A comparison was made of the adsorption and transport characteristics of one-site and five-site molecular models of methane in silica nanopores, using grand canonical Monte Carlo and equilibrium molecular dynamics simulations. It was found that while the two models had similar effective molecular sizes, based on similar high-pressure densities in the bulk and nanopore fluids, the conventional parameters of the one-site model yielded somewhat stronger intermolecular and pore wall interaction. This led to higher densities in the bulk and adsorbed fluids at intermediate pressures, for the one-site model. However, the self- and collective-diffusion coefficients were similar for the two models for most nanopores, except at low densities in large mesopores. In this case, the five-site model exhibited a slightly larger low-density diffusivity, due to its weaker interaction with the pore surface. On the basis of comparison with molecular dynamics simulations for the five-site model fluid, the predictive ability of a frictional theory of transport in nanopores was confirmed over a wide range of densities and pore diameters, using only the low-density diffusivity from a single simulation. Exceptions were found in the region of the critical point where the correlation length of the fluid diverged and when intermolecular interactions became significant in narrow nanopores where the fluid was nearly one-dimensional. In such cases, the local average density model used to estimate local transport properties became inaccurate.

Adsorption and Diffusion of Methane in Silica Nanopores: a Comparison of Single-Site and Five-Site Models. S.K.Bhatia, D.Nicholson: Journal of Physical Chemistry C, 2012, 116[3], 2344-55