Reverse-Micelle Formation in the Partitioning of Trivalent f-Element Cations by Biphasic Systems Containing a Tetraalkyldiglycolamide

• Published in: Langmuir Vol. 23; no. 9; pp. 4765 - 4774
• Main Authors: Jensen, Mark P, Yaita, Tsuyoshi, Chiarizia, Renato
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 24-04-2007
• Subjects: Cations - chemistry; Chemistry; Colloidal state and disperse state; Exact sciences and technology; General and physical chemistry; Glycolates - chemistry; Hydrochloric Acid - chemistry; Micelles; Micelles. Thin films; Molecular Structure; Neodymium - chemistry; Neutron Diffraction; Nitric Acid - chemistry; Physical and chemical studies. Granulometry. Electrokinetic phenomena; Scattering, Small Angle; Solutions - chemistry; Water - chemistry; Diluent; Attraction; Lanthanide; Hydrocarbon; Vapor pressure; Acidity; Nitric acid; Hydrochloric acid; Biphasic system; Acids; Small angle neutron scattering; Alkane; Affinity; Trivalent cation; Dimer; Extraction; Models; Osmometry; Polydispersed particle; Aqueous solution; Thermodynamic properties; Reverse micelle; Monomer
• ISSN: 0743-7463; 1520-5827
• DOI: 10.1021/la0631926

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.