XAFS measurements of Rb-O bonds in ambient and supercritical water

XAFS measurements and analyses of Rb⁺ ions in 0.5 molal RbBr at ambient and several supercritical (P > P_c = 220 bar, T > T_c = 374°C) conditions are presented. The first coordination shell of Rb⁺ ion is seen to be dominated by oxygen from nearby water molecules. Upon going from ambient to supercritical conditions, the Rb-O coordination number decreases significantly, and the Rb-O distance decreases by ∼ 0.10 Å relative to the ambient Rb-O distance (∼ 2.93 Å). Although ion pairing has been predicted for supercritical conditions (due in part to a substantial decrease of the dielectric constant at the critical point), and has been observed with XAFS in other aqueous solutions, we see no direct evidence of pairing of Br^- with Rb⁺ in our measurements.