Adsorption of nonelectrolytes from dilute aqueous solutions in carbon micropores: a condensation approximation approach

The Dubinin model of micropore volume filling for gas/solid adsorption is extended to describe the adsorption of nonelectrolytes from dilute aqueous solutions. Adsorption of poorly soluble organic compounds leads to the complete displacement of water from micropores and to formation of an interface between an adsorbed phase and the solution. The maximum molar work supplied by the field of the adsorption forces is considered to be made up of the work of adsorbate compression, A(c), the work of water desorption, A(d), the work associated with the interface formation, A(i), and the work required for overcoming solute-solvent interactions, A(a). The work of the adsorption forces is found from the relationship between the adsorption energy and slit-like micropore sizes; A(d) is determined using the affinity coefficient of water desorption derived from immersion heats; A(i) is taken to be, the product of the interface tension and the interface area; and A(a) is approximated by the interchange energy. The expression for a micropore filling concentration as a function of micropore sizes (a local isotherm) is derived from the condition of thermodynamic equilibrium on the basis of a condensation approximation approach. The generalization of the local isotherm to encompass a heterogeneous adsorbent with a Normal distribution of micropore sizes leads to a liquid/solid isotherm equation. It is emphasized that there is an urgent need for the independent experimental verification of the present approach. (C) 2000 Elsevier Science Ltd. All rights reserved.