MODELING MACROMOLECULAR DIFFUSION THROUGH A POROUS-MEDIUM

The diffusion of large spherical molecules through a porous medium is studied by means of a two-dimensional computer model developed previously [Int. J. Thermophys., 9 (1988) 1061]. For two well-characterized, bimodal, pore size distributions, a random walk procedure is used to determine the diffusivity under several conditions of wall-molecule interaction, represented by a residence time, and pore connectivity, or tortuosity. To simulate a driving force on the molecules, a constant bias in one direction is imposed on the random walk. The unbiased case is also studied. Pore sizes are chosen either at random from each distribution or from one distribution at a time to form a layered structure. The selectivity of the diffusion with respect to molecular size is monitored. The wall-molecule interaction and the molecule size both have an important effect on diffusion, as expected. The arrangement of the pore size distributions, whether in layers or at random, has only a small effect.