ON ORIGIN OF SMALL-ANGLE MAXIMA ON X-RAY SCATTERING INTENSITY CURVES OF AQUEOUS ELECTROLYTE SOLUTIONS

Abstract

At the X-ray diffraction analysis of high concentrated aqueous electrolyte solutions with multiply charged cations small angle maxima or pre-peaks are arising on scattering intensity curves (IC) (at about 1 Å). Their existence is connected with structural appearance so called “intermediate-range order”. Previously for explanation of this phenomenon some not completely correct hypotheses were suggested. On the example of raw X-ray diffraction data of some aqueous electrolyte solutions in wide concentration range collected by using MoKα-radiation the dynamic of small angle maxima shape change has been studied. It has been noted that pre-peaks absence on IC of cesium iodide and chloride solutions is not supporting the hypothesis about pure small angle contribution from “heavy” ions [1, 2]. In the case of Al3+, Sc3+, Fe3+ chloride solutions relative similarity of IC shapes has been noted for solutions with similar concentrations. The common tendency of pre-peaks position and intensity change has also been followed up. At the same time at comparing of saturated neodymium and ytterbium chloride and bromide solutions IC unexpectable pre-peaks position relative shift for bromide solutions to the long range distance comparing with chloride solutions has also been noted. These observations do not find complete agreement with hypothesis on realization in the structure mainly inter cationic distances [3, 4] and on existence in solutions “liquid type quasi-close-packing for cations” [5, 6]. On the basis of mainly realistic explanation of the pre-peak appearance reason from [7] the authors of this paper propose their own version. Noted shift of pre-peak position with concentration decreasing has been made in accordance with consideration described earlier for lanthanide solutions [8, 9]. The conclusion about pre-peaks on IC arising nature is conditioned by ordered interionic distribution which depends not only on ionic nature but also on solution concentration. For high concentrated solutions with considerable solvent deficit the quasi-periodic inter complex distribution has been supposed. The composition of structure determined cationic complexes has also been discussed. The diluted aqueous electrolyte solutions structure is determined by simple cationic and anionic complexes equidistant distribution.