Thermodynamics of Ion Solvation and Differential Adsorption at Liquid-Liquid Interfaces and Membranes

TL;DR

This paper develops a mean-field thermodynamic model to analyze ion solvation and differential ion adsorption at liquid-liquid interfaces, revealing spontaneous electric double-layer formation and relating ion distributions to Born free energy.

Contribution

It introduces a novel mean-field framework linking ion solvation, adsorption, and electrostatics at liquid-liquid interfaces, including membrane effects.

Findings

Electric double-layer formation is spontaneous at the interface.

Ion distributions are related to Born free energy of polarization.

Membrane charge polarity influences ion selectivity and adsorption.

Abstract

We construct a mean-field formulation of the thermodynamics of ion solvation in immiscible polar binary mixtures. Assuming an equilibrium planar interface separating two semi-infinite regions of different constant dielectric medium, we study the electrostatic phenomenon of differential adsorption of ions at the interface. Using general thermodynamic considerations, we construct the mean-field $\Omega$-potential and demonstrate the spontaneous formation of an electric double-layer around the interface necessarily follow. In our framework, we can also relate both the bulk ion densities in the two phases and the distribution potential across the interface to the fundamental Born free energy of ion polarization. We further illustrate this selective ion adsorption phenomenon in respective examples of fully permeable membranes that are neutral, negative, or positive in charge polarity.