Ion-Solvent and Solvent-Solvent Interactions. X-ray Study of Aqueous Alkali Chloride Solutions

The first neighbour model, FNM, used frequently for the description of hydration has been modified by dropping the assumption that the structure of the "free" solvent is identical with that of the pure solvent. The modified model, FNM2, reproduces the experimental X-ray structure functions of alkali chloride solutions quite well and enables the study of perturbed solventsolvent interactions. The refined structural parameters reasonably indicate the dependence of the hydration of the alkali cations on their radius and concentration. The hydration of Cl~ has been found to be almost independent of both concentration and the type of counter cation. The model assumes regular symmetry for the first neighbour coordination sphere, but the high rms deviations of the water-water distances within the shells of the aggregates indicate considerable individual asymmetries. A further modification of the FNM will be discussed in a following paper.