School of Chemical Sciences, University of Auckland, Private Bag, 92019 Auckland, New Zealand.
Langmuir. 2011 Nov 1;27(21):12930-7. doi: 10.1021/la201775c. Epub 2011 Oct 7.
The effect of ionic strength on reactions at aqueous interfaces can provide insights into the nature of the chemistry involved. The adsorption of H(4)SiO(4) on iron oxides at low surface silicate concentration (Γ(Si)) forms monomeric silicate complexes with Fe-O-Si linkages, but as Γ(Si) increases silicate oligomers with Si-O-Si linkages become increasingly prevalent. In this paper, the effect of ionic strength (I) on both Γ(Si) and the extent of silicate oligomerization on the ferrihydrite surface is determined at pH 4, 7, and 10, where the surface is, respectively, positive, nearly neutral, and negatively charged. At pH 4, an increase in ionic strength causes Γ(Si) to decrease at a given H(4)SiO(4) solution concentration, while the proportion of oligomers on the surface at a given Γ(Si) increases. At pH 10, the opposite is observed; Γ(Si) increases as I increases, while the proportion of surface oligomers at a given Γ(Si) decreases. Ionic strength has only a small effect on the surface chemistry of H(4)SiO(4) at pH 7, but at low Γ(Si) this effect is in the direction observed at pH 4 while at high Γ(Si) the effect is in the direction observed at pH 10. The pH where the surface has zero charge decreases from ≈8 to 6 as Γ(Si) increases so that the surface potential (Ψ) is positive at pH 4 for all Γ(Si) and at pH 7 with low Γ(Si). Likewise, Ψ < 0 at pH 10 for all Γ(Si) and at pH 7 with high Γ(Si). The diffuse layer model is used to unravel the complex and subtle interactions between surface potential (Ψ) and chemical parameters that influence interfacial silicate chemistry. This analysis reveals that the decrease in the absolute value of Ψ as I increases causes Γ(Si) to decrease or increase where Ψ is, respectively, positive or negative. Therefore, at a given Γ(Si), the solution H(4)SiO(4) concentration changes with I, and because oligomerization has a higher H(4)SiO(4) stoichiometry coefficient than monomer adsorption, this results in the observed dependence of the extent of silicate oligomerization on I.
离子强度对水相界面反应的影响可以深入了解所涉及化学的本质。在低表面硅酸盐浓度（Γ(Si)）下，H(4)SiO(4)在氧化铁上的吸附形成具有 Fe-O-Si 键的单体硅酸盐配合物，但随着 Γ(Si)的增加，具有 Si-O-Si 键的硅酸盐低聚物变得越来越普遍。在本文中，在 pH 值为 4、7 和 10 下确定了离子强度（I）对针铁矿表面的 Γ(Si)和硅酸盐低聚物程度的影响，其中表面分别带正、近中和负电荷。在 pH 4 时，在给定的 H(4)SiO(4)溶液浓度下，离子强度的增加导致 Γ(Si)减小，而在给定 Γ(Si)下表面低聚物的比例增加。在 pH 10 时则相反；随着 I 的增加，Γ(Si)增加，而在给定 Γ(Si)下表面低聚物的比例减小。离子强度对 pH 7 时 H(4)SiO(4)的表面化学性质只有很小的影响，但在低 Γ(Si)时，这种影响与在 pH 4 时观察到的方向相同，而在高 Γ(Si)时，这种影响与在 pH 10 时观察到的方向相同。表面零电荷的 pH 值从约 8 降低到 6，随着 Γ(Si)的增加，因此在所有 Γ(Si)下 pH 4 的表面电位（Ψ）为正，在低 Γ(Si)下 pH 7 的表面电位（Ψ）为正。同样，在所有 Γ(Si)下 pH 10 的 Ψ<0 和在高 Γ(Si)下 pH 7 的 Ψ<0。使用扩散层模型揭示了影响界面硅酸盐化学的表面电位（Ψ）和化学参数之间复杂而微妙的相互作用。该分析表明，随着 I 的增加，绝对值 Ψ 的减小导致 Γ(Si)的减小或增加，其中 Ψ 分别为正或负。因此，在给定的 Γ(Si)下，溶液 H(4)SiO(4)浓度随 I 而变化，并且由于低聚物化具有比单体吸附更高的 H(4)SiO(4)化学计量系数，这导致观察到硅酸盐低聚物化程度对 I 的依赖。
