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) measu rements on different sorbate-zeolite systems suggest that there exist at le ast five different types of dependence of self-diffusivity of the sorbate, D, on the concentration, c, of the sorbate. Sorbate-zeolite systems have be en modeled as a two-dimensional lattice gas and studied by carrying out Mon te Carlo simulations under different conditions. Among the different factor s 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 yie ld more than one type of D vs c dependence. The present study provides insi ght into the possible reasons for a given type of D vs c dependence. (C) 19 99 American Institute of Physics. [S0021-9606(99)71927-6].