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 canons (Ca2+ and Mg2+) on the zeta potential and on the isoelectric point of a-alumina in aqueous medium has been studied. The effect of the anions is highly ion specific even at salt concentrations as low as 5 x 10(-4) M. This unexpected finding is in line with a recent report [Bostrom 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 alpha-alumina in aqueous medium, more precisely; S2O3 produced large negative zeta potential (similar to-12 to -47 mV) within the pH range of the study without the isoelectric point (IEP) of alpha-alumina. However, the SO42- decreased the zeta potential of alpha-alumina of different magnitudes (maximum similar to 25 mV at both ends of the experimental acidic and basic pH scale) with a minor shift of the IEP (similar to 0.5 unit) toward lower pH. Ca2+ and Mg2+ 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-hydroxybenz oate 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: S2O32- < SO4(2-) < Cl- > Br- > l(-) > NO3-. The divalent cations (Ca2+ and Mg2+) exhibit a roughly equivalent effect on the adsorption of p-hydroxybenzoate onto a-alumina surfaces. Using the frequency shifts of v(as)(-COO-) and v(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 5 x 10(-4) M NaCl(aq). (C) 2009 Elsevier Inc. All rights reserved.