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

The influence of inorganic anions (NO(3)(-), I(-), Br(-), Cl(-), SO(4)(2-), and S(2)O(3)(2-)) and of divalent cations (Ca(2+) and Mg(2+)) on the zeta potential and on the isoelectric point of alpha-alumina in aqueous medium has been studied. The effect of the anions is highly ion specific even at salt concentrations as low as 5x10(-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 NO(3)(-). Divalent anions (SO(4)(2-) and S(2)O(3)(2-)) decrease the magnitude of the zeta potential of alpha-alumina in aqueous medium, more precisely; S(2)O(3)(2-) produced large negative zeta potential (approximately -12 to -47 mV) within the pH range of the study without the isoelectric point (IEP) of alpha-alumina. However, the SO(4)(2-) decreased the zeta potential of alpha-alumina of different magnitudes (maximum approximately 25 mV at both ends of the experimental acidic and basic pH scale) with a minor shift of the IEP (approximately 0.5 unit) toward lower pH. Ca(2+) and Mg(2+) produce zeta potentials of alpha-alumina roughly equal to that of neat alpha-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 alpha-alumina surfaces. The sequence of anions (with common cation) for the adsorption density of p-hydroxybenzoate on the alpha-alumina surfaces follows the Hofmeister series sequence: S(2)O(3)(2-) < SO(4)(2-) < Cl(-) > Br(-) > I(-) > NO(3)(-). The divalent cations (Ca(2+) and Mg(2+)) exhibit a roughly equivalent effect on the adsorption of p-hydroxybenzoate onto alpha-alumina surfaces. Using the frequency shifts of nu(as)(-COO(-)) and nu(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 alpha-alumina surfaces at pH 5 and 6 and inner-sphere complexes at pH 7, 8, and 9 in the presence of 5x10(-4) M NaCl(aq).