Hydration and contact ion pairing of Ca ²⁺ with Cl ^- in supercritical aqueous solution

X-ray absorption fine structure (XAFS) spectroscopy was used to measure the first-shell structure about Ca ²⁺ in high-temperature aqueous solution. XAFS spectra were acquired at the Ca K edge at temperatures up to 400 °C and pressures up to 350 bars. For the system at 400 °C, both Ca (4038.5 eV) and Cl (2822.4 eV) K-edge data were acquired and a global model was used to fit the two independent sets of XAFS data. Measurements were made at the bending magnet beamline (sector 20) at the Advanced Photon Source, Argonne. Above 250 °C, a significant number of Ca ²⁺-Cl ^- direct contact ion pairs form in agreement with existing thermodynamic data for this system. For aim CaCl ₂ solution at 400 °C, the mean coordination structure about Ca ²⁺ contains 3.2±0.6 water molecules at an average Ca-O distance of 2.356±0.026 Å and 1.8±0.7 Cl ^- at a Ca-Cl distance of 2.677±0.007 A. An evaluation of the Ca and Cl preedge and near-edge (x-ray absorption structure) spectra provided further confirmation of the change in the Ca ²⁺ first-shell structure and symmetry. Overall these measurements provide a structural basis for understanding solvation of Ca ²⁺ in hydrothermal systems. These results also provide important new insights into the structural aspects of Ca ²⁺ ion pairing that are the basis of many biological processes under ambient conditions.