Premium
Kinetic Study of Ni 2+ and Co 2+ Complexation by PADA in AOT‐Based Water‐in‐Oil Microemulsions
Author(s) -
Fernández Esperanza,
GarcíaRío Luis,
Knörl Astrid,
Leis José Ramón
Publication year - 2005
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/ejoc.200400538
Subject(s) - chemistry , microemulsion , reaction rate constant , aqueous solution , dissociation (chemistry) , kinetic energy , inorganic chemistry , analytical chemistry (journal) , pulmonary surfactant , kinetics , organic chemistry , biochemistry , physics , quantum mechanics
A kinetic study was carried out into the complexation of Ni 2+ and Co 2+ 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 Ni 2+ ‐PADA and Co 2+ ‐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 Ni test 2+ test 2+ and the Co 2+ 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, k i 1 , and decomposition, k i –1 , of the complexes for various values of W ( W = [H 2 O]/[AOT]). The results show that k i –1 is independent of the composition of the microemulsion and that k i 1 increases as W decreases. The results also show that the rate constants for the formation of the Co 2+ ