Anion Layering and Steric Hydration Repulsion on Positively Charged Surfaces in Aqueous Electrolytes. Issue 21 (16th October 2017)

• Record Type: Journal Article
• Authors: Hu, Qingyun; Weber, Christian; Cheng, Hsiu‐Wei; Renner, Frank Uwe; Valtiner, Markus
• Is Part Of: Chemphyschem. Volume 18:Issue 21(2017)
• Page Start: 3056
• Page End: 3065
• Publication Date: 2017-10-16
• Subjects: atomic force microscopy -- anions -- hydration forces -- mica -- water layering; Chemistry, Physical and theoretical -- Periodicals; 541.05
• DOI: 10.1002/cphc.201700865
• Languages: English
• ISSNs: 1439-4235

Abstract

Abstract: 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 −, ClO4 −, NO3 −, SO4 2− and PO4 3− ) 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. Abstract : Repell or attract? For a set of anions (Cl −, ClO4 −, NO3 −, SO4 2− and PO4 3− ) structuring steric forces were studied on positively charge surfaces using atomic force microscopy with a Si tip at pH≈1 in pure acids. It was found that anions with lower hydration energies exhibit structural hydration forces due to the specific adsorption effects within the Stern layer, while anions with high hydration energies show exponentially repulsive hydration forces.