Solvation Helmholtz Energy Group Additivity

Conditional Solvation Helmholtz Energy Group Additivity 

Researchers in solvation thermodynamics have long attempted to assign group contributions of various parts of a solute molecule to the thermodynamic quantities of solvation. In this section we examine the molecular basis of such a group-additivity approach. As we shall see, the problem is essentially the same as that treated in the previous section i.e., it originates from a split of the solute-solvent intermolecular potential function into two or more parts. 

Let us start with a solute of the form X—Y, where X and Y are two groups, say CH3 and CH3 in ethane or CH3 and OH in methanol. We assume that the solute—solvent interaction may be split into two parts as follows  

As in the preceding section, where we had split the interaction energy into a hard and a soft part, we also have here an element of ambiguity as to the exact manner in which this split may be achieved. However, assuming that an equation of the form 

We see that, as in the preceding section, we have again an average of a product of two functions. This, in general, may not be factorized into a product of two average quantities. If this could have been done, then relation (6.14.29) could have been written as a sum of two terms, i.e., 

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Conditional Solvation Helmholtz Energy Group Additivity... 

Clearly, since the second group Y is brought to a location very near X, one cannot ignore the effect of X on the (conditional) solvation of Y. In other words, in the first step, we insert X in pure solvent. In the second, we insert Yin a solvent which has been perturbed by the presence of X. It is only in very extreme cases when X and Y are very far apart that we could assume that their solvation Helmholtz energies will be strictly additive, as in equation (7.131). 

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