Anion Layering and Steric Hydration Repulsion on Positively Charged Surfaces in Aqueous Electrolytes

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 , 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.

Forces between solid surfaces in aqueous electrolyte solutions

“…In symmetric systems, short-ranged repulsive non-DLVO forces are omnipresent for strongly hydrated and highly charged mineral surfaces, such as mica, silica, and metal oxides [70,78,[125][126][127][128]. The range of these forces is normally small, typically below one nanometer.…”

Oxoanion Imprinting Combining Cationic and Urea Binding Groups: A Potent Glyphosate Adsorber

“…In the presence of water, the relative hydration energies of the two monoanions seem instead to be the dominating factor. Assuming monoanions of inorganic carbonate 35 and phosphate 36 as estimates (HCO 3 = −380 kJ/mol; H 2 PO 4 = −522 kJ/mol), BA is less strongly hydrated than PPA, for which the strong hydration will prevent its interaction with the binding site.…”

Applicability of the Linearized Poisson-Boltzmann Theory to Contact Angle problems and Application to the Carbon Dioxide-Brine-Solid Systems

“…The point of zero zeta potential obtained via high to low pH titration was reported to be located at pH 4.2 (Nosrati et al, 2009). Hu et al, (2017) also report that at pH ∼ 3, which was measured for a 50 mM solution, the surface of mica surface is close to its point of zero charge pH. Since Jafari &Jung (2018) measured contacts angle on the smooth basal plane, our calculated value of 3.64, which is 0.56 pH unit lower for one molar sodium chloride solution is theoretically and experimentally meaningful.…”

“…Since Jafari &Jung (2018) measured contacts angle on the smooth basal plane, our calculated value of 3.64, which is 0.56 pH unit lower for one molar sodium chloride solution is theoretically and experimentally meaningful. Moreover, at pH approximately equal to 3, the surface of mica is close to its point of zero charge pH (Hu et al, 2017;Kristiensen et al, 2011). Therefore, the closeness of our calculated point of zero charge pH of mica using in situ experimental data and Eq.…”