Adsorption, isosteric heat and commensurate-incommensurate transition of methane on graphite

Abstract

We report a molecular simulation study of methane adsorption and phase transitions on a graphite substrate. Grand canonical Monte Carlo simulations have been carried out to study the adsorption isotherm and heat of adsorption at 77·5 K, using a uniform 10–4–3 substrate potential. Good agreement with experiment is found. Several layering transitions are observed at this temperature. Freezing transitions in the monolayer are found in the simulations, and are in qualitative agreement with experiment. Canonical Monte Carlo simulations have been used to study the commensurate-incommensurate transition of methane on a graphite substrate at conditions near the complete monolayer at 40·0 K, using a periodically varying adsorbate-adsorbent potential. The results are in qualitative agreement with experiments. A new method has been proposed to make a more realistic choice of the dimensions of the simulation cell. The effect of varying the corrugation of the fluid-wall potential on the commensurate and incommensurate phases is explored. Several sets of potential parameters have been used in the literature, and we compare results obtained with these sets throughout our simulations.