Understanding the effect of alkyl chain on extraction of Np4+ and Pu4+ with hexaalkyl nitrilotriacetamides into an ionic liquid vis-à-vis a molecular solvent

Extraction of actinides using exotic ligands in room temperature ionic liquids is one of the most preferred areas of research these days. Though the amides are well studied for the extraction of actinides from nitric acid feeds, multiple amides (such as tripodal amides) are of recent origin, and reports on solvent extraction with these ligands in room temperature ionic liquids are very rare. In this work, the extraction of Np⁴⁺ and Pu⁴⁺ was studied with a series of N,N,N′,N′,N″,N″-hexaalkyl nitrilotriacetamides (NTAamides) in an ionic liquid with an objective to understand the effect of the alkyl chains length on their extraction ability. Bumim⋅Tf₂N was chosen as ionic liquid due to its favourable viscosity for distribution studies. The extraction of tetravalent actinides Np⁴⁺ and Pu⁴⁺ increased linearly with increasing the alkyl chain of the NTAamides from n-butyl via n-hexyl to n-octyl. The extraction pattern was surprisingly opposite to the most observed extraction patterns in an ionic liquid, where the extraction generally decreases sharply upon increasing the feed acidity. The extracted species were ascertained as M(NO₃)₅⋅L (LH), where M = Np⁴⁺ or Pu⁴⁺, for each ligand. Consequent to the formation of an entirely divergent extracted complex, the extraction pattern is completely different than that expected in an ionic liquid.