XAFS Measurements of Rb-O Bonds in Ambient and Supercritical Water

M. Newville; J.L. Fulton; D.M. Pfund; S.L. Wallen; E.A. Stern; Y. Ma

doi:doi:10.1051/jp4:19972118

Numéro		J. Phys. IV France Volume 7, Numéro C2, Avril 1997 Proceedings of the 9th International Conference on X-Ray Absorption Fine Structure


Page(s)		C2-1007 - C2-1008
DOI		https://doi.org/10.1051/jp4:19972118

Proceedings of the 9th International Conference on X-Ray Absorption Fine Structure

J. Phys. IV France 7 (1997) C2-1007-C2-1008
DOI: 10.1051/jp4:19972118

XAFS Measurements of Rb-O Bonds in Ambient and Supercritical Water

M. 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.

Abstract
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.