Thermodynamics and Problems of Taking into Account Deformations of Porous Adsorbents

Abstract

An adsorbate can simultaneously affect the external and internal surfaces of an adsorbent, changing the volume of a sample with a fixed amount of the adsorbent. This process is discussed from the standpoints of mechanics of continuous media, thermodynamics, and molecular models. The microscopic theory of solids distinguishes between external and internal deformations, which makes it possible to associate mechanical characteristics with adsorption ones. Based on the lattice gas model, the principles of two-level structural models are formulated for deformable porous bodies. They enable the self-consistent description of changes in their volumes and adsorption isotherms as a function of the adsorbate external pressure at a fixed temperature. For the simple presentation of the calculation scheme, it is accepted that the atomic sizes of a solid and components of the adsorbate mobile phase are comparable. The molecular level reflects the volume of molecules and their lateral interaction in the quasi-chemical approximation. The supramolecular level of the model is presented as a granule/grain of a porous material with the set distribution function of different (in size and type) pores connected with each other. A relationship between internal deformations and the nonequilibrium state of a solid is shown. A calculation procedure for local mechanical modules with regard to internal and external deformations characterizing the mechanical properties of solids is exemplified by the compression modulus. The procedure takes into account the effect of the nonequilibrium of an adsorbent on adsorbent isotherms and its volume. The approach proposed can be also applied to related systems in which the adsorption of active mobile solutions by highly dispersed compounds changes their volume.