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The conditions for reverse-micelle formation were studied for solutions of tetra-n-octyldiglycolamide (TODGA) in alkane diluents equilibrated with aqueous solutions of nitric or hydrochloric acids in the presence and absence of Nd3+. Small-angle neutron scattering, vapor-pressure osmometry, and tensiometry are all consistent with the partial formation of TODGA dimers at the lowest acidities, transitioning to a polydisperse mixture containing TODGA monomers, dimers, and small reverse-micelles of TODGA tetramers at aqueous nitric acid acidities of 0.7 M or higher in the absence of Nd. Application of the Baxter model to the samples containing 0.005-0.015 M Nd reveals the persistence of tetrameric TODGA reverse-micelles with significant interparticle attraction between the polar cores of the micelles that increases with increasing organic phase concentrations of acid or Nd. Our experimental findings suggest that the peculiar behavior of TODGA with respect to the extraction of trivalent lanthanide and actinide cations arises from the affinity of these metal cations for the preformed TODGA reverse-micelle tetramers.

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This paper provides a comprehensive review of synergistic solvent extraction using crown ethers with a focus on the role of both extractants in facilitating cation-specific separations. An introduction to the various equilibria affecting synergistic extraction using crown ethers is followed by a review of the work published in this field during the 1972-1999 time period. The influence of various solvent extraction parameters on the potential for cation-selective synergism is critically examined. Those synergistic extractant combinations showing cation selectivity are highlighted, as are the fundamental investigations that are the foundation of the current understanding of synergistic solvent extraction using crown ethers.

Small-angle neutron scattering (SANS) data for the tri-n-butyl phosphate (TBP)−n-dodecane, HNO3−UO2(NO3)2 solvent extraction system have been interpreted using the Baxter model for hard spheres with surface adhesion. The increase in the scattering intensity in the low Q range observed when increasing amounts of HNO3 or UO2(NO3)2 are transferred into the organic phase has been interpreted as arising from interactions between solute particles. The SANS data have been reproduced using a 12−16 Å diameter of the hard sphere, d hs, and a 5.6k B T−7.1k B T stickiness parameter, τ-1. When in contact with an aqueous phase, TBP in n-dodecane forms small reverse micelles containing three TBP molecules. Upon extraction of water, HNO3, and UO2(NO3)2, the swollen micelles interact through attractive forces between their polar cores with a potential energy of about 2k B T and an effective Hamaker constant of about 4k B T. The intermicellar attraction, under suitable conditions, leads to third-phase formation. Upon phase splitting, most of the solutes in the original organic phase (water, TBP, HNO3, and UO2(NO3)2) separate in a continuous phase containing interspersed layers of n-dodecane.

Small-angle neutron scattering (SANS) data for the tri-n-butyl phosphate (TBP)-n-octane, HNO(3)-Zr(NO(3))(4) solvent extraction system, obtained under a variety of experimental conditions, have been interpreted using the Baxter model for hard spheres with surface adhesion. The increase in scattering intensity in the low Q range observed when increasing amounts of Zr(NO(3))(4) were extracted into the organic phase was interpreted as arising from interactions between small reverse micelle-like particles containing two to three TBP molecules. Upon extraction of Zr(NO(3))(4), the particles interact through attractive forces between their polar cores with a potential energy that exceeds 2 k(B)T. The interparticle attraction, under suitable conditions, leads to third phase formation. A linear relationship exists between the derivative of the potential energy of attraction with respect to the concentration of nitrate ions in the organic phase and the ionization potential or the hydration enthalpy of the extracted metal cations.