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arXiv · 1812.11235

Direct Multilayer Adsorption of Vapor in Solids with Multiscale Porosity and Hindered Adsorbed Layers in Nanopores

Zdenek P. Bazant·Hoang Thai Nguyen
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Abstract

Hindered adsorbed layers completely filling the nanopores must cause significant deviations from the classical BET isotherms for multimolecular adsorption of vapor in porous solids. Since the point of transition from free to hindered adsorption moves into narrower nanopores as the pore vapor pressure (or humidity) decreases, the area of the surface of multimolecular adsorption layer exposed to vapor gets modified by an area reduction factor that decreases with increasing thickness of the layer, and thus also with increasing vapor pressure. The area reduction factor does not affect the rates of direct adsorption or condensation from the vapor, which represent a local process, but imposes a lateral constraint on the total area and volume of the free portion of the adsorption layer in direct contact with vapor. Assuming a power-law for the dependence of the area reduction factor on the number of molecular layers, one can express the modified isotherm in terms of logarithmic or polylogarithmic (a.k.a Jonqui\`ere) functions. Comparisons with some published experimentally obtained isotherms show that the present modification of the BET theory for hindered adsorption goes in the right direction. Detailed calibration of the theory and an extension for indirect communication of vapor molecules with the molecules adsorbed in nanopores less than a few nm wide will require further research.

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Zdenek P. Bazant, Hoang Thai Nguyen. 2018-12-28. Direct Multilayer Adsorption of Vapor in Solids with Multiscale Porosity and Hindered Adsorbed Layers in Nanopores. https://arxiv.org/abs/1812.11235

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