Adsorption and Diffusion of Methane in Silica Nanopores: A Comparison of Single-Site and Five-Site Models

We report a comparison 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 is found that while the two models show similar effective molecular sizes, based on similar high-pressure densities in the bulk and nanopore fluids, the conventional parameters of the one-site model yield somewhat stronger intermolecular and pore wall interaction. This leads to higher densities in the bulk and adsorbed fluids at intermediate pressures, for the one site model. However, the self- and diffusion coefficients are similar for the two models for most nanopores, except at low densities in large mesopores. In this case, the five-site model shows 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 our recent frictional theory of transport in nanopores is confirmed over a wide range of densities and pore diameters, using only the low-density diffusivity from a single simulation. Exceptions are found in the region of the critical point where the correlation length of the fluid diverges and when intermolecular interactions become significant in narrow nanopores where the fluid is nearly one-dimensional. In such cases, the local average density model used to estimate local transport properties becomes inaccurate.