Third-Phase Formation in Diglycolamides Systems: Extraction and Coordination Chemistry, Colloid Chemistry, and Critical Theory

Diglycolamide (DGA) stands out as one of the most viable extractants for actinide partitioning, but the advancement of the associated separation technology has been hampered by third-phase formation, which limits organic-phase loading capacity and threatens process stability. The mechanisms behind the third-phase formation are significantly complex, and involve multiple disciplines and theories spanning molecular coordination, colloidal assembly, and critical phase transition.This review dissects the mechanisms from three perspectives:1、Extraction/Coordination Chemistry: Phase modifiers (e.g. DHOA, TBP) and unsymmetrical DGAs (UDGAs) enhance the Limiting Organic Concentration (LOC), suppressing phase splitting by improving metal complex solubility. UDGAs, with tailored alkyl substituents, offer superior performance over symmetrical analogs. 2、Colloidal Chemistry: Third-phase formation stems from the self-assembly of amphiphilic DGA molecules into reverse micelles or microemulsion networks. Techniques like DLS and SAXS reveal that high metal/acid loading triggers aggregate growth, morphological transition and enhanced inter-aggregate attraction, eventually leading to phase separation. 3、Phase Transition Theory: The phenomenon is a spontaneous critical transition with its scaling behavior aligning with the universality class of the 3D Ising model. Critical fluctuations in the vicinity of the transition point drive macroscopic phase separation, providing a unified physical framework for the phenomenon.The review concludes by advocating for future advancements through multiscale research that integrates advanced in-situ characterization, computational modeling, and rational extractant design to develop more efficient and stable actinide partitioning processes.