First Report on the Complexation of Uranyl Ion with Two Diglycolamide Ligands in a Room Temperature Ionic Liquid: Optical Spectroscopy and Calorimetric Studies

The complexation of UO22+ with N,N,N ',N '-tetraoctyl diglycolamide (TODGA) and N,N,N ',N '-tetra-2-ethylhexyl diglycolamide (T2EHDGA) was investigated in ionic liquid using UV-vis absorption spectrophotometry and isothermal titration calorimetry. Spectrophotometric titration suggested the formation of two successive complexes, [UO2L](2+) and [UO2L2](2+) where L=TODGA or T2EHDGA. The formation constant values obtained in ionic liquid medium were several orders of magnitude higher than those observed with the analogous DGA ligands in aqueous medium. The stability constant (log beta) for UO22+/TODGA complex was at least an order of magnitude higher than those observed for UO22+/T2EHDGA complex. The speciation of UO22+/DGA complex obtained by spectrophotometric titration was in conformity with those observed in biphasic solvent extraction. Energetically the UO22+/DGA complexation was mainly driven by negative enthalpy and large positive entropy changes as a result of the release of entire water molecules from the solvation sphere of the hydrated UO22+ ions. The results from this work gave an insight understanding of the complexation behaviour of UO22+ ions with DGA ligands in ionic liquid. It is anticipated that such understanding will give valuable information for understanding the extraction behavior of uranyl ions by DGA ligands in ionic liquid.