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-plan e surface of graphite have been performed to investigate the effect of an anisotropic force field on the melting process. In two sets of sim ulations, which are carried out for a number of temperatures ranging f rom the solid to the fluid state of the monolayer, the molecules are d escribed either by a skeletal model, where the interaction sites are r epresented by a ''united atom'' model, or by a model where the interac tion sites are shifted away from the mass site (the ''anisotropic unit ed atom'' model). Independent of the model, the low temperature config uration is an orientationally ordered herringbone structure, which on heating undergoes an orientational phase transition to a rectangular-c entered structure where the molecules tend to align along one directio n. The solid subsequently melts at approximately 175 K. However, the a nisotropic force field promotes a perpendicular orientation of the bac kbone of the hexane molecules and, in contrast to the ''united atom'' model, molecules exhibit a smaller tilt and a low er percentage of gau che defects at melting. This is compensated by increased librational m otion in the plane of the substrate.