Recent solvent extraction investigations have used a N,N,N’,N’-tetraoctyldiglycolamide (TODGA) extractant for lanthanide (Ln) separations as TODGA has been shown to have unique selectivities across the 4f period. Current studies suggest that three TODGA molecules coordinate to Ln with co-extracted water and nitrate molecules located in the outer sphere, forming a complex hydrogen bonding network. At high concentrations of nitric acid in the presence of Lns, TODGA notoriously forms a third phase. Previous work has shown third phase formation can be prevented by adding small volumes of phase modifiers, such as 1-octanol, to the organic phase. The addition of modifiers like 1-octanol has been thought to mitigate third phase formation by solubilizing Ln-TODGA aggregates through interactions with the hydrogen bonding network. This research reveals the effects of varying 1-alcohol alkyl chain length on extraction of Lns, H₂O, and H⁺ in TODGA systems. A multi-faceted approach was used to provide a holistic view of organic phase structure through coupling solvent extraction with both aqueous (ICP-OES) and organic phase techniques (Karl Fischer, FT-IR, and SAXS). Under acidic conditions, 1-alcohols have competing interactions regarding their ability to solubilize aggregates and extract large quantities of H₂O consistent with third phase formation. At low concentrations (5 vol%) of 1-alcohol phase modifiers, shorter alkyl chains suppress aggregation resulting in lower H₂O concentrations. Increasing the concentration of 1-alcohol to 30 vol%, results in the opposite trend with longer alkyl chains suppressing aggregation. The systems that are best at suppressing aggregation also form smaller aggregates. It was found that increasing the ratio of phase modifier per extracted H₂O results in smaller aggregates as the phase modifiers can more effectively solvate H₂O molecules at larger concentrations. Results from this study show that the combination of multiple techniques to investigate organic phase structure is an effective approach to determine trends in extracted outer-sphere molecules and particle size.

Shane Galley

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