Lanthanide-Dependent Self-Assembly of Ion-Extractant Complexes at Liquid Interfaces: Inversion or Exclusion?

The separation of lanthanide elements by solvent extraction is traditionally governed by differences in the thermodynamic stability of extracted complexes; however, ion-specific interfacial complexation can play an important role in determining selectivity in kinetic separations. Nevertheless, the structural and chemical origins of such interfacial selectivity remain poorly understood. In this work, we combine sum frequency generation (SFG) vibrational spectroscopy with X-ray scattering to resolve interfacial structures of extractant-ion complexes at liquid surfaces. Measurements across pH and lanthanide concentration regimes reveal how changes in aqueous conditions alter the molecular organization of interfacial bis(2-ethylhexyl) phosphoric acid (HDEHP)-lanthanide ion complexes. Across the lanthanide series, light lanthanide ions, e.g., those from La to Dy, tend to form relatively ordered monolayers with HDEHP as characterized by strong CH and weak OH vibrational signals with corresponding electron density profiles and surface ion concentrations. In contrast, heavy lanthanide ions, e.g., those from Er to Lu, tend to produce more disordered interfacial assemblies as reflected by weaker CH stretches, stronger OH signals that are accompanied by surprisingly low lanthanide ion surface densities. These systematic, structural, and chemical differences suggest that HDEHP-heavy lanthanide complexes are more readily organized into increasingly centrosymmetric structures that are poised for transport, explaining their apparent absence at liquid-liquid interfaces at equilibrium. These results establish interfacial organization as a key factor governing lanthanide extraction chemistry and point to kinetic handles to impart unconventional selectivity.