A Predictive Model of Surface Charge on Oxides in Aqueous Electrolyte Solutions: The Extended Triple Layer Model (ETLM)

Abstract

Although experimental studies of the surface protonation of oxides in electrolyte solutions have been carried out for many years (Davis and Kent, 1990), few attempts have been made to place the experimental measurements into a coherent framework that permits prediction for systems that have yet to be studied experimentally. The enormous range of different natural and synthetic oxides of interest, and the correspondingly large range of electrolyte types and ionic strengths of relevence to industrial and natural processes, accentuate the need for such a predictive model. The purpose of the present study is to review progress made towards a comprehensive model for the prediction of surface charge on oxides in electrolyte solutions. Experimental data clearly show that surface charge depends on pH, ionic strength, and on the specific electrolyte in solution. Consequently, the triple layer model is used to analyse existing experimental data. It is extended with crystal chemical and Born solvation theory to make predictions of surface charge for an oxide in any specific 1:1 electrolyte over a wide range of ionic strengths. In order to do this, the key parameters of the triple layer model (Westall and Hohl, 1980) must be estimated in a consistent way. These include a site density for each mineral; the surface protonation equilibrium constants Kt and K2; the electrolyte adsorption constants KM+ and KL-; and the inner layer integral capacitance Cv