FREE-ENERGY, ENTROPY, AND INTERNAL ENERGY OF HYDROPHOBIC INTERACTIONS - COMPUTER-SIMULATIONS

Hydrophobic interactions are investigated by molecular dynamics computer simulations of the free energy, entropy, and internal energy of association of two methane molecules in water. Recently we reported a computer simulation calculation of the entropy of association of methane in water [J. Am. Chem. Soc. 114, 5875 (1992)], where entropy drives nonpolar solutes together at short distances. Here this method is compared with two other general methods for the calculation of the entropic contribution to the free energy. The calculated thermodynamic quantities for methane association are in good agreement with available experimental measurements. Solute contact configurations are found to be of greater importance than solvent-separated configurations, in conflict with earlier theoretical and simulation studies of similar systems. In some cases, this conflict may be understood in terms of differences in the assumed, model intermolecular potential energies.