Monovalent Cation Selectivity Quantitatively Modeled According to Hard/Soft Acid/Base Theory

Abstract

The development of a quantitative cation-selectivity model to predict cation-exchange equilibria has been a desirable but elusive goal in the field of surface chemistry of soil clay minerals. We critically examine several cation-selectivity models in this study to establish a predictive, quantitative model for monovalent cation exchange. Two of the models—the cation hydration model and the anion field-strength model—consider only “purely electrostatic” interactions between cation and surface and, thus, can predict the selectivity sequences only for hard (acid)/hard (base) interactions. A third model, based on the hard and soft acid and base (HSAB) principle, takes both electrostatic and covalent interactions into account, but has been applied only qualitatively to cation selectivity. The quantitative prediction of cation selectivity from the HSAB principle requires both a suitable relation between cation selectivity and the HSAB softness and hardness parameters, and a quantitative scale for these parameters. We present a quantitative HSAB-based model to predict cation selectivity with two fundamental cation parameters, absolute electronegativity and absolute softness, and two fitting parameters that are related to the exchanger surface characteristics. The potential advantages of this model are discussed.