Molecular simulation of a concentrated aqueous KCl solution

A 1.0 M aqueous KCl solution was studied by molecular dynamics simulations at 293 K in order to study the influence of the ionic concentration on the hydration structure of the ions as well as the formation of ion clusters. The hydration structures of the ions are almost independent of the ionic concentration unless in respect to the perturbation that appears due to ionic clustering. Fractions equal to 31.9% of the anions and 37.8% of the cations are associated. Clusters constituted by two, three and four ions were detected. Their mean lifetimes are always affected by thermal effects, reorientational relaxation while the longest lifetimes are a consequence of ionic cloud relaxations. The pairs constituted by two anions or two cations are stabilized by water molecules belonging to the solvation shells of both ions. The neutral K⁺Cl⁻ pairs are formed under the influence of the electrostatic attraction that, however, is small due to the ionic radii of these ions. Consequently, this kind of pairs contains only 8.8% of the ions while the fraction of ions in the negative and positive pairs are equal to 29.2 and 39.3%, respectively, when the same ion can pertain to more than one pair.