On the solvent isotope effect in hydrophobic hydration

It is a general result that the transfer of noble gases and aliphatic hydro carbons from H2O to D2O is characterized by negative Gibbs energy changes, but positive heat capacity changes. These experimental evidences are diffic ult to reconcile with the classical view of hydrophobic hydration, which cl aims a reinforcement of II-bonds around nonpolar moieties. In contrast, acc ording to a new emerging theory, the poor solubility of nonpolar compounds in water is determined by a balance between the excluded volume effect due to cavity creation for solute insertion and the attractive solute-water van der Waals interactions, whereas the reorganization of H-bonds is a compens ating process. The hydration thermodynamics of argon, selected as suitable representative solute, in H2O and D2O, in the temperature range 5-100 degre es C, is analyzed by means of the new approach, which proves able to satisf actorily explain the experimental data. Since the strength of van der Waals interactions is the same in the two liquids, the slightly larger solubilit y of argon in D2O, in the whole temperature range 5-100 degrees C, is due t o the lower volume packing density of D2O with respect to H2O, which decrea ses the excluded volume effect for cavity creation. On the other hand, the positive transfer heat capacity change is related to the peculiar features of H-bond reorganization for H2O and D2O molecules constituting the hydrati on shell. By using the modified Muller's model to describe such H-bond reor ganization, the positive heat capacity change is reproduced quite well in t he temperature range 5-100 degrees C.