Comprehensive structural modelling of aqueous solutions using neutron diffraction and X-ray absorption spectroscopy

The ability to combine the complementary characteristics of experimental probes is often highly advantageous for advancing our understanding of the physical and chemical properties of structurally disordered systems such as aqueous solutions. Two techniques that have been very influential in this field are neutron diffraction with isotopic substitution and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. By taking advantage of the recent significant developments in computational power and associated analytical methods, it is now possible to produce comprehensive three dimensional atomistic models of these systems that are consistent with both sets of experimental data. One such method is Empirical Potential Structure Refinement, and this study will illustrate its capabilities in an investigation of a 1:500 aqueous solution of rubidium bromide in water. Hydrogen-deuterium isotopic substitution is used to probe the details of the structural correlations between the solvent water molecules, and Rb and Br K-edge EXAFS is used to validate the structural aspects of the incorporation of the solute ions into the bulk solvent matrix. The resulting model allows an estimate to be made of the extent of ion pairing that occurs in the dilute solution.