High-resolution Measurement of Contact Ion-pair Structures in Aqueous RbCl Solutions from the Simultaneous Corefinement of their Rb and Cl K-edge XAFS and XRD Spectra

Abstract

In concentrated solutions of aqueous RbCl, all of the Rb+ and Cl– ions exist as contact ion pairs. This full structural assessment is derived from the refinement of three independent experimental measurements: the Rb and Cl K-edge X-ray absorption fine structure (XAFS) and the X-ray diffraction spectra (XRD). This simultaneous refinement of the XAFS and XRD data provides high accuracy since each method probes the structure of different local regions about the ions with high sensitivity. At high RbCl concentration (6 mol·kg–1) the solution is dominated by Rb+–Cl– contact ion pairs yielding an average of 1.5 pairs at an Rb–Cl distance of 3.24 A. Upon formation of these ion pairs, approximately 1.1 waters molecules are displaced from the Rb+ and 1.4 water molecules from Cl–. The hydration shells about both the cation and anion are also determined. These results greatly improve the understanding of monovalent ions and provide a basis for testing the Rb+–Cl– interaction potentials used in molecular dynamics (MD) simulation.