Ion specificity of the zeta potential of α-alumina, and of the adsorption of p-hydroxybenzoate at the α-alumina–water interface

The influence of inorganic anions (NO3-, I−, Br−, Cl−, SO42-, and S2O32-) and of divalent cations (Ca2+ and Mg2+) on the zeta potential and on the isoelectric point of α-alumina in aqueous medium has been studied. The effect of the anions is highly ion specific even at salt concentrations as low as 5 × 10−4 M. This unexpected finding is in line with a recent report [Böstrom et al., J. Chem. Phys. 128 (2008) 135104]. It is also in agreement with an earlier theoretical prediction [B.W. Ninham, V.V. Yaminsky, Langmuir 13 (1997) 2097]. The results are consistent with the classical Hofmeister series, except for the case of NO3-. Divalent anions (SO42- and S2O32-) decrease the magnitude of the zeta potential of α-alumina in aqueous medium, more precisely; S2O32- produced large negative zeta potential (∼−12 to −47 mV) within the pH range of the study without the isoelectric point (IEP) of α-alumina. However, the SO42- decreased the zeta potential of α-alumina of different magnitudes (maximum ∼25 mV at both ends of the experimental acidic and basic pH scale) with a minor shift of the IEP (∼0.5 unit) toward lower pH. Ca2+ and Mg2+ produce zeta potentials of α-alumina roughly equal to that of neat α-alumina but slightly higher than that of Na+ at both sides of the IEP. We have shown further that the same ion specificity or equivalently competitive ion effects occur with the adsorption density of p-hydroxybenzoate onto α-alumina surfaces. The sequence of anions (with common cation) for the adsorption density of p  -hydroxybenzoate on the α-alumina surfaces follows the Hofmeister series sequence: S2O32- < SO42- < Cl− > Br− > I− > NO3-. The divalent cations (Ca2+ and Mg2+) exhibit a roughly equivalent effect on the adsorption of p-hydroxybenzoate onto α-alumina surfaces. Using the frequency shifts of νas(–COO−) and νs(–COO−) in the DRIFT spectra of p-hydroxybenzoate after adsorption and other characteristic peaks, we have demonstrated that p-hydroxybenzoate forms outer-sphere complexes onto α-alumina surfaces at pH 5 and 6 and inner-sphere complexes at pH 7, 8, and 9 in the presence of 5 × 10−4 M NaCl(aq).

Graphical abstractThe effect of anions on the zeta potential of α-alumina is highly ion specific. The same ion specificity occurs with the adsorption density of p-hydroxybenzoate onto α-alumina surfaces.Figure optionsDownload full-size imageDownload high-quality image (106 K)Download as PowerPoint slide