Unraveling Hierarchical Structure in Ionic Liquid-Extractant Mixtures with X-ray Scattering

Liquid-liquid extraction (LLE) is the predominant method for the selective recovery and separation of many critical materials, including rare earth elements and platinum group metals. Ionic liquids (ILs) have been proposed to replace traditional solvents in LLE to enhance or manipulate separation performance. The tunable nanostructure of ILs is a unique and defining characteristic that impacts their function, but how that structure is affected by mixing with the LLE extractant is not understood. To investigate this, we combine small-angle X-ray scattering (SAXS) and molecular dynamics (MD) simulations. SAXS profiles computed from the MD simulations are in good agreement with the experimental data. Known pure IL nanostructures persist upon the addition of the malonamide extractant. Decomposition of the simulation structure factors shows extractant distribution between the domains of the IL solvent. In addition, these mixtures also show strong composition fluctuations at low q, indicating inhomogeneity related to phase instability between IL and the extractant. In a trend exactly opposite to that observed in conventional molecular alkane solvents, increasing the alkyl tail length in ILs depresses the fluctuations, suggesting that enhanced IL hydrophobicity promotes mixture stability. These findings suggest that traditional ligand design strategy is, in fact, counterproductive and needs to be reevaluated for LLE processes using ILs. This study provides the basis for understanding the fundamentals of IL-extractant mixtures and will lead to the rational design of both IL and the extractant for more efficient separation processes.