Copper(II) complexation kinetics with hydroxyoximes in CTAB micelles - Effect of extractant hydrophobicity and additives

Micellar solutions can be considered as pseudo-biphasic systems in order to investigate the kinetics and mechanisms of metal ion complexation/extraction by hydrophobic chelating extractants. The kinetics of complexation of copper with a series of 2-hydroxy-5-alkylbenzophenone oximes of different hydrophobicities (the alkyl chains were methyl, tert-butyl and tert-octyl) in cationic micelles have been studied. The absorbance change recorded for the absorption band of the complexes followed first-order kinetics in only a few cases and only when the pH was buffered. In most cases the kinetic curves were best fitted with a biexponential function, with a slow absorbance change following faster complex formation. The influence of different parameters has been considered, i.e. surfactant concentration, copper concentration, pH, effect of added oc-acyloin oxime. The results indicate a very strong partitioning of the three 2-hydroxy-5-alkylbenzophenone oximes in favour of the micellar pseudo-phase, and the apparent rate constants for complex formation decrease in the order methyl > tert-butyl > tert-octyl. The comparison with salicylaldoxime suggests that the mechanism is interfacial, whereas a bulk aqueous mechanism can take into account the data relative to salicylaldoxime itself. Possible origins of the slow relaxation process have been discussed. The trends observed in the change of the reaction rate with different parameters are compared with the behaviour previously reported for the rate of extraction in biphasic toluene-water systems using the ascending-drop method.