Computer simulation of the structural effect of pressure on the hydrophobic hydration of methane

Abstract

Monte Carlo simulations have been carried out on solutions containing one methane solute in 499 water molecules, at pressures of 1, 100, 200, 400, 800 and 1600 atm and 320 K. We observe that, at all pressures, hydration-shell water molecules have a smaller number of hydrogen bonds, a smaller number of near neighbours and larger tetrahedral bond-order parameters than bulk water molecules; however, these differences tend to decrease as pressure increases. In both the hydration shell and the bulk, the rise of pressure produces an increase in the number of hydrogen bonds and near neighbours per water molecule, accompanied by an increase in the tetrahedrality. Enhanced structuring of the hydration-shell water molecules is revealed by an increase of the first peak in the water oxygen—oxygen radial distribution function as pressure is increased. At low pressures, a weak second hydration shell around methane develops, and only at higher pressures does it become increasingly significant.