A DFT-based model for water adsorption at aluminosilicate surfaces. Comparison with experimental data extracted from dielectric relaxation spectroscopy

When applied to adsorption phenomena, Gibbs-Duhem equation shows that surface energy associated to exchangeable cations located at the aluminosilicate surface decreases with the increasing number of adsorbed molecules. In this work, we propose a microscopic representation of this phenomenon, i.e. the adsorption of water molecules at the cationic sites, and its evolution upon water adsorption. Accordingly, the DFT-based model proposed in this paper provides a simple mathematical expression in which the system, i.e. cation/surface, energy is a functional of the electronic density. Therefore, the adsorption of small molecules as water which results in modification of the surface electronic density and, hence, of the electrical potential modifies the barrier for cation hopping. The model presented here is favourably confronted to experimental data of energy barrier for cation hopping measured by dielectric relaxation, i.e. complex impedance spectroscopy and thermally stimulated current, on various aluminosilicate solids: two zeolites (Na-faujasite and Na-mordenite) and clay mineral (Na-montmorillonite) upon water adsorption.