<p>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. </p>
Resonant Inelastic X-ray Scattering of Rare-Earth and CopperSystems