THE MOLECULAR-DYNAMICS SIMULATIONS OF THE MELTING OF A HEXANE BILAYER

Molecular dynamics simulations of a hexane bilayer on the basal-plane surface of graphite have been performed for a number of temperatures, ranging from the solid to the fluid state of the layers, to study the melting behaviour. The molecules are described by a skeletal model, wh ere the interaction sites are represented by an ''anisotropic united a tom'' model. At low temperature, the layers form perfectly ordered her ringbone structures, which on heating undergo an orientational phase t ransition to a rectangular-centred structure which is in coexistence w ith isotropic fluid. The first and second layers subsequently melt at approximately 170 and 190 K, respectively. The features of the transit ions are markedly less pronounced for the second layer reflecting the diminishing influence of the surface potential and hence, the higher m obility of the molecules, The melting of the second layer is induced b y tilting of the molecules out of the plane and by creating gauche def ects. Molecules in the first layer exhibit a smaller tilt and a lower percentage of gauche defects. Compared to a monolayer of full coverage , the presence of the second layer shifts the transition temperatures by similar to 20 K.