Electric double layer composed of an antagonistic salt in an aqueous mixture: Local charge separation and surface phase transition

TL;DR

This study investigates the complex behavior of electric double layers with antagonistic salts in aqueous mixtures, revealing charge separation, surface phase transitions, and the importance of ion size effects.

Contribution

It introduces a model showing how antagonistic salts induce charge separation and phase transitions in electric double layers, emphasizing the role of solvent composition and ion size.

Findings

Anions are trapped in oil-rich layers on hydrophobic walls.

A nonmonotonic potential drop occurs at high solvent interaction parameters.

First order phase transitions between different double layer states are observed.

Abstract

We examine an electric double layer containing an antagonistic salt in an aqueous mixture, where the cations are small and hydrophilic but the anions are large and hydrophobic. In this situation, a strong coupling arises between the charge density and the solvent composition. As a result, the anions are trapped in an oil-rich adsorption layer on a hydrophobic wall. % while the cations are expelled from it. We then vary the surface charge density $\sigma$ on the wall. For $\sigma>0$ the anions remain accumulated, but for $\sigma<0$ the cations are attracted to the wall with increasing $|\sigma|$. Furthermore, the electric potential drop $\Psi(\sigma)$ is nonmonotonic when the solvent interaction parameter $\chi(T)$ exceeds a critical value $\chi_c$ determined by the composition and the ion density in the bulk. This leads to a first order phase transition between two kinds of electric double layers with different $\sigma$ and common $\Psi$. In equilibrium such two layer regions can coexist. The steric effect due to finite ion sizes is crucial in these phenomena.