Molecular dynamics studies of diffusion in model cylindrical pores at verylow densities

Molecular dynamics simulation has been used to study diffusion of methane a t ambient temperature in cylindrical pores at very low densities. The cylin ders were modelled as a continuum solid which interacts with the methane in the radial direction only. At the lowest densities, the VACF method does n ot yield reliable values of the self diffusion coefficient, D-s, but a suit able choice of time step and run length enables values of D-s to be found f rom MSD plots that are below the classical Knudsen diffusion coefficients. When density is increased, D-s passes through a maximum although the adsorp tion isotherm remains inside the Henry law region. Maxima are found for two cylinder radii and for two adsorbent field strengths. The existence of a m aximum is attributed to transient intermolecular interactions. Analysis of a molecular trajectory demonstrates that long diffusion paths can be trigge red by the rare event of an intermolecular encounter which forces a molecul e into the repulsive part of the wall potential. At sufficiently high densi ty, subsequent collisions quench the tendency towards long paths, and D-s d ecreases again. The issue of simulation artefact as a source of these obser vations is discussed.