Structure of the double layer at the oxide/water interface

Abstract

The colloidal oxides of the amphoteric metals Al, Si and Ti exhibit anomalies compared with the more familiar AgI and Hg sols, which are difficult to reconcile with the usual pictures of the electrostatic double layer. It has been suggested that such oxides, particularly SiO₂, are coated with a gel layer of hydrolysed material. We determine here the properties of a particular model of such a system, one in which the gel is of infinite extent. We are able to account for the deviations observed from the Nernst equation ΔΨ_o=–2.3 kT/ΔpH, relating changes in surface potential Ψ₀ to changes ΔpH in the concentration of potential determining ions. We are also able to account for the observed change of 25 mV per tenfold change in the concentration of indifferent electrolyte, and the observation that potentiometric titrations of the surface charge σ_t show a strong dependence on indifferent electrolyte, and rise to much higher values than those predicted by the Stern–Gouy–Chapman theory.