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

Abstract

Concentrated aqueous solutions of salts constituted by divalent ions combinedwith monovalent counter-ions were investigated by x-ray diffraction and Ramanspectroscopy at room temperature. The salts studied were strontium, barium, andmagnesium chlorides and bromides, and lithium and caesium sulphates. For manyof these solutions, intensity maxima were detected in their x-ray diffractionpatterns 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 onconcentrated aqueous solutions of trivalent cations. In these solutions thepre-peaks appear to be narrower and much more intense. In the ionic solutionsstudied here, a slight contrast between values of the scattering power of twospatial domains is suggested as the origin of the observed pre-peak. One of thesecomes from an accumulation of scattering power around the ion with the higherelectric charge, locally organized into a subtle close packing; the other comes fromthe holes of this structure. Molecular models of the structure of theseelectrolytes are used to demonstrate the plausibility of this interpretation.