Application of the Mixture-Model Approach to Water

The idea that a solute changes the structure of the solvent is very old. We have mentioned an application of this idea in Sec. 2.3 to explain some puzzling observations (Chadwell, 1927). The addition of solutes such as ether or methyl acetate to water was found to decrease the compressibility of the system in spite of the fact that the compressibilities of these pure solutes are about three times larger than the compressibility of pure water. Similar attempts to explain the effect of solute on viscosity, dielectric relaxation, self-diffusion, and many other properties have been suggested in the literature.  

There are essentially two fundamental questions that have been the subject of extensive research. The first is concerned with the type of structure that water molecules are assumed to form around the solute molecules. Progress in this field was mainly due to comparison of the thermodynamics of dissolution of gases in water with the thermodynamics of gas-hydrate formation [see, for example, Glew (1962, 1968), and a review by Ben-Naim (1974)]. The second problem is concerned with the mechanism by which a simple solute such as argon enhances the structure of the solvent.  

Perhaps, one of the most striking pieces of evidence that a simple solute has a significant effect on the structure of water comes from a comparison of the entropy of solution of argon and KCl in water. We have discussed these findings in Sec. 3.1, (Frank and Evans, 1945 Friedman and Krishnan, 1973)  

The fact that the entropy of solution of two moles of argon is more negative than that of KCl is quite surprising (see Sec. 3.1). The molecular explanation for this effect is far from trivial. We shall devote most of the remaining part of this chapter to discussing this effect. 

The effect of the solute on the structure of water has been investigated by numerous authors.In most studies, one assumes an ad hoc mixture model for water, and then examines the shift in chemical equilibrium between the various species involved. 

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