Intermediate-range order in aqueous solutions of salts constituted of divalent ions combined with monovalent counter-ions

Concentrated aqueous solutions of salts constituted by divalent ions combined with monovalent counter-ions were investigated by x-ray diffraction and Raman spectroscopy at room temperature. The salts studied were strontium, barium, and magnesium chlorides and bromides, and lithium and caesium sulphates. For many of these solutions, intensity maxima were detected in their x-ray diffraction patterns close to 0.6–1 Å⁻¹. Interpretation of these maxima, pre-peaks, is discussed, taking into account the results of previous investigations of the authors on concentrated aqueous solutions of trivalent cations. In these solutions the pre-peaks appear to be narrower and much more intense. In the ionic solutions studied here, a slight contrast between values of the scattering power of two spatial domains is suggested as the origin of the observed pre-peak. One of these comes from an accumulation of scattering power around the ion with the higher electric charge, locally organized into a subtle close packing; the other comes from the holes of this structure. Molecular models of the structure of these electrolytes are used to demonstrate the plausibility of this interpretation.