Deciphering the Third Phase Structure in Uranium Extraction with Aliphatic Amines Using Combined Small-Angle X-ray and Neutron Scattering

Organic phase demixing remains a major issue of liquid-liquid extraction processes due to an incomplete understanding of its underlying mechanisms. In the context of uranium extraction with aliphatic amines diluted in linear alkanes, we investigated this phase separation by characterizing the multiscale structure of the demixed phases. We determined the extractant phase stability threshold, defined as the maximum concentration of extractant before macroscopic phase splitting occurs. Using small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS), we further analyzed the microstructure of the two resulting organic phases. With all aliphatic diluents, the heavier phase usually named as the third phase, consists of weak water-in-oil (w/o) aggregates with interdigitated hydrocarbon chains, while the lighter phase is mainly composed of monomeric amines and of amines aggregates containing water. Even after centrifugation, SANS detects residual deuterated diluent molecules in the third phase. A fraction of these w/o aggregates contains uranium(VI) cations (U-loaded aggregates), which exhibit stronger attractive interactions than unloaded aggregates. Phase demixing is therefore the results of the segregation of U-loaded aggregates into large clusters, detected as a Porod-type decay in SAXS at low <i>q</i>-values.