Electrokinetic modeling of metal oxides

The focus of this study is the electrokinetic transport of metal oxide colloidal particles. These transport properties include electrophoretic mobility and the primary electroviscous effect of dilute suspensions of spherical particles that may contain a "gel layer" of uniform density on their outer surface. Charge on the surface or in the gel layer is assumed to arise from specific protonation, deprotonation, or complex formation reactions of oxide sites. Charge regulation, or the modulation of the equilibrium constants due to high absolute charge densities, is examined. Both the electrostatics and electrokinetic transport properties are solved numerically within the framework of the continuum-primitive model. With regards to the electrokinetics, detailed account is taken of the finite size of the particle as well as the "electrophoretic effect" and the "relaxation effect." The methodology is then applied to two metal oxide systems: Ludox (a silica sol) and goethite. The charge density, electrophoretic mobility, and primary electroviscous effect of Ludox at pH 8.7 in KCl salt solutions, can be well explained by a gel layer model that expands as the salt concentration is lowered. A spherical model containing no gel layer is unable to account for the electrophoretic mobility of goethite in 0.01 M KNO(3). However, a "sparse" gel layer model that contains 7.5% of the charge sites and consists of 96% solvent can account for both the experimental charge density and mobility over the pH range 4.0 to 11.0.