DIFFUSIVE TRANSPORT ACROSS AN INTERFACE

The kinetics of material diffusion across an interface is analyzed wit h special emphasis on the effect of the interface permeability on the transport characteristics, such as the mean-square displacement and th e residence probability in each of the two phases separated by the int erface. Two practically important cases are considered, namely, one-di mensional diffusion in a stepwise potential and three-dimensional diff usion under a spherically symmetric square-well potential. The transpo rt characteristics are shown to be independent of the permeability at asymptotically long times, unless the interface is absolutely impermea ble. However, the way the asymptotic behavior is approached can be con trolled by the interface permeability. At low permeability values, the kinetics exhibit a characteristic intermediate stage that can last fo r such a long time that the true asymptotic behavior may not be observ ed experimentally at all. Along with exact analytical solutions of the diffusion problems at hand, useful approximate expressions are presen ted for different stages of the kinetics.