Dual control-volume grand canonical molecular dynamics simulations permitted the direct simulation of diffusion at the molecular level. Such simulations of CH4/CF4 mixtures in carbon nanotubes were carried out here. An approach for obtaining composition-dependent transport diffusivities directly from these simulations was introduced. Composition-dependent diffusivities and fluxes were calculated for varying driving forces in order to investigate the influence of the very large driving force in simulations; which was some four orders of magnitude larger than in real experimental systems. While the flux depended upon the driving force, the transport diffusivity was independent of it; so the present simulations could be used to simulate transport under experimental conditions. The results for composition-dependent diffusivities at four different temperatures were presented. A linear function described the composition dependence and reproduced very well the simulated concentration profiles. Analysis of the temperature dependence indicated that transport in the investigated system was due to liquid-like molecular diffusion and not to activated diffusion.

Composition Dependent Transport Diffusion Coefficients of CH4/CF4 Mixtures in Carbon Nanotubes by Non-Equilibrium Molecular Dynamics Simulations. T.Düren, F.J.Keil, N.A.Seaton: Chemical Engineering Science, 2002, 57[8], 1343-54