Evaluation of equilibrium parameters of the anatase/aqueous electrolyte solution interface by introducing surface potential data

Abstract

The single crystal anatase electrode (SCrE) was used to measure the inner surface potential ( φ 0) at the anatase/aqueous electrolyte interface as a function of pH. Measurements were performed in sodium perchlorate and sodium chloride aqueous solutions. Absolute values of the surface potential were calculated from electrode potentials using the electrokinetic isoelectric point as the point of zero potential. The slope of φ 0 (pH) function, with respect to pH, was found to be lower than the Nernstian. These results were accompanied with surface charge density and electrokinetic potential data and were interpreted simultaneously on the basis of the Surface Complexation Model. Both 1-pK and 2-pK mechanisms were employed. Data in the acidic region were interpreted since they were more reliable. Simultaneous interpretation of three sets of data enabled calculation of surface concentrations of all species, and thus thermodynamic equilibrium constants and the inner layer capacitance for each pH value separately. It was found that the equilibrium constants of surface protonation did not depend on the pH and the nature of the counterions, supporting the applicability of both 1-pK and 2-pK models. Thermodynamic equilibrium constants for association of perchlorate ions with positively charged surface groups did not change with pH, as expected, but the inner layer capacitance changed from 0.5 to 1.5 F m −2 by increasing the pH. Chloride ions showed an increase in the association equilibrium constant with pH and a less pronounced increase of capacitance. The advantage of the applied procedure lies in the possibility of avoiding the model assumptions of the constancy of parameters describing the interfacial equilibrium. It was shown that the thermodynamic equilibrium constants of surface protonation are real constants as defined by the Surface Complexation Model. The capacitance, however, changes with pH and surface potential and its constancy is just an approximation introduced by necessity when only one set of experimental data, such as surface charge density is quantitatively interpreted.