Simple Theory of Ionic Activity in Concentrated Electrolytes

TL;DR

This paper extends the Debye-Hückel theory to concentrated electrolytes by incorporating many-body Coulomb correlations and solvation energy, providing a simple yet effective model that fits experimental activity data with minimal parameters.

Contribution

It introduces a straightforward model that accounts for electrostatic correlations and solvation effects to accurately predict ionic activity in concentrated solutions.

Findings

The model fits diverse electrolyte activity data with only one adjustable parameter.

Electrostatic forces such as charge screening, solvation, and Coulomb correlations dominate ionic activity.

The model captures phenomena like over-screening and under-screening in electrolytes.

Abstract

The Debye-H\" uckel formula for ionic activity coefficients is extended for concentrated solutions by solving a simple model of many-body Coulomb correlations and adding the Born solvation energy. Given the bulk permittivity, our formula is able to fit activity data for diverse electrolytes with only one parameter to adjust the correlation length, which interpolates between the Bjerrum length and the mean ion spacing. The results show that ionic activity in most electrolytes is dominated by three types of electrostatic forces: (i) mean-field charge screening, (ii) solvation, and (iii) Coulomb correlations, both "over-screening" (charge oscillations) and "under-screening" (extending beyond the Debye screening length).