Molecular dynamics simulations of the melting of a hexane monolayer: Isotropic versus anisotropic force fields

Molecular dynamics simulations of a hexane monolayer on the basal-plane surface of graphite have been performed to investigate the effect of an anisotropic force field on the melting process. In two sets of simulations, which are carried out for a number of temperatures ranging from the solid to the fluid state of the monolayer, the molecules are described either by a skeletal model, where the interaction sites are represented by a ''united atom'' model, or by a model where the interaction sites are shifted away from the mass site (the ''anisotropic united atom'' model). Independent of the model, the low temperature configuration is an orientationally ordered herringbone structure, which on heating undergoes an orientational phase transition to a rectangular-centered structure where the molecules tend to align along one direction. The solid subsequently melts at approximately 175 K. However, the anisotropic force field promotes a perpendicular orientation of the backbone of the hexane molecules and, in contrast to the ''united atom'' model, molecules exhibit a smaller tilt and a low er percentage of gauche defects at melting. This is compensated by increased librational motion in the plane of the substrate.