Electrolyte layering at the calcite(104)-water interface indicated by Rb super(+)- 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 super(-1) RbCl or 0.5 mmol L super(-1) Se(vi). Results indicate that Rb super(+) 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 super(+) adsorption species: one 1.2 Aa above the surface, the second associated with surface adsorbed water molecules 3.2 Aa above the surface, and the third adsorbed in an outer-sphere fashion 5.6 Aa 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).