Studies of mixed solvent organic electrolyte solutions

Estimation and correlation of the physical properties of mixed-solvent electrolyte solutions is important because of their relevance to, e.g., salt-effect distillation, ion-pair extraction operations, ion-pairing chromatography, and electrochemical and biological systems. In this work, a combination of experiment and molecular simulation is used to explore some of the thermodynamic and microscopic properties of two model mixed-solvent organic electrolyte model systems: tetraalkylammonium bromide solutions in water/2-propanol and water/1-propanol mixtures. Vapor-liquid equilibria (VLE) of these systems show interesting salting-in to salting-out transitions of the nonelectrolyte, which cannot be predicted using existing engineering correlations. These solutions also exhibit interesting volumetric behavior as seen in salt apparent molar volume measurements [1, 2, 3]. Molecular simulation of the mixed solvent systems is employed to identify trends in ion-pairing, cation alkyl chain conformation, and preferential solvation which underly the VLE and volumetric behavior observed experimentally. Aqueous solutions of the tetraalkylammonium salts have been the subject of an enormous number of experimental studies due to their role as simple "soluble hydrocarbon" analogues of biomolecules. These solutions are studied for the first time using molecular dynamics simulations, and direct comparisons are made with recent neutron scattering experiments on the same systems at the same concentrations and state conditions. The simulation results lead to conclusions concerning the origin of "hydrophobic hydration" in agreement with those arrived at from the scattering experiments.