On the concentration dependence of surface diffusion coefficients in capillary porous materials

The influence of structural heterogeneity, in the form of a non-uniform pore size distribution, on the isotherms and surface diffusion coefficients for monolayer physical adsorption is studied. A pore size dependent langmuirian isotherm is used along with consideration of equality of chemical potentials at the pore mouths at an intersection. The diffusion is modelled by a recently developed random walk formulation. It is found that the surface diffusion coefficients are strongly influenced by the heterogeneity and have a stronger increase with overall coverage than that predicted by the Darken equation. The results are found to match the experimental data of P. C. Carman and F. A. Raal on the diffusion of carbon dioxide in carbon black without the use of a fitting parameter.