Rubidium ion hydration in ambient and supercritical water

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%. A slight contraction of this first shell distance by about 0.10 Å is also observed under supercritical conditions. The reduction in the number of 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.