Thermodynamics of primitive model electrolytes in the symmetric and modified Poisson-Boltzmann theories. A comparative study with Monte Carlo simulations

TL;DR

This study compares symmetric and modified Poisson-Boltzmann theories with Monte Carlo simulations to evaluate their accuracy in predicting electrolyte solution properties, finding the modified version generally aligns better with simulation data especially for multivalent salts.

Contribution

The paper provides a detailed comparison of symmetric and modified Poisson-Boltzmann theories against Monte Carlo simulations for primitive model electrolytes, highlighting the improved accuracy of the modified approach.

Findings

Modified Poisson-Boltzmann closely matches Monte Carlo results for monovalent salts.

Symmetric Poisson-Boltzmann performs well for monovalent but less so for multivalent salts.

Theories show good agreement with some experimental activity coefficients.

Abstract

Osmotic coefficients, individual and mean activity coefficients of primitive model electrolyte solutions are computed at different molar concentrations using the symmetric Poisson-Boltzmann and modified Poisson-Boltzmann theories. The theoretical results are compared with an extensive series of Monte Carlo simulation data obtained by Abbas et al. [Fluid Phase Equilib., 2007, 260, 233; J. Phys. Chem. B, 2009, 113, 5905]. The agreement between modified Poisson-Boltzmann predictions with the "exact" simulation results is almost quantitative for monovalent salts, while being semi-quantitative or better for higher and multivalent salts. The symmetric Poisson-Boltzmann results, on the other hand, are very good for monovalent systems but tend to deviate at higher concentrations and/or for multi-valent systems. Some recent experimental values for activity coefficients of HCl solution (individual and mean activities) and NaCl solution (mean activity only) have also been compared with the symmetric and modified Poisson-Boltzmann theories, and with the Monte Carlo simulations.