Ionic structure in aqueous electrolyte solution by the difference method of X-ray diffraction

Abstract

Neutron diffraction difference methods based on isotopic substitutions offer a means of directly probing the detailed structure around a solute dissolved in a host solvent¹, but there are two main drawbacks to their implementation. First, they rely on the use of separated isotopes which are expensive and sometimes of limited availability. Second, the experiments have to be performed at a sophisticated central facility to obtain sufficiently intense beams of thermal neutrons. We have therefore initiated a series of experiments aimed at introducing difference methods to X-ray diffraction studies of liquid and amorphous systems. Difference methods have been attempted before², but the absence of definitive neutron data did not allow the method to be made fully quantiative. We now present the first results obtained from such a study in which the difference technique is applied to a concentrated aqueous electrolyte solution and the cation–cation distribution function g_MM(r) is determined directly. The success of the method rests on the ability to identify nearly isomorphic pairs of elements, whose isomorphism can be quantified by reference to similar results obtained from the first-order difference method of neutron diffraction³.