How Much Structural Change Is Induced by the Solute

We present here an approximation similar to that used in section 7.6 to estimate the amount of structural changes induced by the solute. This estimate is based on the assumption that light and heavy water may be represented by a molecular model with a pair potential of the form (7.4.5). We further assume that light and heavy water have the same pair potential, differing only in the HB parameter hb, which we denote by Su and Sh for D2O and H2O, respectively. We also assume that the solute s interacts identically with both D2O and H2O. 

The estimate of the average change in the structure of water depends on the experimental values of the solvation Gibbs energies and an estimate of the difference d Sh Alternatively, we can use the experimental or the theoretical evidence that simple solutes do increase the structure of water to conclude that 

This means that increasing the HB energy (- hb 0) will decrease AG J (or increase the solubility of s). This finding helps to resolve the apparent puzzling fact that the solubility of small inert solute in D2O is larger than in H2O. Traditionally, the low solubility of simple solutes in water is attributed to the high degree of structure of water. If that is true, then one would expect that the solubility in D2O will be even lower than in H2O. The inequality (7.13.39) shows that the contrary conclusion is correct i.e., the 

As in section 7.6, this value seems to be quite low. The main reason is that the first-order expansion (7.13.38) is probably unjustified. 

We note that if we take larger solutes such as krypton and xenon, we obtain a negative value for Nhb s A hb o i.e., these solutes destabilize the structure of water. 

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