Mechanics of shrinkage-swelling transition of microporous materials at the initial stage of adsorption

Abstract

This work extends the recently developed surface poromechanics theory to explain the unusual contraction behaviour of some microporous materials at the early stage of adsorption. In micropores, the overlap of adsorbed layers near opposing solid walls can give rise to forces that are formerly disregarded in the analysis of macro/meso porous materials. The effect of these interactions is studied at the microscale with reference to a slit pore geometry through pore-scale thermodynamic analysis. As a result, an extra pressure normal to the pore walls known as the disjoining pressure, in addition to the bulk fluid pressure that is typically considered for macro/meso pores, is obtained for micropores. This term also modifies the surface tension parallel to the pore walls. The disjoining pressure and the modified surface tension together create a competing effect depending on which, the material can exhibit shrinkage or swelling during adsorption at low gas pressures, which has been experimentally observed in various microporous solids since the 1950s. By adopting proper adsorption and microstructure models with physically meaningful parameters, the theory is validated against the recent adsorption-deformation data of microporous carbon interacting with nitrogen, argon, and carbon dioxide gases. For the first time, the abnormal contraction of porous materials upon initial gas uptake is quantitatively modelled through poromechanics that considers surface forces developed in micropores. The proposed theory offers a rigorous procedure to upscale the disjoining pressure and surface tension operating at pore-scale to the adsorption stresses felt by the overall solid skeleton, effectively preserving the characteristics of solid-adsorbate interaction in the continuum modelling of microporous materials.