Computer simulation studies of the hydration and aggregation of simple hydrophobic molecules

Molecular dynamics computer simulation has been used to study the structure and dynamics of concentrated solutions of methane molecules in water, at temperatures between 270 and 380 K. We find that at the lowest temperatures the methane particles repel each other weakly. The hydrophobic solute-solute interactions only become attractive above 300 K, and reach a maximum at ca. 340 K. Our model solutions therefore mimic quite well the behaviour of many real systems in which hydrophobic bonding is thought to be influential. Among the solvent molecules we find little evidence for enhanced hydrogen bonding, except at the lower temperatures. This observation is consistent with recent neutron diffraction data, and with studies of hydrophobic hydration in more dilute systems. Energetically, we propose that water molecules prefer to be in the hydration shell at lower temperatures, but in the bulk at higher temperatures. In this case, the observed hydrophobic interactions can be explained by a subtle balance between entropy and enthalpy.