MOLECULAR-DYNAMICS STUDY OF THE SELF-DIFFUSION OF SUPERCRITICAL METHANE IN SLIT-SHAPED GRAPHITIC MICROPORES

Equilibrium transport properties of methane in a carbonaceous slit-shaped pore have been investigated using an equilibrium NVT molecular dynamics simulation. Self-diffusion coefficients, as a function of pore density, were calculated from the Einstein relation for pores of width H/sigma = 2.5 and 3.0 at the supercritical temperature kT/epsilon = 2.0, where epsilon and sigma are the Lennard-Jones parameters for methane. The simulations used two different wall reflection conditions: specular reflection and diffuse reflection. A finite value of self-diffusivity at the low concentration limit was only found 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 using the grand canonical Monte Carlo technique. A value for the transport diffusivity was calculated using the Darken relation. It was found that while self-diffusion decreases with concentration, the dependence of transport diffusivity on concentration differs with pore size. Velocity auto-correlation functions and their Fourier transforms were computed for pore widths H/sigma = 2.0, 2.5 and 3.0 at low adsorbate loading. These are interpreted in terms of the shape of the adsorbate-wall potential-energy profile.