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 ord er to investigate the kinetics and mechanisms of metal ion complexatio n/extraction by hydrophobic chelating extractants. The kinetics of com plexation of copper with a series of 2-hydroxy-5-alkylbenzophenone oxi mes of different hydrophobicities (the alkyl chains were methyl, tert- butyl and tert-octyl) in cationic micelles have been studied. The abso rbance change recorded for the absorption band of the complexes follow ed first-order kinetics in only a few cases and only when the pH was b uffered. In most cases the kinetic curves were best fitted with a biex ponential function, with a slow absorbance change following faster com plex formation. The influence of different parameters has been conside red, i.e. surfactant concentration, copper concentration, pH, effect o f added oc-acyloin oxime. The results indicate a very strong partition ing of the three 2-hydroxy-5-alkylbenzophenone oximes in favour of the micellar pseudo-phase, and the apparent rate constants for complex fo rmation decrease in the order methyl > tert-butyl > tert-octyl. The co mparison with salicylaldoxime suggests that the mechanism is interfaci al, whereas a bulk aqueous mechanism can take into account the data re lative to salicylaldoxime itself. Possible origins of the slow relaxat ion process have been discussed. The trends observed in the change of the reaction rate with different parameters are compared with the beha viour previously reported for the rate of extraction in biphasic tolue ne-water systems using the ascending-drop method.