Surface ionization and complexation at the oxide/water interface: I. Computation of electrical double layer properties in simple electrolytes

Abstract

A new method for determination of intrinsic ionization and complexation constants of oxide surface sites from potentiometric titration data is reported. Using these experimental properties and the stoichiometry of surface reactions, surface charge, σo, adsorption density, Γi, and diffuse layer potentials, ψd, at the oxide/water interface are calculated. The numerical method permits simultaneous calculation of all surface and solution equilibrium states in both simple and complex electrolyte/colloid systems. This allows generalization to dilute ion adsorption in systems with complex solution chemistry. In electrolyte solutions of moderate or high concentration, the surface charge is dominated by surface complex formation of ionizable surface sites and electrolyte ions. In very dilute simple salt solutions the surface ionization reactions are more important. H+ and OH− are the principal potential determining ions (p.d.i.), but the electrolyte ions have a minor effect on the calculated surface potential, ψ0. The electrolyte ions and p.d.i. have a joint role in determining the magnitude of the surface charge via their reactions with surface sites. The important experimental parameters which characterize these contributions are ΔpKcumptex = p*Kcationint − p*Kanionint and ΔpKa = pKa2int − pKa1int.