(23f) Thermodynamic Approach to Adsorption Deformation

Molecules adsorbed in pores cause elastic deformations of the solid matrix leading to either contraction or swelling of the material. Although experimental manifestation of adsorption-induced deformation in clays, coals, carbons, silicas, and other materials has been known for a long time, a rigorous theoretical description of this phenomenon is lacking. We suggest a thermodynamic approach based on the nonlocal density functional theory (NLDFT) calculations that reproduce almost quantitatively the adsorption and strain isotherms of Kr and Xe on zeolite X. This system exhibits characteristic contraction at low vapor pressures and swelling at high vapor pressures. We show that the experimentally observed changes of the adsorbent volume are proportional to the solvation (disjoining) pressure caused by the adsorption stress exerted on the pore walls. The proposed NLDFT model can be used for interpretation of adsorption measurements in micro- and mesoporous materials, and for characterization of their mechanical properties.