Ion-Specific Clustering of Metal-Amphiphile Complexes in Rare Earth Separations

Aggregation and clustering of metal-amphiphile complexes formed during solvent extraction of lanthanides have been studied with small angle X-ray scattering. The nanoscale structure of the complex fluid strongly depends on the counter-ion (NO3- or SCN-) and the lanthanide being extracted. As a result, it is possible to selectively transport light or heavy lanthanides from the aqueous phase into the organic phase by simply choosing NO3- or SCN- as the background anion, respectively. While the organic phase containing TOMA-NO3 always shows clustering, indicating the presence of stronger attractive interactions between metal-amphiphile aggregates, TOMA-SCN shows clustering as a function of the metal loading. These qualitative differences suggest that the extraction efficiency is driven by the aqueous phase conditions in NO3- solutions, while it is driven by the organic phase structuring in SCN- solutions. A clustering model, that accounts for the hard sphere repulsions and short-range attractions between the aggregates, has been developed to model the X-ray scattering results. The new model successfully describes the nanoscale structure and helps understanding the mechanisms responsible for amphiphile assisted ion transport and complexation between immiscible liquids.