A simulation study of the low temperature phase diagram of the methane monolayer on graphite: a test of potential energy functions.

We have used molecular simulation with two intermolecular potential models, TraPPE-UA and TraPPE-EH , to study the effects of molecular shape on methane adsorption on graphite at low temperatures. The first model represents methane as a spherical molecule and the second includes the hydrogen atoms and accounts for the tetrahedral shape. Both models give good descriptions of the vapour-liquid equilibria in the bulk phase, but adsorption on graphite is better described by the TraPPE-EH model at temperatures below the bulk boiling point where the structure of the monolayer is modified by registry of methane with respect to the carbon hexagons in the outermost graphene layer. Molecular configurations in the monolayer, show the variation with temperature of the registry sites for the carbon and hydrogen atoms of the methane molecules. At temperatures below 70K, the centre of mass (COM) of the molecules is in registry with the centre of the carbon hexagons, as for the united atom model, and the molecule adopts a tripod configuration with its C-H bonds pointing to carbon atoms on the graphene layer. At temperatures above 70K, a commensurate monolayer is initially formed as at low temperatures, then as the loading is increased beyond the commensurate monolayer density, the onset of the second layer begins and the first layer remains in registry, but the COM of the methane molecules in the first layer shifts to the top of the graphite carbon atoms with the C-H bond pointing to carbon atoms in the second shell of a C-hexagon. At temperatures above 93K, the first adsorbate layer goes through these two commensurate states and then undergoes a transition to an incommensurate solid at higher loadings. Finally, for temperatures greater than 110K there is no longer a preferred orientation and methane behaves like a pseudo spherical molecule. This study suggests that TraPPE-EH should be used for monolayer studies at temperatures below 110K, but that TraPPE-UA could be used at higher temperatures to reduce the computation time, without any loss of accuracy in the description of the isotherm and the isosteric heat.