Zeta-potential and radiotracer adsorption measurements on EFG α-Al 2O 3 single crystals in NaBr solutions

Abstract

Radiotracer adsorption measurements ( 24Na 82Br tracer) and ζ-potential measurements were performed on α-Al 2O 3 single crystals, grown by the EFG process, in 0.01 and 0.05 mole dm −3 NaBr and with varying pH. Evidence was obtained for the adsorption of sodium as well as of bromide ions in the compact part of the electrical double layer. A gel layer is not formed. Surface charge density curves were derived from the adsorption and ζ-potential data. The point of zero charge is at pH ≈ 4.5, whereas the isoelectric point (iep) occurs at pH 3.1–3.5, depending on the NaBr concentration. This difference between iep and pzc is attributed to a greater complexation strength of Al 2O 3 for Br − ions than for Na + ions. This explanation is corroborated by the radiotracer adsorption measurements and the low iep therefore cannot be attributed to silica contamination. An acceptable description of the experimental data can be given in terms of the site-binding model, following the approach of Davis et al. [ J. Colloid Interface Sci. 63, 480 (1978)]. The inner layer capacitance, required to describe the pH region of predominating Br − adsorption, is about twice as large as that required to describe the pH region of predominating Na + adsorption.