed solvents uskg water 8s one C O D ponent have been used rather than strictly nonaqueous media for systems using organic exchangers. Equilibrium is reached very slowly in most pure organic the presentinvestigation, the dimethylsolvents. fomamide-water (DMF-H20) system was chosen since this system provided reasonably good salt sohbilities, a good range of dielectric constants, and waterlike properties' A low cross-1inked exchanger was to attainment Of in the DMFrich mixtures where resin swelling was limited.
Experimental Section MateriaZs. Solutions were prepared using reagent
grade DMF and ion-free water. Potassium chloride, sodium chloride, lithium chloride, and cesium chloride were all reagent grade and were dried under V a m m n before use. The radioactive sodium-22 and cesium134 were carrier free in the chloride f0m-1. The ex(1) Presented before the Division of Physical Chemistry of the American Chemical society at the Southwest Regional Meeting, Shreveport, La.,Dec 1964. (2) Taken in part from the dissertation of Teh-hsuen Chen to the Graduate School of Kansas State College of Pittsburg in partial fda m e n t of the requirements for the Master of Science degree. (3) National Science Foundation undergraduate research participant. (4) R. G. Fessler and H. A. Strobel, J. Phys. Chem., 67,2562 (1963). ( 5 ) P. C. Huang, A. Mizany, and J. L. Pauley, ibid., 68,2575 (1964). (6) R. Gable and H. Strobel, ibid., 60,513 (1956). (7) D. D. Bonner and J. C. Moorefield, {bid., 58, 555 (1954). (8).A. Materova, Zh. L. Verts, and G. P. Grinberg, Zh. Obshch. Kham., 24,953 (1954).
Volume 70,Number I February 1966
522
A. GHODSTINAT, J. PAULEY, T. CHEN,AND M. QUIRK
changer used was Dowex 50W X-1 (50-100 mesh) obtained in the hydrogen form from Bio-Rad Corp. The resins were converted to the appropriate salt form with the hydroxide or chloride of the desired salt as convenient. Resin capacities were determined by adding excess base and back titrating the excess. Capacities in milliequivalents per dry gram of resin were 5.20, 5.00, 4.66, 4.34, and 3.08 for the hydrogen, lithium, sodium, potassium, and cesium exchangers, respectively. Samples of the various salt forms of the exchanger were spiked with carrier-free sodium-22 or cesium-134 (activity approximately lo6 counts/ min per g) and dried under vacuum at 120' before use. Equilibrium Systems. Weighed samples of approximately 1 g of the dry, spiked exchanger in the salt form were transferred to glass serum bottles and permitted to equilibrate a t 30' with 25 ml of an approximately 0.1 M solution of the salt corresponding to the ionic form of the resin. When equilibrium had been reached, aliquots of the solution phase were taken and counted to determine radioactivity eluted from the resin phase. Later aliquots were counted to determine that equilibrium had truly been reached. A total activity balance was determined in several cases to assure that there were no losses of activity due to adsorption. Selectivities. Selectivities calculated corresponded to the exchange A + B*R = AR B*f, where A + was sodium, lithium, potassium, or cesium and B* was radioactive sodium or cesium. Selectivity coefficients were calculated according to
+
K*B
=
+
(AR, mequiv/g) (B*+, counts/min per ml) (B*R, counts/min per g) (A+, mequiv/ml)
The normality of the metal salt solution, (A+), and the capacity of the resin, (AR), were assumed to remain constant during the exchange. The activity of the resin phase was corrected for the activity in the equilibrium solution phase. Rate Determinations. To determine the rates of reaction, the same procedure mas used as for the equilibrium studies except that samples were withdrawn periodically, centrifuged, and the aliquots were counted to determine tracer activity in the solution phase. Solvent Distribution. Solvent distribution between the resin and solution phases was obtained by equilibrating the appropriate salt form of the resin with the various solvent mixtures. The excess solvent was then removed by filtration, followed by blotting the resin phase. The solvent retained in the resin phase was removed by distillation under vacuum. The composition of the distillate was determined by measuring its refractive index. The JOUTnal of Physical Chemistry
-0. -0. -0.5 -0. 0
1
I
IO
20
I
30
I
40
I
I
I
50
60
10
PE K E N T DMF
00
90
180%
Figure 1. Logarithm of selectivity coefficient for exchange reactions us. composition of the DMF-H*D solvent mixture: A, Na-Li exchange; m, K-Na exchange; 0,Cs-Na exchange; V, Na-Cs exchange. The points ( A ) for Na-Li exchange represent the inverse of the experimentally determined selectivity coefficients for the Li-Na exchange using radioactive sodium as the tracer element.
