Monte Carlo simulation of methyl chloride monolayer on the surface of graphite

摘要

Monte Carlo simulation of an all-atom model of a methyl chloride monolayer adsorbed on the surface of graphite reveals a striking degree of orientational correlations among the adsorbed molecules, both with respect to the tilt angle away from the surface perpendicular and the methyl rotor angle. The lowest energy configuration of the high density (HD) antiferroelectric adsorbate was determined. As the temperature is raised, a continuous drop of the tilt order parameter and a peak of the calculated heat capacity were found near 91 K, indicating a tilt order/disorder phase transition. The behavior of the tilt order parameter and heat capacity is consistent with the Ising model universality class. In the neighborhood of the phase transition domains of similarly tilted molecules could be identified.Two crystal structures proposed for the low density (LD) methyl chloride monolayer were studied. The configuration proposed by Morishige, Tajima, Kittaka, Clarke and Thomas was found to be lower in energy than an alternative structure proposed by Shirazi and Knorr. The first-order melting transition of the monolayer crystal was found to occur at about 90 K, in qualitative agreement with experiments. Above the melting point, short-range orientational order within the LD methyl chloride fluid phase was found. (c) 2005 Elsevier B.V. All rights reserved.