Hu Qingyun, Weber Christian, Cheng Hsiu-Wei, Renner Frank Uwe, Valtiner Markus
Department for Interface Chemistry and Surface Engineering, Max Planck Institut für Eisenforschung GmbH, D-40237, Düsseldorf, Germany.
Institut für Physikalische Chemie der TU Bergakademie Freiberg, 09599, Freiberg, Germany.
Chemphyschem. 2017 Nov 3;18(21):3056-3065. doi: 10.1002/cphc.201700865. Epub 2017 Oct 16.
The molecular structure at charged solid/liquid interfaces is vital for many chemical or electrochemical processes, such as adhesion, catalysis, or the stability of colloidal dispersions. How cations influence structural hydration forces and interactions across negatively charged surfaces has been studied in great detail. However, how anions influence structural hydration forces on positively charged surfaces is much less understood. Herein we report force versus distance profiles on freshly cleaved mica using atomic force microscopy with silicon tips. We characterize steric anion hydration forces for a set of common anions (Cl , ClO , NO , SO and PO ) in pure acids at pH ≈1, where protons are the co-ions. Solutions containing anions with low hydration energies exhibit repulsive structural hydration forces, indicating significant ion and/or water structuring within the first 1-2 nm on a positively charged surface. We attribute this to specific adsorption effects within the Stern layer. In contrast, ions with high hydration energies show exponentially repulsive hydration forces, indicating a lower degree of structuring within the Stern layer. The presented data demonstrates that anion hydration forces in the inner double layer are comparable to cation hydration forces, and that they qualitatively correlate with hydration free energies. This work contributes to understanding interaction processes in which positive charge is screened by anions within an electrolyte.
带电固/液界面的分子结构对许多化学或电化学过程至关重要，如粘附、催化或胶体分散体的稳定性。阳离子如何影响跨带负电表面的结构水合力和相互作用已得到深入研究。然而，阴离子如何影响带正电表面的结构水合力却鲜为人知。在此，我们使用带有硅尖的原子力显微镜报告了新鲜劈开云母上的力与距离曲线。我们表征了在pH≈1的纯酸中一组常见阴离子（Cl⁻、ClO₄⁻、NO₃⁻、SO₄²⁻和PO₄³⁻）的空间阴离子水合力，其中质子是共离子。含有低水合能阴离子的溶液表现出排斥性的结构水合力，表明在带正电表面的前1-2纳米内存在显著的离子和/或水结构。我们将此归因于斯特恩层内的特定吸附效应。相比之下，具有高水合能的离子表现出指数级排斥的水合力，表明斯特恩层内的结构程度较低。所呈现的数据表明，内双层中的阴离子水合力与阳离子水合力相当，并且它们在性质上与水合自由能相关。这项工作有助于理解电解质中阴离子屏蔽正电荷的相互作用过程。
