Molecular dynamics simulation was used to study diffusion of methane at ambient temperature in cylindrical pores at very low densities. The cylinders were modelled as a continuum solid which interacted with the methane in the radial direction only. At the lowest densities, the VACF method did not yield reliable values of the self diffusion coefficient, Ds, but a suitable choice of time step and run length enables values of Ds to be found from MSD plots that were below the classical Knudsen diffusion coefficients. When density was increased, Ds passed through a maximum although the adsorption isotherm remained inside the Henry law region. Maxima were found for two cylinder radii and for two adsorbent field strengths. The existence of a maximum was attributed to transient intermolecular interactions. Analysis of a molecular trajectory demonstrated that long diffusion paths could be triggered by the rare event of an intermolecular encounter which forces a molecule into the repulsive part of the wall potential. At sufficiently high density, subsequent collisions quench the tendency towards long paths, and Ds decreased again.

Molecular Dynamics Studies of Diffusion in Model Cylindrical Pores at Very Low Densities. Papadopoulos, G.K., Nicholson, D., Suh, S.H.: Molecular Simulation, 1999, 22[4-5], 237-56