Role of attractive methane-water interactions in the potential of mean force between methane molecules in water

TL;DR

This paper investigates how attractive methane-water interactions influence methane hydration and the potential of mean force between methane molecules in water, using a gaussian quasi-chemical model that emphasizes packing and mean-field effects.

Contribution

It introduces a non van der Waals model highlighting the dominance of packing and mean-field energies in methane hydration, and analyzes the statistical distribution of binding energies.

Findings

Hydration is dominated by packing and mean-field energies.

Attractive interactions contribute a net repulsive force.

Binding energy distributions are super-gaussian, modeled by Gumbel distributions.

Abstract

On the basis of a gaussian quasi-chemical model of hydration, a model of non van der Waals character, we explore the role of attractive methane-water interactions in the hydration of methane and in the potential of mean force between two methane molecules in water. We find that the hydration of methane is dominated by packing and a mean-field energetic contribution. Contributions beyond the mean-field term are unimportant in the hydration phenomena for a hydrophobic solute such as methane. Attractive solute-water interactions make a net repulsive contribution to these pair potentials of mean force. With no conditioning, the observed distributions of binding energies are super-gaussian and can be effectively modeled by a Gumbel (extreme value) distribution. This further supports the view that the characteristic form of the unconditioned distribution in the high-e tail is due to energetic interactions with a small number of molecules. Generalized extreme value distributions also effectively model the results with minimal conditioning, but in those cases the distributions are sufficiently narrow that details of their shape aren't significant.