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 resin. These values appeared to be more reproducible and more easily interpreted than volume ratios.
Results Equilibrium results for several exchanges are shown in Figure 1, in which the logarithm of the selectivity coefficient is plotted against solvent composition. I n the case of the Li-Na exchange, the reciprocals of experimental exchange coefficients, corresponding to the calculated coefficient for the Na-Li exchange, are plotted for convenience in comparing the results for the various systems. I n the case of Na-Li, KNa, and Cs-Na exchanges, there is an increase of log K in going from water to high concentrations of DMF. This corresponds to the change expected for ion-ion interactions since the dielectric constant decreases rather regularly with increasing Dh4F concentration. I n the cases of the Cs-Na and K-Na exchanges, however, a minimum was observed in the plot of log K vs. concentration of DPtlF. To investigate the reality of the minimum, the Na-Cs exchange was also investigated, and a maximum was observed a t about the
ION-EXCHANGE PROCESSES IN AQUEOUS DIMETHYLFORMAMIDE MIXTURES
Table I : Dielectric Constant as a Function of Composition for the H20-DMF System Compn, 70 D M F
Dielectric constant, D
1/D X 102
0 10 25 50 75 90 100
78.2 72.2 63.7 56.7 46.8 40.0 36.7
1.18 1.38 1.57 1.76 2.14 2.50 2.73
same solvent composition as the minimum for the Cs-Na system. Plots were made also of log K us. the reciprocal of the dielectric constant of the solvent mixtures. The general shapes of the curves were the same. The increase of K with decreasing dielectric constant agrees generally with the effects noted by Fessler and Strobe14for exchanges in methanol-water and ethanolwater mixtures. For the Na-Li exchange they noted no maximum as solvent composition varied although a leveling off of K values was observed at higher methanol concentrations. They noted maxima for all exchanges involving hydrogen ion at dielectric constants of about 40 and 29 for ethanol-water mixtures and methanol-water mixtures, respectively. The presence of a minimum or maximum in selectivity coefficients as the per cent of DMF increases implies the operation of two opposing factors. The decrease in dielectric constant accompanying the increase in per cent of D M F would be expected to result in a regular increase of selectivity for the preferred ion. However, if ionic solvation changed in an opposing manner as the solvent composition changed, the observed minima or maxima could be accounted for on the basis of a relatively simple electrostatic interaction model9 or on the basis of competition between coulomb interaction and ionic solvation as proposed by Eisenman.lo It may be of interest to note that a t low DRIF concentrations the solvent distribution results (Table 11) suggest that the cesium resin does not selectively absorb water as effectively as do the potassium and sodium resins. It is possible that at these concentrations the cesium ion on the resin is at least partially solvated by the larger D n l F molecules while potassium and sodium ions are either solvated only by water, or at least are less solvated by DMF. This would result in a change of relative, solvated ionic sizes in the direction required to account for the observed results, It can be seen from Table I1 that above
523
