Micellar extraction of europium (III) by a bolaform extractant and parent compounds derived from 5-pyrazolone

Micellar ultrafiltration is used to determine the extraction of europium (III) by a series of micelle-solubilized extractants derived from 5-pyrazolone. The extractants, solubilized in CTAB (cetyltrimethylammonium bromide) micelles, have either a single complexing site: 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone (HPMBP), 1-phenyl-3-methyl-4-lauroyl-5-pyrazolone (HPMLP) or they are bis-ligands which can be called ‘bolaform’ extractants: 1,6-bis(1′-phenyl-3′-methyl-5′-pyrazolone-4′-)-1,6-hexanedione (HP-4-PH), 1,12-bis(1′-phenyl-3′-methyl-5′-pyrazolone-4′-)-1,12-dodecanedione (HP-10-PH). The yield of extraction is measured as a function of the pH and of the extractant/metal ratio. The efficiencies of the extractants are in the order HP-10-PH>HPMLP>HPMBP and the complex stabilities are larger than in standard solvent extraction, the former ligand giving a yield of europium extraction close to 100% at pH 2. A theoretical model is used to determine the stoichiometries of the extracted complexes, which are then compared with the situations encountered in standard solvent extraction. For the single site complexing agents the complexes formed involved three ligand molecules (L) for one metal. For the bolaform extractants the complex stoichiometries are found to be either EuL2H (case of HP-4-PH) or Eu2L3 (case of HP-10-PH). The stoichiometries and stabilities of the complexes are discussed in relation with the confinement effect offered by the micellar particles and the effect of the local concentrations. The association of a CTA+ surfactant molecule to the complex, which was suggested by previous works to ensure neutrality, appears here to be unlikely.