J. Phys. IV France 7 (1997) C2-1007-C2-1008
XAFS Measurements of Rb-O Bonds in Ambient and Supercritical WaterM. Newville1, J.L. Fulton2, D.M. Pfund2, S.L. Wallen2, E.A. Stern3 and Y. Ma3
1 Lawrence Livermore National Laboratory, L-219, Livermore, CA 94550, U.S.A.
2 Pacific Northwest National Laboratory, Richland, WA 99352, U.S.A.
3 Department of Physics, Box 351560, University of Washington, Seattle, WA 98195, U.S.A.
XAFS measurements and analyses of Rb+ ions in 0.5 molal RbBr at ambient and several supercritical (P > Pc = 220 bar, T < Tc = 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.
© EDP Sciences 1997