1477
SELECTIVE SOLVATION OF IONS BY WATERIN PROPYLENE CARBONATE
Selective Solvation of Ions by Water in Propylene Carbonate by David R. Cogley, James N. Butler,”’ and Ernest Grunwald Tyco Laboratories, Inc., and Brandeis University, Waltham, Massachusetts 02164
(Received December 17, 1970)
Publication costs assisted by Harvard University
Proton magnetic resonance (chemical shift) measurements of water at low concentrations in propylene carbonate (PC) containing various salts have been made at 60 MHz and 36.4’. From these data the equilibrium constants for association of water with individual ions have been obtained under a relatively mild set of extrathermodynamic assumptions. The molal association constants K I are 6.5 for Li+, 1.4 for NaT, 0.4 for K+, and 6.2 for C1-. Additional order-of-magnitude estimates are 0.3 (Et4N+), 0.3 (BFd-), 0.3 (Clod-), 0.1 (BPh4-), and 0.4 (LiCl ion pair). Stepwise constants for addition of 2 and 3 water molecules to Li+ are Kt = 2.7 and K S = 1.9. The chemical shift of protons on a water molecule associated with an ion, compared with a water molecule in PC alone, varies from approximately 150 Hz for BPh4- to $95 Hz for Li+. The affinity of mater at low concentrations in PC for alkali metal cations correlates well with the free energy of transfer of these ions from PC to bulk water; but the free energy of transfer for a chloride ion (-11.3 kcal) is more than 10 times the free energy of formation of a chloride-water bond in PC. This reflects the stabilization of chloride ion in water by cooperative interactions which are at least as important as the direct coordination of the ion by HZO molecules.
Introduction
oxide-water mixtures, one may infer that the water is specifically coordinated to the hydroxide ion and deThe wide variation in reactivity of ions in different activates it,12but this need not be the only possible exsolvents is well known, and is usually discussed in terms planation in other cases. One approach to the elucidaof solvation energies and medium effects, or thermotion of this problem is to measure the first association dynamic functions of transfer from one pure solvent to constant of water with a series of ions in dipolar aprotic another.2-6 I n mixtures of solvents, where the first solvents, and we report here a relatively direct method solvation shell of different ions may have substantially of obtaining this information. different compositions, the additional concept of seFor these studies we chose as solvent propylene carlective solvation becomes an important part of underbonate [4-methyl-l,3-dioxolan-2-one, PC ], a dipolar standing chemical interaction^.^+-^ From the study of ionic reactions such as nucleophilic s u b s t i t ~ t i o n , ~acid ~ ~ ~d~i”s ~ o c i a t i o n , ~ solu~ ’ ~ ~ ’ ~ (1) Visiting lecturer on applied chemistry, Harvard University, 1970-1971. Division of Engineering and Applied Physics, Harvard bility, l4 and electrochemical potentials,’j it has become University, Pierce Hall, Cambridge, Mass. 01238. clear that a fruitful distinction can be made between (2) B. Gutbezahl and E. Grunwald, J . Amer. Chem. Soc., 75, 565 dipolar aprotic solvents (dielectric constant greater (1953). than 30) and hydroxylic or hydrogen-bonding solvents. (3) E. Grunwald, G. Baughman, and G. Kohnstam, {bid., 82, 5801 (1960) I n particular, the reactivity of small anions is dramati(4) A. J. Parker, Chem. Rep., 69, 1 (1969). cally greater in dipolar aprotic solvents compared to (5) R. G. Bates in “The Chemistry of Nonaqueous Solvents,” hydroxylic solvents, whereas the reactivity of cations is Val. 1, J. J. Lagowski, Ed., Academic Press, New York, N. Y., 1966; J. F. Coetzee and C. D. Ritchie, much less sensitive to the nature of the ~ o l v e n t . and ~ ~in “Solute-Solvent ~ ~ ~ ~ ~ Interactions,” ~ ~ Ed., Marcel Dekker, New York, K.Y., 1969. For instance, the nucleophilic displacement of iodide (6) P. Debye, Z. Phys. Chem., 130, 56 (1927). from CH31by chloride ion proceeds lo6 to lo7 times as (7) G. Scatchard, J . Chem. Phys., 9 , 34 (1941). fast in dipolar aprotic solvents (dimethylformamide, (8) H. Schneider in “Solute-Solvent Interactions,” J. F. Coetzee dimethylacetamide, acetone) as in hydroxylic solvents and C. D. Ritchie, Ed., Marcel Dekker, New York, N, Y . , 1969. (water, m e t h a n ~ l ) . Another ~ example is the marked (9) L. S. Frankel, C. H. Langford, and T. R. Stengle, J . Phys. Chem., 74, 1376 (1970). reduction in basicity when small amounts of water are (10) R. F. Rodewald, K. Mahendran, J. L. Bear, and R. Fuchs, added to benzyltrimethylammonium hydroxide in diJ. Amer. Chem. Soc., 90, 6698 (1968). methyl sulfoxide. n (11) R. Alexander, E. C. F. KO, A. J. Parker, and T. J. Broxton, ibid., 90, 5049 (1968). However, a more general question may be raised. (12) C. D. Ritchie in “Solute-Solvent Interactions,” J. F. Coetzee Is the high affinity of small anions for water a result of and C. D. Ritchie, Ed., Marcel Dekker, New York, N. Y., 1969. specific coordination effects (which should be apparent (13) B. W. Clare, D. Cook, E. C. F. KO,Y . C. Mac, and A. J. Parker, at low water concentrations in a dipolar aprotic solvent) J. Amer. Chem. Soc., 88, 1911 (1966). (14) R. Alexander, E. C. F. KO,Y . C. Mac, and A. J. Parker, ibid., or is it a cooperative phenomenon which becomes ap89, 3703 (1967). parent only when one approaches the structure of pure (15) J. N. Butler, Advan. Electrochem. Electrochem. Eng., 7, 77 water? I n the case of hydroxide ion in dimethyl sulf(1970) . I
