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

Quiñones Quiñones,Bhuiyan Bhuiyan,Outhwaite Outhwaite

doi:10.5488/cmp.21.23802

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.

Highlights

Designing many of the important industrial chemical processes involving aqueous electrolytes requires an understanding of the thermodynamics of these systems
We have employed the symmetric Poisson-Boltzmann and the modified Poisson-Boltzmann theories of statistical mechanics to characterize the thermodynamics of electrolyte solutions
The osmotic coefficient, the individual activity coefficients and the mean activity coefficients of 104 primitive model electrolyte systems with arbitrary ionic sizes and ionic valences were calculated and the results were compared with the corresponding Monte Carlo simulations

Summary

Introduction

Designing many of the important industrial chemical processes involving aqueous electrolytes requires an understanding of the thermodynamics of these systems. A few years ago, Abbas et al [24, 25] made extensive MC simulations for a series of symmetric (in ion size and valence) and asymmetric PM electrolytes and reported their osmotic coefficient, single ion and mean activity coefficient results. They treated 104 electrolyte systems covering a wide range of concentration and different valence combinations. Experimental measurements of osmotic and mean activity coefficients of electrolytes abound in the literature [2,3,4] These two quantities are related by the famous Gibbs-Duhem equation of physical chemistry [1], which has been extended to multicomponent fluids [26]. We will make comparisons of the SPB and MPB predicted activity coefficients with the experimental results for HCl due to Sakaida and Kakuichi [28] and for NaCl taken from the literature [2]

Model and Methods

Results

Comparison with experiments

Conclusions