Towards a Statistical Mechanical Theory of Mixed Solvent Electrolyte Solutions.

Electrolyte solutions play a fundamental role in both naturally occurring and industrial processes such as geochemistry, fuel cells, electrochemistry, corrosion, pollution, and natural biological and batch biochemical processes. Therefore, in view of their importance, it is useful to develop a model to predict the physical properties of such systems. The crucial role played by water in electrolyte solutions suggests that any statistical mechanical approach must begin by identifying a model for water-water interactions. Three models (SPC, TIP4P, and TIPS2) were chosen for study. Monte Carlo Simulations using the Ewald sum method to account for the long range interactions were performed. Among the properties calculated are the internal energy, dielectric constant, and the site-site correlation functions. It was found that all models had similar values except for the dielectric constant which was most accurately reproduced by SPC at 63 +/- 10. Therefore, SPC was used as the water potential in all of the following simulations. Vapor-liquid simulations were then carried out with SPC to examine its behavior along the vapor-liquid coexistence curve via Gibbs ensemble Monte Carlo simulation. It was found that the calculated coexistence curve moved further from the experimental coexistence curve as the critical point is approached. The critical temperature and critical density were calculated to be 314^circ C and 0.27 g/cc, respectively. A mixture simulation was written for a salt and water where the potentials between the salt ions are those of Fumi and Tosi. The liquid densities are too low for the given state points, but the internal energies are qualitatively predicted correctly. Water -methanol systems where the methanol potential is a model developed by Jorgensen and modified by Haughney et al. were then simulated with good prediction of the vapor-liquid equilibrium. Lastly, a water-methanol-salt simulation was performed with correct prediction of the salting out effect. One drawback to simulations is that they are extremely computationally intensive so it would be useful to be able to predict the various properties of concern in a less computationally intensive manner. Therefore, an analytical solution to the OZ equation with the MSA closure using Baxter's factorization for a system of hard ions and dipoles to represent the water-salt-methanol system was undertaken.