Hard and Soft Parts of the Dissolution Process

We now turn to a more detailed elaboration of the two parts of the dissolution process from the molecular point of view. The importance of this particular splitting is twofold. First, substantial evidence exists showing that the anomalous properties of water are already revealed in the first step, the creation of a cavity. Second, powerful theoretical tools have been developed recently to compute the work required to create such a cavity. This will be described later in this section. 

For convenience, we consider a system in the T, V, N ensemble. (A similar treatment can be carried out in the T, P, N ensemble see, for example, Appendix 9-F.) The chemical potential of the solute S at infinite dilution in water is given by 

Using this coupling parameter, we can rewrite the chemical potential of the solute as 

In this particular ease, we have qs = F, since only one solute particle is present in the system. 

It is clear that insofar as we are interested in the solvation properties of the system, the first term on the rhs of (7.20) carries no relevant information. In fact, it would be the same for different solvents at the same solute density Qg. Therefore, we focus our attention on the pseudo-chemical potential, defined by 

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