On the Solvent Isotope Effect in Hydrophobic Hydration

Abstract

It is a general result that the transfer of noble gases and aliphatic hydrocarbons from H2O to D2O is characterized by negative Gibbs energy changes, but positive heat capacity changes. These experimental evidences are difficult to reconcile with the classical view of hydrophobic hydration, which claims a reinforcement of H-bonds around nonpolar moieties. In contrast, according 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 compensating process. The hydration thermodynamics of argon, selected as suitable representative solute, in H2O and D2O, in the temperature range 5−100 °C, is analyzed by means of the new approach, which proves able to satisfactorily explain the experimental data. Since the strength of van der Waals interactions is the same in the two liquids, the slightly larger solubility of argon in D2O, in the whole temperature range 5−100 °C, is due to the lower volume packing density of D2O with respect to H2O, which decreases 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 hydration shell. By using the modified Muller's model to describe such H-bond reorganization, the positive heat capacity change is reproduced quite well in the temperature range 5−100 °C.