Langmuir 1995,11, 1984-1986
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Microosmosis, a Chaotic Phenomenon of Water and Solutes in Gels Philippa M. Wiggins Department of Medicine, University of Auckland School of Medicine, Private Bag, 92019 Auckland, New Zealand Received November 30, 1993. Zn Final Form: March 15, 1995@ Gels of the Biogel P series, which have previously been shown to contain low-density water with highly selective solvent properties, have been used to examine the question of how solutes and water come to equilibrium in such environments. The internal water content of gel beads was measured as a function of time in the presence of no solutes, single solutes, and combinations of solutes. In the presence of 10 mM KNO3 (or of KC1 or KHzPO4)which are selectively accumulatedinto stretched water adjacent to poorly hydrogen bonding surfaces, internal volume increased steadily for up to 17 days. In the absence of small solute or in the presence of 100 mM NaC1, internal volume became constant after two days. With mixed solutes, such as a bicarbonate-buffered Ringer's solution or 5 mM K phosphate pH 7, the internal volume oscillated with a period of days.
Introduction Uniquely among liquids, water exists as three-dimensional networks of mutually hydrogen-bonded molecules. The existence of this network dominates all physical and chemical properties of the liquid. For example, other hydrides (HzS, HZSe, HZTe, SbH3, AsH3, HBr, HC1, CH4, NH3, and PH3) and oxides (CO, COZ,SOZ, and NO) are gases a t ambient temperatures. Liquid water, however, cannot vaporize until thermal energy releases molecules from their intermolecular bonds, an access of energy which is only reached a t approximately 100 "C higher than the boiling points of similar nonnetworking liquids. It follows that any change in the number or strength of waterwater hydrogen bonds can be expected to change all the properties of the liquid. I t has been p r o p o ~ e d l -that ~ the origin of hydrophobic hydration and the hydrophobic force can be traced to the inability of water molecules adjacent to a hydrophobic surface to make as many or as strong hydrogen bonds as can more distant molecules which hydrogen-bond with one another. Since hydrogen-bondingis cooperative, weak bonding persists over several layers of molecules, generating a region of water of higher enthalpy than bulk water. The responses of the system to its high enthalpy state are first, if possible, to squeeze out some of the highenthalpy water. This is the origin of the attractive hydrophobic force between surfaces separated by water. Second, a s molecules exchange freely across the interface between the two regions of water, those populations of molecules must tend toward the same chemical potential; in order to do so, it is suggested, molecules of the highenthalpy water move apart, lowering the local chemical potential by doing work of expansion. The two populations of water molecules thus equilibrate, but they coexist in states of different enthalpy and density. Sciortino et al.,5 using molecular dynamics simulations, found that a decrease in density of water led to a slowing down of the rotational and translational single particle diffusion; i.e., @
Abstract published in Advance A C S Abstracts, May 15, 1995.
(1)Wiggins, P.M.;van Ryn, R. T. J. Macromol. Sci., Chem. 1986, A23, 875. (2) Wiggins, P.M.; van Ryn, R. T. Biophys. J . 1990,58, 585. (3) Wiggins, P.M., MacClement, B. A. E. Int. Rev. Cytol. 1987,108, 249. (4) Wiggins, P.M. Microbiol. Rev. 1990,54, 432. (5) Sciortino, F.;Geiger, A.; Stanley, H. E. J. Chem. Phys. 1992,96, 3857.
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changes in density do, indeed, change dynamic properties of the network. We have measured the density of water in polyamide beads of the Biogel P- seriesaZ Pores have diameters ranging from 1to 3 nm in diameter, and surfaces carrying weakly hydrogen-bonding C=O and N-H groups, sparse carboxyl groups, and stretches of hydrocarbon chainsa6 Densities of internal water ranged from 0.98 (P-30) to 0.973 (P-2). Still lower densities were measured when internal water was under osmotic stress (i.e. beads were bathed with a solution containing 20 M polyethylene glycol which was too big to enter the pores). Density decreased down to 0.96 with increasing osmotic stress. While this might be perceived as merely a 4% change in a property of the liquid, it is, perhaps, more revealing to see it as a change to a state midway between water (density 1)and ice (density 0.92). We have also found that the solvent properties of water change significantly with its d e n ~ i t y . ~Highly , ~ , ~ hydrated solutes (Mg2+,Ca2+,H+,and Na+)and small hydrophobic molecules were relatively excluded from low density (stretched water), while larger cations (eg. K+) and univalent anions, were selectively accumulated into it. Contact between low density water in a gel and normal aqueous solutions, therefore, induces gradients in solute concentrations and hence in water activity. This phenomenon, which has been called microosmosis, is examined in this communication using microporous beads of the Biogel P- series.
Experimental Section Biogels P-2 (0.4 gin 2 mL), P-4 (0.3 gin 2.1 mL), and P-6 (0.3 gin 3.2 mL), obtained from BioRad Laboratories Pty. Ltd., were mixed with solutions in screw-topped Pierce vials with silicone/ Teflon septa. Each solution contained 1%polyethylene glycol 3350 (PEG),which is too big to enter the pores of these gels, with or without 14C-PEGas an external solution marker. Vials were rotated evenly and slowly at constant temperatureon a verticallyoriented turntable, so that the contents inverted completely in each cycle. In order to obtain enough experimental points in a normal working day, two sets of vials were used in most experiments, one set starting 4 h before the other. Vials were removed serially and supernatants removed, spun down, and transferred to clean tubes for analysis. When I4C-PEGwas (6)Fischer, L. Gel Filtration; Elsevier, North Holland Biomedical: Dordrecht, The Netherlands, 1980; p 41. (7) Wiggins, P. M.; van Ryn, R. T.; Ormrod, D. G. C.Biophys. J.1991, 60, 8.
0 1995 American Chemical Society
Microosmosis
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Time (days) Figure 1. Internal volume of Biogel P-2 in contact with 100 mM NaCl for eight days. Each point is the mean and standard deviation of duplicate determinations.
T i m e (days)
Figure 3. Changes in internal volume of Biogel P-6 (150-300 pm) (m) and external pH (A)in the presence of 5 mM m i , pH 7. Each volume point is the mean and standard deviation of duplicate determinations; pH was reproducible to 0.01 unit.
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