Langmuir 1986, 2, 797-800
797
Ion Selectivity by Solvents in Narrow Pores: Physical and Biophysical Significance Frank M. Etzlerh College of Pharmacy, University of Kentucky, Lexington, Kentucky 40536-0082
Terry L. Liles Department of Chemistry, East Carolina University, Greenville, North Carolina 27834 Received J u n e 9, 1986. I n Final Form: August 12, 1986 We report the temperature dependence of the selectivity coefficient for K+ relative to Li+ (KKILi)by various solvents confined to silica gel pores. The solvents studied were HzO,DzO,and CH30H. The thermal dependence of KK L1 is discussed in regard to the earlier results of Wiggins (as well as those of Hurtado from aqueous solutions. The and Drost-Hansenj who observed maxima near 15,30, and 45 "C for KKINa relation of these results to the nature of vicinal (interfacial) water is also considered: specifically, it is found that KKILi exhibits notably non van't Hoff temperature dependencies in HzO and DzO. Thus vicinal DzO apparently closely resembles vicinal HzO although transition temperatures observed are somewhat lower for DzO than for HzO. Maxima in KKiLiobserved in these systems have been previously ascribed to structural transitions in vicinal water, the maxima occurring at the temperature of the thermal transitions. No extrema were observed in KKpi vs. temperature for ions in methanol confined to silica pores, but the value for KK L1 (about 2.3 over the entire range of temperatures studied) is notably larger than 1.0. Perhaps because CH3dH is not capable of building extensive tetrahedral structures, its vicinal structure does not resemble vicinal water structure sufficiently to exhibit the thermal transitions observed with both HzO and D,O.
Introduction The properties of water near solid surfaces are known to differ from those of the bulk. For comprehensive reviews the reader is directed to the papers of Drost-Hansen.ld Among the unusual properties of vicinal water it is noteworthy that the heat capacity (in 14-nm diameter silica gel pores) is approximately 2530% greater than the bulk value7Band that the density is 2-370 lower than the bulk den~ity.~JOFrom measurements by Peschel and Adlfinger,'l-13 it appears that the viscosity of water between quartz plates is 2-20 times that of the bulk. In an attempt to correlate some of the properties of vicinal water, Etzler14 has proposed a statistical thermodynamic model for water confined to narrow pores. The model considers water in terms of both a bond percolation model, similar to that proposed for bulk water by Stanley and Teixeira,15 and a bimodal single particle enthalpy distribution calculated earlier by Stey.16 Etzler's model is able to correlate some thermodynamic and dynamic properties of water in silica pores. From the model it appears that vicinal water is similar to water in the supercooled region or under negative pressure. In other words, hydrogen bonding between water molecules is enhanced near solid surfaces. Recently, from measurements of the density of water in silicas of various pore diameters, Etzler and FagunduslO have estimated that vicinal structuring extends a t least 3-5 nm from a surface and that this structuring decays in an approximately exponential manner. The distance over which vicinal structuring occurs appears to be consistent with the properties of water in m o n t m ~ r i l l o n i t e , ' ~the - ~ ~apparent properties of intracellular water as discussed by Clegg,20-23and the disjoining pressure of water between quartz plates as measured by Peschel et al.24 Braun and Drost-Hansen7 have studied the heat capacities of water near a variety of surfaces. Their results are consistent with later measurements of Cianci8for water in 14-nm diameter silica gel pores. From the above work, it appears that the heat capacity of vicinal water is inde*To whom all correspondence should be addressed.
0743-7463/86/2402-0797$01.50/0
pendent of the solid material studied and about 25-30% greater than the bulk. This observation suggests that the properties of vicinal water are independent of the precise physicochemical nature of the surface. The apparent independence of vicinal water properties on the nature of the surface has been termed the "paradoxical effect" by Drost-Hansen. In addition to structural modification induced by propinquity to solid surfaces, vicinal water appears to undergo structural transitions near 15,30,45,60,and probably 75 "C. Thus, Peschel and Ad1finge1-ll-l~have reported maxima near 15,30, and 45 "C in the viscosity and disjoining (1) Drost-Hansen, W. Ind. Eng. Chem. 1969, 61, 10. (2) Drost-Hansen, W. In Chemistry of the Cell Interface; Brown, H.
