ON THE PROBLEM OF REACTION SITE IN BIPHASIC EXTRACTION OF METAL-IONS - NMR VERSUS SPECTROPHOTOMETRIC EVIDENCE IN MICELLAR MODEL SYSTEMS

A much debated question in solvent extraction is that of the locale of the metal-extractant reaction: in the homogeneous aqueous phase or the organic/aqueous interface? Due to the existence of microscopic interfaces, micellar solutions and microemulsions can serve as model systems to try answering this question. Taking advantage of the very slow rate of complexation between an alkylated derivative of 8-hydroxyquinoline and Ni2+ in CTAB/butanol micelles, we were able to make use of NMR as well as UV-visible spectrophotometry for kinetic investigations. Whereas UV-visible spectroscopy directly monitors complex formation, NMR spectroscopy has permitted us to detect the interaction of Ni2+ with the surface of CTAB/butanol micelles in which the extractant is solubilized. Kinetic data obtained from both techniques turn out to be identical, tending to demonstrate that, viewed from the micelles, the reaction takes place at the microscopic interface. Critical examination shows that the answer might not be so simple.