Surveying Phase\nModifier Functional Groups for Applications\nto Ln(III) Separations

The\napplication of <i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′-tetraoctyl diglycolamide\n(TODGA) in solvent extraction systems for lanthanide (Ln) separations\nis well understood. In these systems, the formation of a third phase\nhas motivated the use of phase modifiers to enable higher concentrations\nof H<sup>+</sup> and Ln common to industrial processes. Several different\nphase modifiers with applications to diglycolamide (DGA) systems have\npreviously been reported, with a focus on tri-<i>n</i>-butyl\nphosphate (TBP), <i>N</i>,<i>N</i>′-dihexylactanamide\n(DHOA), <i>N,N</i>-dioctyl-2-hydroxyacetamide (DOHyA), <i>N</i>,<i>N</i>′-dimethyl-<i>N</i>,<i>N</i>′-dioctylhexylethoxy malonamide (DMDOHEMA),\nand octanol. While the primary utility of phase modifiers is the increased\nmetal loading, they can have significant effects on the metal distribution\nratios, which are well described by the energetics of the extraction\nprocess itself. However, the mechanisms by which phase modifiers impact\ndistribution ratios are not generally understood. This work considers\nthe ability of phase modifiers to affect Ln distribution ratios by\nusing phase modifiers with two different functional groups (−Cl\nand −CN) and an octyl alkyl chain in a TODGA and <i>n</i>-dodecane system. Determining the effect of chlorooctane\nand octane nitrile is important for understanding how phase modifier\nfunctional groups and their hydrogen bonding interactions affect Ln\nextraction. Through combining distribution ratio measurements with\norganic phase spectroscopic investigations, the impact of chlorooctane\nand octane nitrile on Ln extraction and their inner-sphere complexes\nis reported. The addition of either chlorooctane or octane nitrile\nto TODGA in <i>n</i>-dodecane decreases Ln extraction while\nmaintaining the same inner-sphere Ln complex. The lack of change in\ninner-sphere Ln-TODGA coordination upon incorporation of phase modifiers\nand the significant impact of these phase modifiers on distribution\nratios suggest the importance of a supramolecular structure. Understanding\nthe role of chlorooctane and octane nitrile on the organic phase structure\nat longer length scales has been identified as an avenue for future\ninvestigations.