XAFS MEASUREMENTS OF RB-O BONDS IN AMBIENT AND SUPERCRITICAL WATER

XAFS measurements and analyses of Rb+ ions in 0.5 molal RbBr at ambien t and several supercritical (P > P-c = 220 bar, T > T-c = 374 degrees C) conditions are presented. The first coordination shell of Rb+ ion i s seen to be dominated by oxygen from nearby water molecules. Upon goi ng from ambient to supercritical conditions, the Rb-O coordination num ber decreases significantly and the Rb-O distance decreases by similar to 0.10 Angstrom relative to the ambient Rb-O distance (similar to 2. 93 Angstrom). Although ion pairing has been predicted for supercritica l conditions (due in part to a substantial decrease of the dielectric constant at the critical point), and has been observed with XAFS in ot her aqueous solutions, we see no direct evidence of pairing of Br- wit h Rb+ in our measurements.