Kinetic Monte Carlo simulations were used to study benzene diffusion through thin (37 to 100nm) NaX zeolite membranes under a gradient of chemical potential. Non-linearities in adsorbate loading near to the membrane boundaries were shown to arise from the difference in adsorbate density between the zeolite and the adjacent fluid phase. Direct extraction of the transport diffusivity from gradient kinetic Monte Carlo simulations permitted testing of the Darken approximation. This rigorous approach revealed limitations of that approximation and the potentially complex non-unique functionality and multiplicity of the transport diffusivity for strongly interacting adsorbates.

The Role of Molecular Interactions and Interfaces in Diffusion: Transport Diffusivity and Evaluation of the Darken Approximation. M.A.Snyder, D.G.Vlachos: Journal of Chemical Physics, 2005, 123[18], 184707