Molecular simulations of concentrated aqueous solutions: Ionic equilibrium structures in solutions

The computer simulation methods have been applied to study the structure of aqueous solutions of simple ionic salts in the region of very high concentrations. The calculations of ionic structures in solutions were performed for NaOH, NaCl, LiCl and MgCl(2) solutions. The concentrations ranged from 0.2 M to saturated solutions, in some cases as much as 19 M. A particularly careful analysis was devoted to the topology of the ionic structures in solution. Up to now, most of the research on ionic solutions was devoted to studies of the ionic hydration shells. However, beyond the Debye - Huckel range of very low concentrations, very little is known about the interionic spatial correlations. Certain theories predict the existence of ionic quasi-lattices in the region of high concentrations. In the present work we used the Molecular Dynamics method combined with such statistical tools as the radial distribution functions, Voronoi tessellations, the running and O'Keeffe coordination numbers, etc., in order to analyze the ionic structures. The radial distribution functions of three types: the cation - anion, cation - cation and the anion - anion type were calculated for each solution. The functions are typical for the quasi-crystalline order within the first 2 - 3 ionic coordination layers around a selected ion. The order is particularly pronounced for the anion - cation RDF's. The distributions of the sphericity factor of the Voronoi polyhedra were calculated for the ionic substructures in the configurations produced by the Molecular Dynamics simulation. The increase of the ton concentration causes evolution of these distributions towards increased signatures of predominant geometries of the Voronoi polyhedra. This, together with the results for RDF's, provides a strong conjecture for existence of the ordered structures of ions in concentrated solutions.