Osmotic coefficients and Activity coefficients of Primitive Model electrolyte solutions in the Symmetric and Modified Poison-Boltzmann theories; A comparative study with Monte Carlo simulations.

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 Zareen Abbas, et al. (J. Phys. Chem. B, vol. 113, pp. 5905-5916 (2009)). This was achieved by adjusting the ion size parameters and the ion valences in the theories to match the corresponding Monte Carlo data. The agreement between Modified Poisson-Boltzmann predictions with the exact simulation results is almost quantitative for monovalent salts, while being semiquantitative 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 for higher valency 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. For HCl, the theories and the simulations reproduce well the experimental results for the mean activity coefficients but reproduce poorly the individual coefficients. For the NaCl solution, the experimental mean activity coefficient matches well with the theories and the simulations