Total Reflection Method Applied to an X-Ray Absorption Fine Structure Study of Aqueous Solution Surface

Abstract

A new method has been developed for investigating the geometrical structure of chemical species present at a solution surface. The method is the X-ray absorption fine structure (XAFS) spectroscopy which is combined with X-ray introduction under a total reflection condition. It was applied to aqueous solutions of bromide and rubidium ions with and without surface-active counter ions. The solvation structure for Br segregated at the surface by surface-active cation has been found to be different from that in the bulk. The coordination numbers for both the cation and anion at the surface have also been found to be different from those in the bulk. Due to the highly polarized X-ray radiation from a synchrotron used in this study, the coordination number determined by this technique must reflect the preferential orientation of the observing atom-atom bond with respect to the solution surface. Thus, the solvation structures and orientations for ions at the surface are discussed.