The computer simulation of adsorbed hydrocarbons

Abstract

The molecular dynamics and Monte Carlo methods have been used to simulate the adsorption of butane and decane on the basal plane of graphite. Monte Carlo simulations of the isolated molecules on the surface give isosteric enthalpies at zero coverage which are in excellent agreement with experiment. Molecular dynamics simulations are performed at room temperature as a function of surface coverage. For fluid butane on graphite, increasing the coverage towards monolayer completion causes tilting of the molecules out of the surface plane, second-layer promotion of molecules, a loss of in-plane orientational ordering in the monolayer and a reduction in the percentage of trans conformational states in the adsorbate. For decane an increase in coverage has little effect on the tilt of the molecules and does not cause second-layer promotion. At high decane coverage there is considerable short-range orientational ordering of the long-axis of the molecules in the plane of the surface. There is a small reduction in the percentage of trans conformational states with coverage, but the molecular flexibility inhibits the formation of a quasi-two-dimensional nematic phase.