COMPUTER-SIMULATION OF HYDROPHOBIC HYDRATION FORCES ON STACKED PLATESAT SHORT-RANGE

The potential of mean force between two large parallel hydrophobic obl ate ellipsoidal plates in liquid water is determined by molecular dyna mics. Each ellipsoid displaces approximately 40 water molecules and ha s major and minor axes of 3.1 and 9.3 Angstrom, respectively, has a su rface area of 650 Angstrom(2), and interacts repulsively with the solv ent water molecules. The potential of mean force is calculated from th ermodynamic perturbation theory for a series of decreasing plate separ ations, using constant-pressure molecular dynamics. As the plates are moved together, they are first separated by three water layers and the n by two, but for shorter distances, a dewetting transition occurs, an d one water layer is never observed despite the fact that one can fit. As the plates are brought together, there is a corresponding weak osc illation in the potential of mean force corresponding to the removal O f each water layer until the dewetting transition takes place, and for closer separations, the surrounding water molecules induce a constant average attractive force of 25 (kJ/mol)/Angstrom between the plates. This hydrophobic attraction is largely entropic in character, and the potential of mean force is found to be proportional to the area of the water-vacuum surface in this dewetting regime. The constant of propor tionality is found to be smaller than the gas-liquid surface tension o f the water model used. There is a very strong short-range driving for ce toward contact pairing.