Electrostatic correlation induced ion condensation and charge inversion in multivalent electrolytes

TL;DR

This paper develops a modified Gaussian fluctuation theory with boundary layer modeling to better understand ion condensation and charge inversion in multivalent electrolytes, capturing nonmonotonic behaviors observed experimentally.

Contribution

It introduces a boundary layer approach and includes excluded volume effects to accurately model ion correlations and charge inversion phenomena.

Findings

Predicts formation of a condensed ion layer near surfaces.

Shows non-monotonic dependence of charge inversion on salt concentration.

Reproduces experimental and simulation observations qualitatively.

Abstract

The study of the electrical double layer lies at the heart of colloidal and interfacial science. The standard mean-field Poisson-Boltzmann (PB) theory is incapable of modeling many phenomena originated from ion correlation. An important example is charge inversion or overcharging of electrical double layers in multivalent electrolyte solutions. Existing theories aiming to include correlations cannot capture the nonmonotonic dependence of charge inversion on salt concentration because they have not systematically accounted for the inhomogeneous nature of correlations from surface to the bulk and the excluded volume effect of ions and solvent molecules. In this work, we modify the Gaussian renormalized fluctuation theory by including the excluded volume effect to study ion condensation and charge inversion. A boundary layer approach is developed to accurately model the giant difference in ion correlations between the condensed layer near the surface and the diffuse layer outside. The theory is used to study charge inversion in multivalent electrolytes and their mixtures. We predict a surface charge induced formation of a three-dimensional condensed layer, which is necessary but not sufficient for charge inversion. The value of the effective surface potential is found to depend non-monotonically on the bulk salt concentration. Our results also show a non-monotonic reduction in charge inversion in monovalent and multivalent electrolyte mixtures. Our work is the first to qualitatively reproduce experimental and simulation observations and explains the underlying physics.