The Journal of Physical Chemistry, Vol. 76, N o . 10, 1071
1478 aprotic solvent with high dielectric constant (65.0 at 250)16 and dipole moment (4.94 D).” Unlike water, P C does not exhibit any strong self-association or well defined intermolecular structure, as evidenced by a Rirkwood g factor near unity (1.02 a t 213’K).16 The main intermolecular forces are strong but nonspecific, and the high dielectric constant makes it possible to dissolve salts up to concentrations of several molal. Most salts are completely dissociated in dilute solutions.18 I n this respect, it has been suggested that P C is a close approximation to an “ideal structureless dielectric” solvent for studies of electrolytes.19 Because P C is only weakly acidic and weakly basic, one does not expect to find highly specific coordination phenomena such as are found in solvents like dimethylformamide or dimethyl sulfoxide. It has a wide liquid range (- 49.2 to 247.1 a t 1 atm) 2o which allows one to study temperature effects easily. It is relatively resistant to chemical attack (except for acid- or basecatalyzed hydrolysis a t high water concentrations), a property which has given it an important place in the development of high-energy batteries using lithium anodes. I n this application, selective solvation of ions by water or organic impurities may play a crucial role in battery system performance.l8lz1 To probe the composition and structure of the first solvation shell, nuclear magnetic resonance provides a valuable method because of the straightforward correspondence between chemical shifts and relaxation times of resonance signals and the details of short-range struct u r e ~2 3 . We ~ ~ have ~ ~ studied ~ ~ chemical shifts of the proton magnetic resonance of small amounts of water in P C containing dissolved salts. On the basis of relatively mild extrathermodynamic assumptions, we have been able to obtain equilibrium constants for association of water with individual ionic species. We find that the association constant for C1- with water in PC is very close to that for Li+ and only slightly greater than that for Na+. This result implies that, at least in the case of C1-, its much lower reactivity in water compared with dipolar aprotic solvents is due to cooperative effects at least as much as to specific coordination.
Experimental Section Solvent. Propylene carbonate (Jefferson Chemical Co.)ZO was purified by distillation a t approximately 2 Torr in a Podbielniak adiabatic vacuum still of approximately 50 theoretical plates. Typical concentrations of impurities (gas chromatographic analysis) were 10 ppm of propylene oxide, 20 ppm of propylene glycol, 2 ppm of allyl alcohol, and less than 2 ppm of water. Details of purification and analysis are given else~ h e r e . 225 ~ Doubly distilled, COz-free water was used in preparation of solutions containing HzO. Salts. Ultrapure lithium perchlorate was obtained from Anderson Physics Laboratories. This sample was prepared from G. F. Smith 99.9% LiC104, recrystalThe Journal of Physical Chemistry, Vol. 76, N o . 10,1971
D. R. COGLEY,J. N. BUTLER,A N D E. GRUNWALD lized three times from water, heated under vacuum for several days, and finally fused under vacuum and sealed in glass under argon. It contained less than 0.0015% HzO and less than O.OOOSOl, C1.26 Lithium tetrafluoroborate (Foote RIineral Company) in the form of a fine, free-flowing white powder was used as received. Flame photometric analysis (Instrumentation Laboratories Model 143A, 1 mM KCI internal standard, 0.02% wetting agent) indicated 0 A 1 mol % Na and 103 f 1 mol % of the expected Li content. The Li calibration n7as made with ultrapure LiCl (see below). The most likely impurity was LiF (Le., 96 mol % LiBF?, 4 mol % LiF). Because hydrolysis and solvate formation occur so easily with BF4- and PFosalt)s, we did not attempt further purification by recrystallization. The salt as received had already been recrystallized several times from a proprietary solvent which showed the least decomposition effects of more than 60 solvents tested.27 Lithium tetraphenylboride was custom synthesized by Veron, Inc. Flame photometric analysis showed that the salt contained 0 i 1 mol % Ka, but only 58 A 5 % of the expected Li content. (Since I
