Molecular dynamics of phenol at the liquid-vapor interface of water

Molecular dynamics results are presented for phenol at the water liquid-vapor interface at 300 K. The calculated excess free energy of phenol at the interface is -2.8 +/- 0.4 kcal/mol, in good agreement with the recent experimental results of Eisenthal and co-workers. The most probable orientation of the phenol molecule at the surface is such that the aromatic ring is perpendicular to the interface and the OH group is fully immersed in water. The hydroxyl substituent has a preferred orientation which is similar to the orientation of OH bonds of water at the pure water liquid-vapor interface. The transition between interfacial and bulk-like behavior of phenol is abrupt and occurs when the center of mass of the solute is located about 6 angstroms from the Gibbs surface of water. In this region the para carbon atom of the hydrophobic benzene ring can reach the interface and become partially dehydrated. This result suggests that the width of the interfacial region in which the behavior of a simple amphiphilic solute in water is influenced by the presence of the surface depends primarily on the size of its hydrophobic part. The role of the OH substituent was investigated by comparing phenol at the interface with two model systems: benzene with and without partial charges on carbon and hydrogen atoms. It is shown that in the absence of the hydrophilic substituent the solute is located further away from the liquid phase and is more likely to be oriented parallel to the interface. However, when the center of mass of the solute is moved into the interfacial region where the density of water approaches that of the bulk solvent, all three molecules become oriented perpendicularly to the surface. In this orientation the work of cavity formation needed to accommodate the hydrophobic ring in aqueous solvent is minimized.