about 50% DRIF there seems to be little difference in selectivities of each of these ions for water over DMF. If this reflects attainment of a relatively stable solvated state, dielectric constant effects should again become dominant which would lead to the observed increase of selectivity with increasing D M F concentration. The reversal of selectivity for potassium and cesium which occurs above about 25% DMF may be related to the observed changes in order of solvent absorption as shown in Table 11. Below about 25% D M F the order of solvent absorption is as expected: Li > Ka > K > Cs. Above about 25% DMF the order is Li > Ya > Cs > K. This may suggest a relative change in the solvation of the ions which would account for the reversal of selectivities. The fact that there may be differences in composition in the solvent composition in the solution and resin phases has not been generally considered for systems of mixed solvents. I t can be seen from Table I1 and would be expected from work on solvent exclusion processes that the resin phase has a significantly higher affinity for water than for DMF. This is particularly true at low DMF concentrations. The Li resin seems to have a much less marked preference for water in the middle region of the composition of solvent. Similarly, as seen from solvent uptake data, the Li resin is swollen to a much larger degree by the solvent mixtures high in DMF than the other resins. This may suggest a t least weak complex formation of the lithium ion with the DMF somewhat analogous t o complexes formed with lithium ions and ethylenediamine. Plots of log K us. composition of the solution in the resin phase were also made. However, the nature of the plots was not significantly different from plots of log K us. composition of the external solution phase although the slopes changed slightly and the maxima or minima were shifted slightly toward lower D M F concentrations. Rate data were obtained for the Li-Na exchange in 10% DIlF-H20, 90% DAIF-HzO, and pure D M F solution.11 As would be anticipated, the rate of exchange decreased with increasing DMF concentration. Equilibrium was reached within 1 hr in 10% DSIF and in less than 2 hr in 90% DMF. However, it is doubtful if a true equilibrium was reached in pure D M F even after 3 days. A plot of the function B(t) as suggested (9) J. L. Pauley, J. Am. Chem. Soc., 76, 1422 (1954). (10) G. Eisenman, “Membrane Transport and Metabolism,” A. Kliemzeller and A. Kotyk, Ed., Academic Press Inc., New York, N. Y., 1961, p 163. (11) F. Helfferich, “Ion Exchange,” McGraw-Hill Book Co., Inc., New York, N. Y., 1962, p 302 ff.
Volume 70,Number 8 February 1966
A. GHODSTINAT, J. PAULEY, T. CHEN,AND M. QUIRK
524
~~
Table 11: Swelling and Changes in Solvent Composition in the Resin and Solution Phases for Li, Na, K, and Cs Resins aa a Function of the Composition of the Solution Phase Compn of soln phase,
% DMF
0 5 10 25 50 75 90
7
L
i
R
-
A
0 2.5 4.5 23 46 73 88
P B
1.1
.*. 1.1 1.1 0.99 0.95 0.91
(A) Compn of the solvent in the reain phase, yo DMF (B) Amount of solvent retained by the resin, g of solvent/mequiv of resin -NaRKR-CsRA B A B A
0 1.5 4.0 14 35 66 84
by Rei~henbergl*>'~ indicated that the rate is diffusion controlled as appears to be the case in water.
Conclusion As shown by this and other investigations, the solvent does significantly affect both the selectivities and the rates of ion-exchange processes. The present results agree with the conclusion of other investigations that the dielectric constant of the media is a most significant factor in solvent effects. However, other effects, including ionic solvation, do play a significant
The J O U Tof ~Physical Chemietry
0.97
...
1.1 0.91 0.81 0.60 0.24
0 2.0 4.0 14 37 66 89
0.95
...
0.96 0.92 0.67 0.15 0.12
0 2.5 6.0 19 41 67 85
B
0.96
... 0.06 0.96 0.79 0.24 0.12
role as evidenced by the existence of maxima or minima in plots of selectivity coefficients vs. solvent composition. These effects appear also to be reflected by changes in solvent composition in the resin and solution phases and by resin swelling results. These observations suggest that further studies may be of value in improving understanding of the interaction of ionic materials as a function of solvent composition. (12) D.J. Reichenberg, J . Am. C h m . SOC.,7 5 , 589 (1953). (13) G.E. Boyd, A. pi. Adamson, and L. S. Myers, Jr., ibid,, 69, 2836 (1947).
