Behavior of hexane on graphite at near-monolayer densities: Molecular dynamics study

Abstract

We present the results of molecular-dynamics studies of hexane physisorbed onto graphite for eight coverages in the range 0.875⩽ρ⩽1.05 (in units of monolayers). At low temperatures, the adsorbate molecules form a uniaxially incommensurate herringbone solid. At high coverages, the solid consists of adsorbate molecules that are primarily rolled on their side perpendicular to the surface of the substrate. As the coverage is decreased, the amount of molecular rolling diminishes until ρ=0.933, where it disappears (molecules become primarily parallel to the surface). If the density is decreased enough, vacancies appear. As the temperature is increased, we observe a three-phase regime for ρ>0.933 (with an orientationally ordered nematic mesophase); for lower coverages, the system melts directly to the disordered (and isotropic) liquid phase. The solid-nematic transition temperature is very sensitive to coverage, whereas the melting temperature is quite insensitive to it, except for at low coverages where increased in-plane space and, ultimately, vacancies soften the solid phase and lower the melting temperature. Our results signal the importance of molecular rolling and tilting (which result from the competition between molecule-molecule and molecule-substrate interactions) for the formation of the intermediate phase, while the insensitivity of the system's melting temperature to changing density is understood in terms of in-plane space occupation through rolling. Comparisons to experimental results are discussed.