An analysis of the fluctuation potential in the modified Poisson-Boltzmann theory for restricted primitive model electrolytes

TL;DR

This paper presents an analytical solution for the fluctuation potential in a modified Poisson-Boltzmann theory, improving predictions of electrolyte structure and thermodynamics, validated against Monte Carlo simulations.

Contribution

It introduces a new approximate analytical solution for the fluctuation potential applicable at all inter-ionic distances in electrolyte solutions.

Findings

Excellent agreement with Monte Carlo data for reduced energies up to 1 mol/dm³.

Improved predictions of osmotic coefficients and contact radial distribution functions.

The solution enhances the theoretical description of electrolyte structure and thermodynamics.

Abstract

An approximate analytical solution to the fluctuation potential problem in the modified Poisson-Boltzmann theory of electrolyte solutions in the restricted primitive model is presented. The solution is valid for all inter-ionic distances, including contact values. The fluctuation potential solution is implemented in the theory to describe the structure of the electrolyte in terms of the radial distribution functions, and to calculate some aspects of thermodynamics, viz., configurational reduced energies, and osmotic coefficients. The calculations have been made for symmetric valence 1:1 systems at the physical parameters of ionic diameter $4.25 \times 10^{-10}$ m, relative permittivity 78.5, absolute temperature 298 K, and molar concentrations 0.1038, 0.425, 1.00, and 1.968. Radial distribution functions are compared with the corresponding results from the symmetric Poisson-Boltzmann, and the conventional and modified Poisson-Boltzmann theories. Comparisons have also been done for the contact values of the radial distributions, reduced configurational energies, and osmotic coefficients as functions of electrolyte concentration. Some Monte Carlo simulation data from the literature are also included in the assessment of the thermodynamic predictions. Results show a very good agreement with the Monte Carlo results and some improvement for osmotic coefficients and radial distribution functions contact values relative to these theories. The reduced energy curve shows excellent agreement with Monte Carlo data for molarities up to 1 mol/dm$^{3}$.