Relationship of interface structure to the dynamics of selective lanthanide extraction

Recent synchrotron X-ray studies have revealed a variety of specific ion effects in the structures formed by lanthanides and surfactants at air-water and solvent-water interfaces. However, the relevance of the interfacial structure to extraction and selective separation processes has been unclear. We have studied solvent extraction from aqueous solutions containing lanthanide ions to organic solvents containing model extractants, and correlated the results to the interfacial structure. The selectivity, extraction rates and extraction efficiencies are consistent with the postulate that when hydrophobic bilayers form at the interface, there is faster and better extraction. Further, selectivity during extraction from mixtures is enhanced when there is less extractant than needed for complete extraction of all the ions present.