The temperature-dependence of hydrophobic association in water. Pair versus bulk hydrophobic interactions

The temperature dependence of hydrophobic interactions of methane-like particles in water is analyzed in terms of free energy, entropy, internal energy, and the second osmotic virial coefficient. A large computational effort (approximately 15 ns cumulative trajectory length at each temperature) has been undertaken ill order to guarantee reliable free energy and entropy data, At 300 K association is controlled by entropy, but as the temperature rises the internal energy takes over and dominates at 500 K. Both internal energy and entropy change sign within this temperature range. Our results correspond qualitatively with the experimentally observed temperature effect for transfer of gaseous hydrophobic substances into water: Delta A shows a weak temperature dependence, while Delta E and Delta S vary strongly with temperature, The second osmotic virial coefficients were calculated al different temperatures. Agreement with osmotic virial coefficients measured by solubility experiments at 300 K was found. Our results indicate that pairwise hydrophobic association studied by molecular dynamics simulation shows:the key effects reported for bulk hydrophobic interactions, At present, there is no evidence for a qualitative difference between pair and bulk hydrophobic interactions. It is demonstrated that the comparison of the second osmotic virial coefficient of the solute particles in water, B-2,B-aq, With that in the pure gas phase, B-2,B-g, is not appropriate for an assessment of the influence of water on pairwise hydrophobic interactions.