Kinetic Study of Ni2+ and Co2+ Complexation by PADA in AOT‐Based Water‐in‐Oil Microemulsions

Abstract

AbstractA kinetic study was carried out into the complexation of Ni2+ and Co2+ with the bidentate ligand pyridine‐2‐azo‐p′‐(N,N‐dimethylaniline) (PADA) in AOT/isooctane/water microemulsions [where AOT = sodium bis(2‐ethylhexyl)sulfosuccinate]. The results obtained show that the macroscopic rate constants for the formation of the Ni2+‐PADA and Co2+‐PADA complexes decrease as the surfactant concentration and the water content of the system increase. The rate constants are in all cases greater than those observed in an aqueous medium, with catalytic effects of up to 25 times. To interpret the kinetic results a model has been proposed which considers the distribution of PADA between the continuous medium and the interface of the microemulsion, and the distribution of the Nitest 2+ test2+ and the Co2+ cations between the aqueous microdroplet and the interface. The formation and dissociation reactions of the complexes take place only at the interface. On the basis of the model we obtained values for the rate constants for the formation, ki1, and decomposition, ki–1, of the complexes for various values of W (W = [H2O]/[AOT]). The results show that ki–1 is independent of the composition of the microemulsion and that ki1 increases as W decreases. The results also show that the rate constants for the formation of the Co2+‐PADA complex are approximately 50 times greater than those of the Ni2+‐PADA complex. These results are compatible with the fact that the reaction follows an Eigen–Wilkins‐type mechanism and that, in the presence of added salt, there is an increase in the rate constant of water exchange as the saline content of the microdroplet increases, that is, as W decreases. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)