XAFS measurements on zinc chloride aqueous solutions from ambient to supercritical conditions using the diamond anvil cell

The structure and bonding properties of metal complexes in subcritical and supercritical fluids are still largely unknown. Conventional high pressure and temperature cell designs impose considerable limitations on the pressure, temperature, and concentration of metal salts required for measurements on solutions under supercritical conditions. In this study, we demonstrate the first application of the diamond anvil cell, specially designed for x-ray absorption studies of first-row transition metal ions in supercritical fluids. Zn K-edge XAFS spectra were measured from aqueous solutions of 1-2m ZnCl₂ and up to 6m NaCl, at temperatures ranging from 25-660 °C and pressures up to 800 MPa. Our results indicate that the ZnCl₄^2- complex is predominant in the 1m ZnCl₂/6m NaCl solution, while ZnCl₂(H₂O)₂ is similarly predominant in the 2m ZnCl₂ solution, at all temperatures and pressures. The Zn-Cl bond length of both types of chlorozinc(II) complexes was found to decrease at a rate of about 0.01 Å/100 °C.