Thermodynamic and Transport Properties of Electrolyte Solutions and Melts from Molecular Simulations

Abstract

Salts are ubiquitous in everything from biological and geological systems to industrial processes. They can exist in all phases, including in the form of a pure crystal, an aqueous solution, a melt, or even as a vapor of ion pairs. In molecular simulations, these ionic systems are challenging to study due to the long-ranged nature of the Coulomb interactions, which leads to large system-size effects and inaccurate behavior of molecular models. Here we investigate these effects, as well as develop new methods to calculate properties of electrolyte systems. We first present corrections to the effect of system size on the chemical potentials and activity coefficients of electrolyte solutions. The system-size effects are found to be especially significant at low salt concentrations. We then investigate the activity coefficients and solubility of calcium chloride using a number of different force fields, including a model employing scaled Coulomb interactions. None of the models are able to capture the behavior of both properties at different temperatures, and scaling the ions' charges is found to be an insufficient way of representing polarizability. We also perform chemical potential calculations in a molten alkali-metal carbonate-hydroxide system in order to determine chemical reaction equilibria. The simulation results suggest that reactions in these high temperature melts will lead to the formation of undesired hydroxide ions. Finally, we calculate ionic conductivity using a new reverse non-equilibrium molecular dynamics method, but conclude that the method suffers from significant shortcomings. The results in this thesis illustrate the challenges in calculating the properties of ionic systems from molecular simulations but also show that simulations can still have a profound effect on our understanding of salts. The methods presented here will allow for more accurate future simulations of these important systems.