Chemically reactive thin diffusion barriers

We assess the degree of protection against diffusion of an environmental pollutant afforded by a thin, chemically reactive surface film on a slab or membrane. For the case of an irreversible quasi-unimolecular reaction in such a surface film we derive a modified radiation boundary condition which adequately describes the presence of such a film. We show that the two extra parameters which appear in this boundary condition, an effective surface conductivity and an effective ambient concentration can be found from the permeation steady state flux and time lag through such a membrane. We compare our approximate boundary condition with that obtained for a chemically passive thin film and a newly derived boundary condition which exactly describes the transport in the composite medium — reactive film and body (slab or membrane). In certain instances such a thin, reactive film is effective as a diffusion barrier.