An XAFS Study of Strontium Ions and Krypton in Supercritical Water

We present the first direct measurement of ion hydration in supercritical water using X-ray absorption fine structure (XAFS). Radial structure functions were determined for strontium ions in supercritical water at 385[degree]C and 269-339 bar at a concentration of strontium of 0.2 M. For supercritical water, at a temperature of 385[degree]C and density of 0.54 g/cm[sup 3], the number of waters of hydration was a factor of 0.52 of the number in liquid water under ambient conditions. The radius of the first solvation shell changes very little at these elevated temperatures. This large local depletion of water around the ion would affect the short-range interactions with counterions and may increase the ion reactivity. We also report XAFS results for krypton in supercritical water and show that, in contrast to strontium ions, the local solvent environment is more gaslike in the first few solvation shells of the krypton atom. 36 refs., 5 figs., 1 tab.