<title>Molecular simulations of concentrated aqueous solutions: ionic equilibrium structures in solutions</title>

Abstract

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, NaC1, LiCI and MgC 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 OKeeffe 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 ion 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.

Restructuring of Hydration Shell Water due to Solvent-Shared Ion Pairing (SSIP): A Case Study of Aqueous MgCl2 and LaCl3 Solutions

“…Next, we have extracted the hydration shell spectrum of the Mg 2+ cation (black curve, Figure b) by the RD-SCF analysis of dilute MgCl 2 solution (0.187 M i.e., 0.375 M in terms of [Cl – ]) relative to the NaCl (0.375 M) solution. At such low concentration of MgCl 2 (0.187 M), the interionic interaction is negligible and the ions largely exist in the fully hydrated state. − The hydration shell water of the Mg 2+ cation has a broad OH-stretch spectrum with three distinct bands at around 3220, 3380, and 3600 cm –1 , corresponding to the water-OH stretches with decreasing order of H-bond strength, , though a contribution of vibrational coupling and Fermi resonance cannot be ruled out for the 3220 cm –1 band. On doubling the concentration of MgCl 2 to 0.375 M, the intensity of the Mg 2+ -hydration shell spectrum increases without a significant change in the band shape (red curve, Figure b).…”