D., Ed.; Academic Press: New York, 1971; Part B. (3) Drost-Hansen, W. Phys. Chem. Liq. 1977, 7, 243. (4) Etzler, F. M.; Drost-Hansen, W. In Cell Associated Water; Drost-Hansen, W., Clegg, J. S., Eds.; Academic Press: New York, 1979; p 125. (5) Drost-Hansen, W. In Biophysics of Water;Franks, F., Ed.; Wiley: New York, 1982. (6) Etzler, F. M.; Drost-Hansen, W. Croat. Chem. Acta 1983,56,563. (7) Braun, C. V.; Drost-Hansen, W. In Colloid and Interface Science; Kerker, M., Ed.; Academic Press: New York, 1976; Vol. 111, p 533. (8) Cianci, J. M. S. Thesis, University of Miami, 1981. (9) Etzler, F. M.; Fagundus, D. M. J . Colloid Interface Sci. 1983, 93, 585. (10) Etzler, F. M.; Fagundus, D. M. J . Colloid Interface Sei., in press. (11)Peschel, G.; Adlfinger, K. H. 2. Naturforsch.,A 1971, 26A, 707. (12) Peschel, G.; Adlfinger, K. H. Naturwissenschaften, 1969, 11, 1. (13) Peschel, G.; Adlfinger, K. H. J. Colloid Interface Sei. 1970, 34, 505. (14) Etzler, F. M. J . Colloid Interface Sci. 1983, 92, 43. (15) Stanley, H. E.; Teixeira, J. J . Chem. Phys. 1980, 73, 3404. (16) Stey, G. C.; Ph.D. Thesis; University of Pittsburgh; 1967. (17) Low, P. F. Soil. Sci. Am. J. 1979, 43, 652. (18) Oliphant, J. L.; Low, P. F. J . Colloid Interface Sci. 1983,95,45. (19) Vani, B. E.; Low, P. F.; Roth, C. B. J. Colloid Interface Sei. 1983, 96, 229. (20) Clegg, J. S. In Cell Associated Water; Drost-Hansen, W., Clegg, J. S., Eds.; Academic Press: New York, 1979. (21) Clegg, J. S. In Biophysics of Water;Franks, F., Ed.; Wiley: New 1982. York. ~ ,~ - - -. - . (22) Clegg, J. S. Am. J . Physiol. 1984, 246, R133. (23) Clegg, J. S. Cell Biophys. 1984, 6, 153. (24) Peschel, G.; Belocheck, P.; Muller, M. M.; Muller, M. R.; Koing, R. Colloid Polym. Sci. 1982, 260, 444. ~
0 1986 American Chemical Society
798 Langmuir, Vol. 2, No. 6, 1986
pressure of water between quartz plates. Cianci8 has observed heat capacity spikes a t the vicinal water transition temperatures. Furthermore, Cianci's results suggest that the AHbm is on the order of 10-50 cal/mo1(4&200 J/mol). Dreyer et alez5have measured the self-diffusion coefficient of thiourea in water in a small capillary a t closely spaced temperature intervals. "Kinks" in the plot of the diffusion coefficient as a function of temperature were observed near 15, 30,45, and 60 "C. Also, Schufle, Huang, and DrostHansenZ6have observed unusual behavior of the energy of activation for surface conductance of aqueous solutions in micron-size capillaries. Further evidence for the existence of vicinal water structural transitions near 15, 30, 45, and 60 "C may be found in the reviews by DrostHansen.1-6 Of particular interest to this paper, wig gin^^^,^^ has measured the thermal dependence of the selectivity coefficient of K+ relative to Na+, K K p a , for water confined to the pores of a silica gel. Wiggins found that K K , N a exhibited a non-van't Hoff temperature dependence with maxima near 15,30, and 45 "C. The temperature maxima have been interpreted by Wiggins to reflect structural transitions in vicinal water. Hurtado and Drost-HansenB later observed similar behavior in a different silica gel. From Wiggins' s t ~ d i e s , ~ ' ~ it*appears * that ions of the soc d e d "structure-making'' type (e.g., Na+, Li+) are excluded from the pores while ions of the "structure-breaking" type (e.g., K+) are selected into the pores. Ling30 has also reported that compounds other than simple ions are selected or excluded from polymer matrices on the basis of their hydration characteristics. Wiggins extended her studies to biological materials. Specifically, she measured K K / N a in a system where rat renal cortex