Materials Science Division, North-East Institute of Science & Technology, CSIR, Jorhat 785 006, Assam, India.
J Colloid Interface Sci. 2010 Apr 15;344(2):482-91. doi: 10.1016/j.jcis.2009.12.048. Epub 2009 Dec 28.
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).
已研究了无机阴离子（NO3-、I-、Br-、Cl-、SO42-和 S2O32-）和二价阳离子（Ca2+和 Mg2+）对α-氧化铝在水介质中的 ζ 电位和等电点的影响。即使在低至 5x10-4M 的盐浓度下，阴离子的影响也具有高度的离子特异性。这一意外发现与最近的一份报告[Böstrom 等人，J. Chem. Phys. 128（2008）135104]一致。它也与早期的理论预测[B.W. Ninham，V.V. Yaminsky，Langmuir 13（1997）2097]一致。结果与经典的 Hofmeister 系列一致，但 NO3-除外。二价阴离子（SO42-和 S2O32-）降低了α-氧化铝在水介质中的 ζ 电位的幅度，更确切地说，在研究的 pH 范围内，S2O32-产生了大的负 ζ 电位（约-12 至-47 mV），而没有α-氧化铝的等电点（IEP）。然而，SO42-以不同的幅度降低了α-氧化铝的 ζ 电位（最大约为 25 mV，在实验酸性和碱性 pH 尺度的两端），IEP 略有向较低 pH 值移动（约 0.5 个单位）。Ca2+和 Mg2+产生的α-氧化铝 ζ 电位大致与纯净α-氧化铝的 ζ 电位相等，但在 IEP 两侧略高于 Na+的 ζ 电位。我们进一步表明，相同的离子特异性或等效的竞争离子效应也发生在对羟基苯甲酸吸附到α-氧化铝表面的吸附密度上。对羟基苯甲酸在α-氧化铝表面的吸附密度的阴离子序列（具有相同的阳离子）遵循 Hofmeister 系列顺序：S2O32- < SO42- < Cl- > Br- > I- > NO3-。二价阳离子（Ca2+和 Mg2+）对 p-羟基苯甲酸在α-氧化铝表面的吸附表现出大致相当的影响。使用吸附后 p-羟基苯甲酸的 DRIFT 光谱中 νas（-COO-）和 νs（-COO-）的频率位移以及其他特征峰，我们已经证明，在 5x10-4M NaCl（aq）存在的情况下，p-羟基苯甲酸在 pH 5 和 6 下形成α-氧化铝表面的外球配合物，在 pH 7、8 和 9 下形成内球配合物。
