Solvation Helmholtz Energy Hard and Soft Parts

Conditional Solvation Helmholtz Energy Hard and Soft Parts 

One of the earliest attempts to interpret the anomalous properties of aqueous solutions was based on splitting the solvation process into two parts a cavity formation and introduction of the solute into the cavity. In the present section we shall discuss the statistical-mechanical basis for such a split of the solvation process. 

In essence, the molecular basis for splitting the solvation into two (or more) steps stems from the recognition of the two (or more) parts of the solute-solvent intermolecular potential function, the so-called hard and soft parts of the interactions. 

For simplicity let us assume that the solute-solvent pair potential is a function of the distance R only, and that this function may be written as 

The average quantity in relation (6.14.17) can be viewed as an average of a product of two functions. If these were independent (in the sense of the independence of two random variables), one could rewrite the average as a product of two averages, i.e., 

---