Ion-pairing in aqueous CaCl2 and RbBr solutions: simultaneous structural refinement of XAFS and XRD data

We present a new methodology involving the simultaneous refinement of both x-ray absorption and x-ray diffraction spectra (x-ray absorption∕diffraction structural refinement, XADSR) to study the hydration and ion pair structure of CaCl(2) and RbBr salts in concentrated aqueous solutions. The XADSR method combines the x-ray absorption fine structure (XAFS) spectral analysis of both the cation and anion as a probe of their short-range structure with an x-ray diffraction (XRD) spectral analysis as a probe of the global structural. Together they deliver a comprehensive picture of the cation and anion hydration, the contact ion pair (CIP) structure, and the solvent-separated ion pair (SSIP) structure. XADSR analysis of 6.0 m aqueous CaCl(2) reveals that there are ~0.26 Ca(2+)-Cl(-) CIP's separated by about 2.71 Å, while there are 3.4 SSIP's separated by about 4.98 Å. In contrast XADSR analysis of 6 m aqueous RbBr yields about 0.7 pair CIP at a bond length of 3.51 Å. The present work demonstrates a new approach for a direct co-refinement of XRD and XAFS spectra in a simple and reliable fashion, opening new opportunities for analysis in various disordered and crystalline systems.