Measurement of surface diffusivities in mesoporous Vycor glass membranes using a modified Wicke–Kallenbach cell with variable cell volume

The purpose of this paper is to suggest a new experimental method for the measurement of surface diffusivities in porous membranes and to demonstrate its applicability in a case study. So far only indirect methods are applied. Hereby the flux contributions due to surface diffusion are extracted from the total fluxes by subtracting calculated gas phase contributions. Since gas and adsorbed phases are coupled by adsorption equilibrium, surface diffusivities cannot be extracted from surface diffusion fluxes without knowing sorption isotherms. Thus, the quality of surface diffusivity data then depends on the quality of the adsorption data. Therefore, it might be worthwhile to look for another method, which allows the direct measurement of surface diffusivities without knowing the adsorption isotherm. We propose to use a modified Wicke–Kallenbach cell (WKC) with a variable chamber volume and to run pressure response experiments with one inert gas and the counter-diffusing adsorbable gas, for which one wants to determine the surface diffusivity.