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

The free energy profile of the transport of a benzene molecule across the liquid/vapor interface of water was investigated using molecular dynamics techniques. A large free energy minimum was found near the Gibbs dividing surface. At this free energy minimum, which is an indication of surface activity, 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 barrier to passage from the bulk solution to the surface-adsorbed site. The hydration free energy estimated from the difference between gas-phase and liquid-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. The results described in this paper demonstrate the validity of our approach, as well as the quality of our potential energy surfaces.