Complexation of Actinyl Ions with DHOA and TBP in an Ionic Liquid and a Molecular Solvent: How Similar or Different Are They?

Complexation thermodynamics of uranyl ions with well-known reprocessing ligands like tributyl phosphate (TBP) and dihexyl octanamide (DHOA) was studied in an ionic liquid (IL) versus a molecular solvent. Whereas 1-butyl-3-methylimidazolium bis(trifluoromethyl sulfonyl)imide (Bumim·TfN) was used as an IL due to its favorable viscosity, acetonitrile was the choice of molecular solvent due to its poor coordinating nature. Optical spectroscopy studies revealed that UO ions formed species of the types ML and ML with both TBP and DHOA, in a stepwise manner. The formation constants (log β) of UO with DHOA in the IL were determined as 3.58 ± 0.03 and 6.54 ± 0.10, while those with TBP were obtained as 3.42 ± 0.05 and 6.38 ± 0.08, respectively. In the case of acetonitrile, on the other hand, these β values were significantly lower than those observed in the IL medium, which was supported by the DFT calculations. Calorimetric titrations of the UO ions with DHOA and TBP confirmed that the complex formation reactions were thermodynamically more favored in the ionic liquid medium than the molecular solvent. The nature of binding through FTIR investigations and DFT calculations suggested that the complexes formed in the IL medium were cationic in nature of the types [UO(HO)(TBP)] and [UO(HO)(DHOA)], but neutral complexes were formed in the molecular solvent of the types [UO(NO)(TBP)] and [UO(NO)(DHOA)].