Dependence of the self-diffusion coefficient on the sorbate concentration: A two-dimensional lattice gas model with and without confinement

Previous pulsed-field gradient (PFG) nuclear magnetic resonance (NMR) measurements on different sorbate–zeolite systems suggest that there exist at least five different types of dependence of self-diffusivity of the sorbate, D, on the concentration, c, of the sorbate. Sorbate–zeolite systems have been modeled as a two-dimensional lattice gas and studied by carrying out Monte Carlo simulations under different conditions. Among the different factors that have been varied are the arrangement of adsorption sites, hop length, nature and strength of the sorbate-sorbate interaction, and the degree of confinement. Surprisingly, even the simple 2-D lattice gas model could yield more than one type of D vs c dependence. The present study provides insight into the possible reasons for a given type of D vs c dependence.