Molecular librations and solvent orientational correlations in hydrophobic phenomena

Abstract

A detailed analysis of the energetic characteristics associated with the solvent in a simulation of a solution containing a pair of apolar spheres immersed in ST2 water solvent is presented. The spheres are separated by a distance corresponding to the so-called solvent-separated hydrophobic interaction. The analysis of local structure is facilitated by the consideration of only near-neighbour solvent–solvent interactions and the use of a coarse-grained time-averaging procedure which removes the high-frequency librational excitation from the underlying hydrogen-bond network. The structure produced is analogous to that termed “V-structure” by Eisenberg and Kauzmann.

The results manifest the expected enhancement in intermolecular interactions within the solvation shell, characteristic of higher orientational correlation, and this enhancement is markedly greater in the region which includes solvent that is shared by solute solvation shells. The V-structure obtained is consistent with an underlying network with further enhancement of intermolecular orientational correlations, upon which is superimposed thermal librational excitation. The intermolecular bonding is consistent with local energetic stabilization of the solvent-separated configuration, reinforcing the clathrate-like picture of solvation.