ELECTROCHEMISTRY OF A MODEL FOR PATCHWISE HETEROGENEOUS SURFACES - THE RUTILE HEMATITE SYSTEM

The electrical double layer properties of aqueous suspensions of rutile and hematite and of mixtures of these two oxides have been studied. The surface charge σ0 of the oxides and their mixtures as a function of pH in various KNO3 concentrations and the isoelectric points (IEP) of rutile and hematite have been measured. From shifts of the points of zero charge (PZCs) and the IEPs of the pure oxides as a function of the KNO3 concentration and by the analysis of the components of charge of the double layer it could be concluded that for both oxides specific adsorption of K+ and NO-3 occurs. The σ0(pH) and the capacitance curves, C*(pH), indicate that rutile and hematite behave similarly but not identically. Modeling of the σ0(pH) and the C*(pH) curves of both oxides with the one pK-Stern-Gouy-Chapman (SGC) model gives good results provided K+ and NO-3 are allowed to adsorb. The experimental σ0(pH) and C*(pH) data obtained for six different mixtures of rutile and hematite can be described as a weighed average of the individual component properties. This indicates that the mixtures can be treated as a patchwise heterogeneous system. Modeling of the mixtures on the basis of the one pK-SGC model extended to patchwise heterogeneity with the parameters obtained for the pure oxides results in a satisfactory description of σ0(pH), C*(pH), and the shift of a mixture PZC as a function of both the heterogeneity and the KNO3 concentration. Qualitative agreement is found between the model predictions and the experimentally observed shift of the capacitance minimum. © 1990.