A Monte Carlo simulation and symmetric Poisson–Boltzmann study of a four-component electrolyte mixture

The structure and thermodynamics of a four-component electrolyte mixture consisting of two electrolytes with a common ion, and a neutral species are studied using the canonical Metropolis Monte Carlo simulations. The steric effects of the neutral species are seen to shape substantially the equilibrium fluid properties, an important result being the existence of an attractive force between like-charged ions at sufficiently high concentrations of the neutral species. Parallel calculations performed utilizing a symmetric Poisson–Boltzmann theory reveal the usefulness of the classical theory; it is able to predict (a) the simulation profiles and the thermodynamics to a very reasonable degree of accuracy up to moderate solution concentrations, and (b) the attractive interaction between like-charge ions under the influence of high contents of neutral particles. These are useful results since the generalizations of the more formal statistical mechanical theories to four or more components require very extensive numerical work.