Features of the cationic hydration and its influence on the structure of electrolytes and their viscosity

Abstract

The interaction of ions with water was considered using the example of alkali metal cations with a common hydroxyl anion. Structural and energetic changes in the ion-dipole system of water were analyzed. A significant influence of the size of the cations on the change in the interaction energy and the polarizability (dielectric permeability) of water around the ion was shown. In this case, at the boundary with the lithium ion, the dielectric constant was close to 1, and at the boundary with the cesium ion, it increased to 1.67. The dielectric constant at the boundary of the first hydrate shell for lithium and cesium ions increased to 2.1 and 2.25, respectively, corresponding to the experimentally obtained value given in the literature. An analysis of energetic and steric factors affecting the amount of ions hydration was carried out. Lithium was shown to have two shells consisting of 4 and 8 dipoles of water, corresponding to primary (positive) hydration. A significant value of the ion-dipole bond energy made it possible to consider the hydrated ion as a quasi-solid particle. The effect of hydrated ions on the water viscosity was calculated using Einstein formula. Two opposite factors were shown to affect the viscosity during hydration: the presence of quasi-solid particles increased the viscosity, and the existence of a secondary hydrated shell with broken bonds between water molecules decreased it. Satisfactory correspondence between theoretically calculated and experimentally determined viscosity values was obtained.