Equilibrium transport properties of methane in a carbonaceous slit-shaped pore were investigated using an equilibrium NVT molecular dynamics simulation. Self-diffusion coefficients, as a function of pore density, were deduced from the Einstein relationship for pores of width H/σ = 2.5 and 3.0 at the supercritical temperature kT/ε = 2.0, where ε and σ were the Lennard-Jones parameters for methane. The simulations used two wall-reflection conditions: specular and diffuse. A finite value of self-diffusivity at the low-concentration limit was found only for the diffuse condition, and differing diffusivity values for the two conditions were observed at all concentrations. Adsorption isotherms for the two pore sizes were simulated by using the grand canonical Monte Carlo technique. A value for the transport diffusivity was calculated by using the Darken relationship. It was found that, whereas self-diffusion decreased with concentration, the dependence of transport diffusivity upon concentration differed with pore size. Velocity auto-correlation functions and their Fourier transforms were computed for pore widths H/σ = 2.0, 2.5 and 3.0 at low adsorbate loading. These were interpreted in terms of the shape of the adsorbate-wall potential-energy profile.

Molecular Dynamics Study of the Self-Diffusion of Supercritical Methane in Slit-Shaped Graphitic Micropores. Cracknell, R.F., Nicholson, D., Gubbins, K.E.: Journal of the Chemical Society, Faraday Transactions, 1995, 91[9], 1377-83