was substituted for silica gel. Again, a non van't Hoff temperature dependence with maxima near 15 and 30 "C was observed. This result strongly suggests that water in silica pores and water in biological cells share similar characteristics. Indeed, the density of intracellular water as discussed by Clegg20-23is similar to that found by Etzler and FagundusgJoin silica pores. Ling's results30 also suggest that exclusion of some solutes by vicinally structured water may be of biochemical, pharmacological, and pharmaceutical importance. The existence of physiological anomalies a t the temperatures discussed above has been examined by Etzler and D r o ~ t - H a n s e n .In ~ an early paper Oppenheimer and D r o ~ t - H a n s e nfound ~ ~ that microorganisms exhibited growth minima near 15, 30, and 45 "C. More recently, Etzler and Drost-Hansen studied the growth of Cyanidium caldarium4and the germination of turnip seeds.32 In each organism abrupt qualitative changes in metabolism were found to occur a t or near some vicinal water temperature. Cyanidium caldarium is particularly interesting in that it exhibits growth minima near both 30 and 45 "C. Additionally, N i ~ h i y a m ahas ~ ~ reported .~~ a large number of (25) Dreyer, G.; Kahrig, E.; Kristein, D.; Erpenbeck, J.; Lange, F. Naturwissenschaften 1969,56, 558. (26) Schufle, J. A.; Huang, C.; Drost-Hansen, W. J . Colloid Interface Sei. 1976, 54, 184. (27) Wiggins, P. M. Biophys. J. 1973, 13, 385. (28) Wiggins, P. M. Clin. E x p . Pharmacol. Physiol. 1975, 2, 171. (29) Hurtado, R. M.; Drost-Hansen, W. In Cell Associated W a t e r ; Drost-Hansen, W.; Clegg, J. S.,Eds.; Academic Press: New York, 1979; p 115. (30) Ling, G. N.; Walton, C.; Bersinger, T. J. Physiol. Chem. Phys. 1980, 12, 111. (31) Oppenheimer, C. H.; Drost-Hansen, W. J. Bacteriol. 1960,80,21. (32) Etzler, F. M.; Drost-Hansen, W. Colloid Interface Sei., Proc. Int. Conf., 50th, 1976 1976, 3, 517. (33) Nishiyama, J. Res. Bull. No. 2, Hokkaido National Agricultural Experimental Station, Japan, 1972.
Etzler and Liles physiological anomalies in plants a t 15, 30, and 45 "C. In the 1920's, Crozier investigated the effect of temperature on a number of biological and biochemical phenomena. In Crozier's ~ o r k numerous ~ ~ - ~ examples ~ of physiological anomalies a t the vicinal water transition temperatures may be found. The need for further research into the origin of thermal anomalies is recognized. Indeed, we know that lipid transitions play an important role in some physiological anomalies which occur a t other temperatures. In summary, it appears that the properties of vicinal water differ from those of the bulk. Furthermore, it has been suggested that vicinal water differs from the bulk in that hydrogen bonding is enhanced by propinquity to solid surfaces. It also appears, that vicinal water undergoes structural transitions near 15, 30, and 45 "C. The exact nature of these transitions remains unknown. Furthermore, a considerable, albeit often circumstantial, body of evidence exists which suggests that the vicinal water transitions are of probable biochemical and physiological significance. In this paper, we report the temperature dependence of K K / L i by solvents confined to the pores of a silica gel. The solvents studied are HzO, D20, and MeOH. The results of this study are discussed with regard to their relevance t o vicinal water structure.
Experimental Section A. Materials. 1. Solvents. The solvents chosen were water, methanol, and deuterium oxide. Water for the experiments was obtained from a BarnstedJSybron 4-cartridge ion exchange system--18 M%cm resistance. Absolute methanol (
