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

Abstract

<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 developed 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>

Origins of Clustering of Metalate–Extractant Complexes in Liquid–Liquid Extraction

“…Figure 3a Solutions of TOMA.Cl in toluene that are not contacted with any aqueous phase show aggregation. 59 SAXS of these solutions show that the aggregates behave like hard-spheres with repulsion-dominant interactions between the spheres. 59 The organic phase obtained after LLE of metalate-free aqueous phase shows significant structuring due to aggregation of extractants (Figure 3a, blue circles).…”

“…Extraction of metalates with Aliquat 336 proceeds with co-extraction of water and acid. 53,[55][56][57] In accordance with the literature, 31,50,[58][59] we define the divalent chloro complexes of platinum and palladium as metalates. The species formed by metalate and two extractant molecules (required for charge balance) are termed metalate-extractant complexes.…”

Origins of Clustering of Metalate-Extractant Complexes in Liquid-Liquid Extraction

“…Solutions of TOMA.Cl in toluene that are not contacted with any aqueous phase show aggregation. 59 SAXS of these solutions show that the aggregates behave like hard-spheres with repulsion-dominant interactions between the spheres. 59 The organic phase obtained after LLE of metalate-free aqueous phase shows significant structuring due to aggregation of extractants (Figure 3a, blue circles).…”

“…This increased low-Q scattering is an indication of increasing attractive interactions between the aggregates. 27,59 The differences between the uncontacted and the contacted metalate-free organic phases are likely due to the differences in water and acid content in the organic phases. 63 When metalates are extracted, there is no appreciable change in the SAXS pattern with PdCl 4 2until the aqueous phase concentration is above 0.01 M (Figure 3a).…”

“…53,82 The atypical selectivity of TOMA-NO 3 towards light lanthanides is hypothesized to be due to the hydration properties of the lanthanides. 59,[83][84] The major difference in the effect of chlorometalates on the organic phase structure is the higher clustering of aggregates in the case of PtCl 6 2compared to that of PdCl 4 2-(Figure 3). Given the similar metal content but different water content, these differences can be due to different geometries of PtCl 6 2and PdCl 4 2and different hydration structures in the reverse micellar aggregates.…”