The ion pairing and hydration structure of Ni2+ in supercritical water at 425 °C determined by x-ray absorption fine structure and molecular dynamics studies

The ion pairing structure of ${\mathrm{Ni(Br)}}_{\mathrm{2}}$ solutions (0.2 and 0.4 molal) under supercritical conditions was determined using x-ray absorption fine structure (XAFS) spectroscopy. These first measurements of the average bulk structure show that approximately one ${\mathrm{Br}}^{\mathrm{-}}$ counterion is associated with each ${\mathrm{Ni}}^{\mathrm{2+}}.$ The ${\mathrm{Ni}}^{\mathrm{2+}}$ -to-${\mathrm{Br}}^{\mathrm{-}}$ distance of 2.40 Å is very accurately determined and the strength of this interaction, as indicated by the Debye–Waller factor ${\mathrm{(\sigma }}^2{\text{=0.009\hspace{0.17em}Å}}^2\mathrm{),}$ shows that the bromine anion is very tightly bound to the nickel cation under these supercritical conditions. In addition to the onset of ion pairing interactions, there is also a dramatic transition in the hydration structure. Results show a loss of about 50% of the waters in the first shell upon going from ambient to a hydrothermal condition of 425 °C and 690 bar. Finally, we use molecular dynamics simulations with refined intermolecular potentials to directly calculate XAFS spectra that are shown to quantitatively reproduce the experimental results for the ion pair and hydration structure.