Eu(III) Solvation and Complexation in Mixed Solvent Systems
Felmy, Heather Michelle
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Luminescence spectroscopy and thermodynamic techniques were used to determine the solvation environment and reaction thermodynamic driving forces for Eu(III) complexation with 2-hydroxyisobutyric acid (HIBA) and 2-aminoisobutyric acid (AIBA) in binary mixtures of 0, 10, 30, and 50% (v/v) methanol (MeOH)-water and N,N-dimethylformamide (DMF)-water. The goal of this project was to understand the role of solvation on the behavior of the trivalent f-elements in mixed solvents. This research has applications to the nuclear fuel cycle where the separation of the trivalent lanthanides (Ln(III)) and actinides (An(III)) is a vital step in reprocessing spent nuclear fuel. The removal of the long-lived minor actinides is also important in the reduction in volume of highly radioactive waste. Separation schemes have been developed around the slight difference in hydration between Ln(III) and An(III) complexes in solvent extraction systems. Mixed aqueous-organic solvents can alter the solvation environment of the trivalent f-elements and an ability to predict these changes will provide important information in the design of new separation systems. The highly charged trivalent f-elements impose significant order in polar solvents such as water and are known to form ordered solvation spheres around the cations in solution. These solvation spheres are disrupted when the metal cations are complexed by organic ligands, resulting in an increase in entropy. This is often the driving force for these complexation reactions. Potentiometry and calorimetry were used to determine how the thermodynamic driving forces for Eu(III) complex formation changed as a function of solvent composition. Luminescence lifetime measurements were used as a direct probe of the solvation environment immediately surrounding the metal cation in solution. With this technique, the composition of the inner solvation sphere of Eu(III) was determined as a function of varying solvent composition as well as complex formation. These solvation changes can be related to the thermodynamic driving forces for complexation, especially reflected in changes in entropy. This research will help to gain a better understanding of how mixed solvents affect the solvation of ions in solution well as thermodynamic driving forces for complex formation, which will add to the understanding of the chemistry of the trivalent f-elements.