Gas‐adsorbate collisional effects and surface diffusion in porous materials

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Abstract
If gas‐adsorbate momentum transfer is ignored, calculated surface fluxes within porous materials often can be grossly incorrect. Methods are introduced for including these interactions when calculating surface fluxes in permeability studies. Developments are carried out for systems with porous materials having both uniform and heterogeneous surfaces. These methods complement one developed earlier for Wicke‐Kallenbach experiments, using porous materials with uniform surfaces. Some previous studies are then considered. In one system, including gas‐adsorbate momentum transfer changes the proposed surface‐diffusion mechanism from a “hopping” phenomenon to the mobility inherent in a two‐dimensional gas. Consideration of other systems shows that, while very frequently these collisional interactions are important, experiments can occur where they are not. A criterion is developed for indicating when this momentum transfer is significant and its inclusion in surface‐flux determinations is necessary.

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