Composition-resolved structure of the complex interfacial region in liquid-liquid extraction using combined X-ray and neutron reflectometry

The liquid-liquid interface is expected to govern ion complexation, extractant self-assembly, and species transfer during solvent extraction. We hypothesize that this interface is not a passive dividing surface but an active, compositionally distinct coordination environment, and that combining complementary X-ray and neutron reflectivity can quantitatively resolve its molecular organization under realistic, strongly absorbing, high-loading extraction conditions.

We combine high-energy X-ray reflectometry and neutron reflectometry to quantitatively reconstruct the composition profiles of a buried liquid-liquid interface under realistic uranyl extraction conditions, a cation of interest in nuclear waste recycling and actinide recovery. Dedicated sample cells and reflectivity analysis accounting for interfacial concentration gradients enabled nanometer-resolution determination of interfacial structure evolution. XRR probed the electron-density profile upon uranyl loading, while NR provided complementary sensitivity to the protonated extractant and deuterated solvent. Both datasets were jointly refined to constrain the interfacial composition.

Quantitative concentration profiles of all species are reconstructed across the interface. DEHiBA is enriched within a 3.5 nm interfacial zone, with a surface excess of 1.3 molecules nm^-2 in the uranyl-free system, decreasing to 1 molecules nm^-2 upon uranyl loading. Uranyl nitrate co-localizes with DEHiBA, with UO₂(NO₃)₂-to-DEHiBA molar ratio of 0.75. The uranyl species shows an apparent free-energy minimum of -3 k_BT relative to the aqueous bulk reference, governing the transfer-process dynamics. These profiles provide molecular-level insight into extractant organization at buried liquid-liquid interfaces under high metal loading.