INFLUENCE OF LOCAL-FIELDS ON MACROSCOPIC TRANSPORT-COEFFICIENTS

The analysis of the influence of local fields on the behaviour of transport coefficients is addressed by means of the exit-time equation deriving from the theory of stochastic processes, transport being considered in terms of a stochastic Newtonian equation of motion at the microscopic scales. Two cases are considered in detail: a one-dimensional model of transport in a periodic distribution of potential walls and barriers; and a two-dimensional model of a unit cell of porous structure in the presence of convective transport. The former indicates the nonmonotonic behaviour of the diffusion coefficient with respect to the dimensionless parameter E0/k(B)T. The latter illustrates the applicability of the proposed methods to transport phenomena in complex porous structures. We present a practical application of the exit-time equation serving to predict the diffusion coefficient in porous structures directly from molecular data on the interaction between particles and pore wall. The results are applied to the determination of the diffusion coefficient of noble gas in noble gas in 5A zeolites.