Entropy of association of methane in water : a new molecular dynamics computer simulation

Hydrophobic interactions are important in clathrate-hydrate formation, protein folding, micelle formation, and other fields. Entropy should play a major role in the interaction of hydrophobic solutes, just as it dominates the thermodynamics of hydrophobic solvation. Theoretical and computer studies have calculated the free energy of association of hydrophobic solutes, yet none have evaluated directly the role of entropy. We present here a computer simulation of the entropy of association of two methane-like molecules in aqueous solution. The entropic contribution to the free energy of association is attractive at short separations, when the solutes are in contact. The authors found that entropy causes an attraction between the solutes at separations less than approximately 5.5 angstroms. The experimental data predicts that as the temperature increases, the magnitude of the attraction at short separations increases and the contact well in the potential of mean force becomes deeper. 25 refs., 2 figs.