Hydration and Distribution of Ions at the Mica-Water Interface

Schematic model depicting the vertical distribution of Rb+ (left) and Sr2+ (right) at the muscovite-water interface along with the adsorbed ion’s hydration shell.

The interactions of cations with mineral surfaces are central to understanding the transport of nutrients and contaminants in soils and ground waters. The textbook picture of ion adsorption has two distinct classes of adsorbed cations: inner-sphere species that adsorb by displacing the cation hydration shell with surface oxygens and outer-sphere species that retain their hydration shell upon adsorption. It is widely assumed that outer- and inner-sphere species are inherently weakly and strongly bound, respectively, although previous studies were unable to directly observe the outer-sphere species. Our results show that this assumption may be inaccurate.

• Park et al. determined the distribution of cations at muscovite surfaces in contact with aqueous electrolyte solutions. The results reveal that Rb+ adsorbs as an inner-sphere species, while Sr²+ adsorbs both as inner- and outer-sphere species. This demonstrates that the interaction of outer-sphere species is not necessarily intrinsically weaker than inner-sphere species.
   
• Perturbations to the distribution of water immediately adjacent to the muscovite surface (i.e., the interfacial hydration layer) were observed with respect to the hydration structure in de-ionized water, and these changes differed substantially for the monovalent and divalent cations.
   
• The ability to directly observe outer-sphere species was obtained with resonant anomalous x-ray reflectivity, which directly probes the element-specific cation distribution.

These results reveal that the actual interfacial ion distribution is determined by a balance between the electrostatic attraction between the ion and the charged substrate and the energy cost dehydration of the adsorbing cation and the substrate surface.

Reference

C. Park, P. A. Fenter, K. L. Nagy and N. C. Sturchio, “Hydration and Distribution of Ions at the Mica-Water Interface,” Physical Review Letters, 97, 016101(1-4) (2006).