Complexation thermodynamics of UO₂²⁺/diglycolamide complex in a room temperature ionic liquid: A study by optical spectroscopy and microcalorimetry

Complexation thermodynamics of UO₂²⁺ and a series of N,N,N’,N’-tetraalkyl diglycolamide (TRDGA) ligands having alkyl chains –CH₃ to –C₆H₁₃ were investigated in an ionic liquid (IL). The complexation constant for UO₂²⁺/DGA complex increased linearly with increasing alkyl chain length of DGA from tetramethyl (logβ₁ = 4.52, logβ₂ = 8.01) to tetrahexyl (logβ₁ = 5.62, logβ₂ = 10.3). There were two types of species recorded in the calorimetric titration for all the four ligands, one having endothermic complexation, and other exothermic. This observation was in contrast with those observed for aqueous medium, where both the stepwise complexation enthalpies were endothermic. The structural feature indicated the formation of [UO₂L₂]²⁺ type of complex without water or Tf₂N⁻ anion in the primary coordination sphere of the uranyl cation.