Complexation of Uranium(VI) and Samarium(III) with Oxydiacetic Acid:  Temperature Effect and Coordination Modes

The complexation of uranium(VI) and samarium(III) with oxydiacetate (ODA) in 1.05 mol kg^-1 NaClO₄ is studied at variable temperatures (25−70 °C). Three U(VI)/ODA complexes (UO₂L, UO₂L₂^2-, and UO₂HL₂^-) and three Sm(III)/ODA complexes (SmL_j^(3-2j)+ with j = 1, 2, 3) are identified in this temperature range. The formation constants and the molar enthalpies of complexation are determined by potentiometry and calorimetry. The complexation of uranium(VI) and samarium(III) with oxydiacetate becomes more endothermic at higher temperatures. However, the complexes become stronger due to increasingly more positive entropy of complexation at higher temperatures that exceeds the increase in the enthalpy of complexation. The values of the heat capacity of complexation (ΔC_p° in J K^-1 mol^-1) are 95 ± 6, 297 ± 14, and 162 ± 19 for UO₂L, UO₂L₂^2-, and UO₂HL₂^-, and 142 ± 6, 198 ± 14, and 157 ± 19 for SmL⁺, SmL₂^-, and SmL₃^3-, respectively. The thermodynamic parameters, in conjunction with the structural information from spectroscopy, help to identify the coordination modes in the uranium oxydiacetate complexes. The effect of temperature on the thermodynamics of the complexation is discussed in terms of the electrostatic model and the change in the solvent structure.