Hydration and contact ion pairing of Ca2+ with Cl- in supercritical aqueous solution
X-ray absorption fine structure (XAFS) spectroscopy was used to measure the first-shell structure about Ca2+ 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 bar. 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 Ca2+-Cl- direct contact ion pairs form in agreement with existing thermodynamic data for this system. For a 1 m CaCl2 solution at 400 C, the mean coordination structure about Ca2+ contains 3.2 ? 0.6 water molecules at an average Ca-O distance of 2.356 ?0.026 ? and 1.8 ? 0.7 Cl- at a Ca-Cl distance of 2.677 ? 0.007 ?. An evaluation of the Ca and Cl pre-edge and near-edge (XANES) spectrum provided further confirmation of the change in the Ca2+ first-shell structure and symmetry. Overall these measurements provide a structural basis for understanding solvation of Ca2+ in hydrothermal systems. These results also provide important new insights into the structural aspects of Ca2+ ion pairing that are the basis of many biological processes under ambient conditions.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 892890
- Report Number(s):
- PNNL-SA-50016; TRN: US0605870
- Journal Information:
- Journal of Chemical Physics, 125(9):094507-1 - 094507-10, Journal Name: Journal of Chemical Physics, 125(9):094507-1 - 094507-10
- Country of Publication:
- United States
- Language:
- English
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