Effects of Permanent Charge on the Electrical Double‐Layer Properties of Clays and Oxides

Abstract

AbstractTwo complimentary models which describe the surface chemistry of, respectively, clay minerals and oxides submersed in an aqueous solution are presented. Both models incorporate isomorphous‐substitution charge, specific adsorption of counter ions at the inner Helmholtz plane, and a diffuse double layer outside the outer Helmholtz plane. In addition, one of the models allows for adsorption of potential‐determining ions at the surface plane, which is located inward of the inner Helmholtz plane. General algorithms to solve surface chemistry models are given and methods to determine the associated empirical parameters are discussed. With the model which includes adsorption of potential‐determining ions, it is shown that isomorphous substitution may be of great importance to the adsorption properties of oxides. Based on published data for NaCl/Al2O3 and KCl/SiO2, computations are made which show that changes in the charge density of the potential‐determining ions adsorbed at the surface plane tend to be equal in magnitude but opposite in sign to changes in the isomorphous‐substitution charge density (σo). However, this cancellation is not perfect, and especially for KCl/SiO2 systems, the counter‐ion exchange capacity and its pH dependency and the zeta potential are strongly dependent on σo. The dependence on σo is particularly strong in the region where σo is small (−0.04 < σo < 0.02 C/m2). This implies that small amounts of isomorphous substitution may have a dramatic effect on the physico‐chemical properties of oxides. The ion‐adsorption behavior of montmorillonite cannot be explained with our generalized model for oxide surfaces. Thus, our calculations confirm the long‐standing view that the basal planes of clay minerals do not adsorb H+ and OH‐ as potential‐determining ions.