Harnessing Outer-Sphere Hydrogen Bonding Interactions for Enhancing Ln(III) Selectivity with Alcohol Phase Modifiers

In recent decades, the extractant N,N,N',N'-tetraoctyldiglycolamide (TODGA) has garnered significant attention for its strong selectivity across the lanthanide (Ln) series. This selectivity has been connected to outer-sphere interactions, particularly hydrogen bonding between TODGA, outer-sphere counteranions, and coextracted acid and water molecules. The addition of phase modifiers to the organic phase that help avoid phase splitting, such as linear alcohols, can also influence the Ln distribution ratios. However, the mechanism by which alcohols affect Ln extraction and their impact on Ln selectivity remain poorly understood. This study investigates the extraction of Ln by TODGA in mixtures of alcohol and n-dodecane to explore how the alkyl structure of the alcohol influences Ln extraction and speciation in both HNO₃ and HCl systems. Our findings demonstrate that incorporating alcohols into n-dodecane organic phases can significantly enhance Ln selectivity, with an order-of-magnitude increase in separation factors for many systems. Using multiple spectroscopic techniques, such as ultraviolet-visible (UV-Vis)-NIR, time-resolved fluorescence spectroscopy (TRFS), extended X-ray absorption fine structure (EXAFS), and NMR, we confirm that alcohols do not alter the inner-sphere coordination of Ln, preserving the 1:3 [Ln(TODGA)₃]³⁺ complex, but instead associate in the outer sphere. This demonstrates a means beyond inner-sphere coordination to control and significantly enhance Ln selectivity with phase modifiers.