Role of Alkyl Chain Branching in the Self-Assembly of Nitrilotriacetamide-Based Lanthanide Extractants

Understanding the structural factors governing the metal ion selectivity of solvent extraction systems is crucial for developing advanced processes for the partitioning and the transmutation of high-level radioactive waste. Here, we investigated the effect of alkyl side-chain branching in nitrilotriacetamide (NTAamide) extractants on the extraction of La³⁺ and Nd³⁺, chosen as representative lanthanides (Ln). The four extractants were examined that each had three amide groups, with two alkyl chains containing eight carbon atoms attached to each amide group. Although they have identical molecular weights, they differ in the degree of alkyl chain branching. Distribution ratios were measured as a function of HNO₃ concentration. The local La³⁺ and Nd³⁺ coordination structures were analyzed by extended X-ray absorption fine structure (EXAFS), whereas supramolecular aggregation in the organic phase was characterized by small-angle neutron scattering (SANS). EXAFS analysis revealed that the inner sphere coordination environments of La³⁺ and Nd³⁺ were unaffected by the degree of alkyl branching. In contrast, SANS showed that extractants with fewer branched alkyl groups formed larger aggregates at low HNO₃ concentrations, particularly for Nd³⁺, where aggregation improved extraction and prevented precipitation of poorly soluble complexes. Multiscale structural analysis using EXAFS and SANS revealed that the extraction of HNO₃ alone has little influence on the aggregation behavior of NTAamide extractants. In contrast, differences in alkyl chain branching were found to govern the solvation stability of Ln(III)-NTAamide complexes in the organic phase, resulting in pronounced differences in their aggregation behavior. These findings clearly indicate that alkyl branching strongly affects supramolecular aggregation, which in turn governs extraction behavior. This work highlights the potential of nanoscale structural control as a new design concept for improving selectivity in Ln and actinide solvent extraction systems.