X-Ray Diffraction Studies of Aqueous Alkali-Metal Halide Solutions

Abstract

Radial distribution functions were calculated for 19 concentrated solutions of eight alkali-metal halides. These are analyzed in terms of the sizes and environments of the aqueous ions. The sizes of the hydrated and associated ions are found to be given by their respective crystal ionic radii. From seven to nine water molecules occupy the first hydration spheres of non-associated halide ions, the numbers being relatively independent of solution concentration but increasing slightly with increasing size of the ion. Second and third hydration spheres are correlated with the halide ions, the region of influence for the anions being larger than for the cations studied. Positive ions are found to lie on the dipolar axes and negative ions on the hydrogen—oxygen bond axes of the primary waters of hydration. In contrast to the lithium and sodium halide solutions, considerable direct contact among ions exists in cesium halide solutions. Increasing association occurs with increasing concentration of the solutions and decreasing size of the halide ion. No preferred spatial arrangement is found for the associated ionic species.