Approaches to separation interfaces

The interaction and separation of ions with zwitterionic layers are reviewed principally based on a series of the author's work. An electrostatic model has allowed us to discuss the chromatographic retention of ions on the zwitterionic stationary phase, and has revealed the ionic interaction occurring at the zwitterionic interface. Similar consideration is applicable to the ionic partition into zwitterionic micelles having the spherical dimension. In the electrostatic models, ion association and solvation changes of ions have been assumed to explain the selectivity in ion recognition. Both assumptions are applicable to polarizable large ions, whereas the former cannot account for the results obtained for small and well-hydrated ions (Cl− and Br−). A special X-ray absorption finestructure (XAFS) measurement, which allows selective access to ions interacting with surface monolayers, has been developed, and applied to ions attracted by a zwitterionic monolayer. The X-ray absorption spectra suggest that Zn2+ attracted by the zwitterionic monolayer is still hydrated. In contrast, the direct interaction of Br− has been confirmed, indicating that the electrostatic model involving either ion association or the solvation change of an ion does not properly explain the observed phenomena but both effects should be taken into consideration.