Molecular dynamics study of water-benzene interactions at the liquid/vaporinterface of water

The free energy profile of the transport of a benzene molecule across the l iquid/vapor interface of water was investigated using molecular dynamics te chniques. A large free energy minimum was found near the Gibbs dividing sur face. At this free energy minimum, which is an indication of surface activi ty, the benzene molecule lies parallel to the surface and has an adsorption free energy of about -4 kcal/mol. In addition, there is a free energy barr ier to passage from the bulk solution to the surface-adsorbed site. The hyd ration free energy estimated from the difference between gas-phase and liqu id-phase free energies is about -0.1 +/- 0.4 kcal/mol, a value that agrees well with the corresponding experimental measurement of -0.767 kcal/mol. Th e results described in this paper demonstrate the validity of our approach, as well as the quality of our potential energy surfaces.