Electrolyte layering at the calcite(104)-water interface indicated by Rb⁺- and Se(VI) K-edge resonant interface diffraction

Calcite-water interface reactions are of major importance in various environmental settings as well as in industrial applications. Here we present resonant interface diffraction results on the calcite(104)-aqueous solution interface{,} measured in solutions containing either 10 mmol L-1 RbCl or 0.5 mmol L-1 Se(vi). Results indicate that Rb+ ions enter the surface adsorbed water layers and adsorb at the calcite(104)-water interface in an inner-sphere fashion. A detailed analysis based on specular and off-specular resonant interface diffraction data reveals three distinct Rb+ adsorption species: one 1.2 A above the surface{,} the second associated with surface adsorbed water molecules 3.2 A above the surface{,} and the third adsorbed in an outer-sphere fashion 5.6 A above the surface. A peak in resonant amplitude between L = 1.5 and L = 3.0 is interpreted as signal from a layered electrolyte structure. The presence of a layered electrolyte structure seems to be confirmed by data measured in the presence of Se(vi).