THE TEMPERATURE-DEPENDENCE OF HYDROPHOBIC ASSOCIATION IN WATER - PAIRVERSUS BULK HYDROPHOBIC INTERACTIONS

The temperature dependence of hydrophobic interactions of methane-like particles in water is analyzed in terms of free energy, entropy, inte rnal energy, and the second osmotic virial coefficient. A large comput ational effort (approximately 15 ns cumulative trajectory length at ea ch temperature) has been undertaken ill order to guarantee reliable fr ee energy and entropy data, At 300 K association is controlled by entr opy, but as the temperature rises the internal energy takes over and d ominates 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 hy drophobic substances into water: Delta A shows a weak temperature depe ndence, while Delta E and Delta S vary strongly with temperature, The second osmotic virial coefficients were calculated al different temper atures. Agreement with osmotic virial coefficients measured by solubil ity experiments at 300 K was found. Our results indicate that pairwise hydrophobic association studied by molecular dynamics simulation show s:tile key effects reported for bulk hydrophobic interactions, At pres ent, there is no evidence for a qualitative difference between pair an d bulk hydrophobic interactions. It is demonstrated that the compariso n 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 appr opriate for an assessment of the influence of water on pairwise hydrop hobic interactions.