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

preprint
revised on 05.08.2020 and posted on 06.08.2020 by Srikanth Nayak, Kaitlin Lovering, Ahmet Uysal

The nanoscale structure of a complex fluid can play a major role in the selective adsorption of ions at the nanometric interfaces, which are crucial in industrial and technological applications. Here we study the effect of anions and lanthanide ions on the nanoscale structure of a complex fluid formed by metal-amphiphile complexes, using small angle X-ray scattering. The nano- and mesoscale structures we observe can be directly connected to preferential transfer of light (La, Nd) or heavy (Er, Lu) lanthanides into the complex fluid from an aqueous solution. While the toluene-based complex fluids containing trioctylmethylammonium-nitrate (TOMA-nitrate) always show the same mesoscale hierarchical structure regardless of lanthanide loading and prefer light lanthanides, those containing TOMA-thiocyanate show an evolution of mesoscale structure as a function of the lanthanide loading and prefer heavy lanthanides. The hierarchical structuring indicates the presence of attractive interactions between ion-amphiphile aggregates, causing them to form clusters. A clustering model, that accounts for the hard sphere repulsions and short-range attractions between the aggregates, has been adapted to model the X-ray scattering results. The new model successfully describes the nanoscale structure and helps in understanding the mechanisms responsible for amphiphile assisted ion transport between immiscible liquids. Accordingly, our results imply different mechanisms of lanthanide transport depending on the anion present in the complex fluid and correspond with anion-dependent trends in rare-earth separations.

Funding

U.S. Department of Energy, Office of Basic Energy Science, Division of Chemical Sciences, Geosciences, and Biosciences, under contract DE-AC02-06CH11357.

History

Email Address of Submitting Author

ahmet@anl.gov

Institution

Argonne National Laboratory

Country

United States

ORCID For Submitting Author

0000-0003-3278-5570

Declaration of Conflict of Interest

Authors declare no competing financial interest.

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