Specific ion binding influences on surface potential of chromium oxide

Abstract

Colloidal force measurements as a function of pH can yield the isoelectric point (IEP) of a surface immersed in an electrolyte. The condition of surface charge-potential regulation imposed by the potential-dependent binding of H + and counter-ions at the interface makes a detailed analysis of the electrostatic force non-trivial. In the current study, the specific ion binding of phosphate ions on to chromium oxide has been investigated. An atomic force microscope (AFM) has been used to measure the force of interaction between a SiO 2 sphere (∼5 mm diameter) and a chromium oxide surface in aqueous media of sodium phosphate buffer or sodium chloride over the pH range 3–11. From the force separation profiles the force at `Jump To' is plotted over the pH range studied for each ionic strength. As the IEP of SiO 2 is around pH 2 the probe interaction with the surface measures its electrostatic properties, and hence can be used to determine the IEP. The comparison of force titration plots shows the IEP of the chrome surface decreases with increasing phosphate ion concentration, from around pH 8 with no phosphate ions present, down to around pH 6 at 0.01 M ionic strength phosphate buffer. This indicates that there is specific ion binding of the phosphate to the chrome oxide surface. We have used approach of DLVO theory, together with a simple model of specific adsorption of ions at the oxide–water interface, to model the long range electrostatic repulsion force measured by the force separation plots at each pH and ionic strength. By comparing this model to the isoelectric points at several ionic strengths, we can estimate surface dissociation constants for the adsorption of protons and phosphate from the electrolyte.