Divalent Ion Specific Outcomes on Stern Layer Structure and Total Surface Potential at the Silica:Water Interface.

Emily Ma,Franz M Geiger

doi:10.1021/acs.jpca.1c08143

Abstract

The second-order nonlinear susceptibility, χ(2), in the Stern layer and the total interfacial potential drop, Φ(0)tot, across the oxide:water interface are estimated from SHG amplitude and phase measurements for divalent cations (Mg2+, Ca2+, Sr2+, and Ba2+) at the silica:water interface at pH 5.8 and various ionic strengths. We find that interfacial structure and total potential depend strongly on ion valency. We observe statistically significant differences between the experimentally determined χ(2) value for NaCl and that of the alkali earth series but smaller differences between ions of the same valency in that series. These differences are particularly pronounced at intermediate salt concentrations, which we attribute to the influence of hydration structure in the Stern layer. Furthermore, we corroborate the differences by examining the effects of anion substitution (SO42- for Cl-). Finally, we identify that hysteresis in measuring the reversibility of ion adsorption and desorption at fused silica in forward and reverse titrations manifests itself both in Stern layer structure and in total interfacial potential for some of the salts, most notably for CaCl2 and MgSO4 but less so for BaCl2 and NaCl.

Highlights

Ion specific interactions at charged interfaces have been explored intensely over the years[1,2,3,4] but they are challenging to incorporate into models
At mineral/oxide interfaces, mobile ions form an electrical double layer (EDL) that extends from the solid surface into the aqueous bulk, modulating electrostatic interactions and balancing the charges that exist at the interface
We find that χ(2) is essentially invariant for the alkali earth chlorides but that their point estimates for F(0)tot decrease with increasing cation radius

Summary

Introduction

Ion specific interactions at charged interfaces have been explored intensely over the years[1,2,3,4] but they are challenging to incorporate into models. Much work has sought to fill in those necessary aspects and to provide a detailed description of both hydration structure of ions[5] and its influence on the electrostatic potential at an interface.[10] An important question that has arisen from such studies concerns whether electrolyte valency (z) is a reasonable description of ion correlations at the interface as well as the overall potential that exists at an aqueous interface Another question that has arisen pertains to the interplay between molecular interactions and electrostatics at the interface. Exploring these questions with divalent ions offers an opportunity to pursue fundamental investigations of ion specific EDL structure and electrostatics at aqueous interfaces

Methods

Results

Conclusion