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^-, SO₄^{2 -}, and S₂ O₃^{2 -}) 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 (SO₄^{2 -} and S₂ O₃^{2 -}) decrease the magnitude of the zeta potential of α-alumina in aqueous medium, more precisely; S₂ O₃^{2 -} 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 SO₄^{2 -} 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: S₂ O₃^{2 -} <SO₄^{2 -} < 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 ν_as(-COO^-) and ν_s(-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).