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Review date: July 24, 2003
PNNL-SA-27883

X-ray Rubidium Ion Hydration in Ambient and Supercritical Water

JL Fulton, DM Pfund, SL Wallen, M. Newville, EA Stern, and Y Ma. J. Chem. Phys. 105, 2161-2166 (1996).

Abstract: X-ray absorption fine structure (XAFS) measurements and analyses are presented for Rb⁺ in supercritical water solutions. The structure of the first hydration shell at ambient conditions is compared to that in the supercritical region at a temperature of 424°C and pressures from 382 to 633 bar. For all reported studies, RbBr at a concentration of 0.5 molal was used. XAFS results show that there is a well-defined hydration shell around the cation even at 424°C but at these high temperatures the extent of hydration of the Rb cation is reduced by about 40 percent. A slight contraction of this first shell distance by about 0.10 Å is also observed under supercritical conditions. The reduction in water-ion bonds is analogous to the reduction in the amount of water-water hydrogen bonding that has been observed by others under supercritical conditions. The reduction in waters-of-hydration under supercritical conditions may also be in part due to formation of contact-ion pairs.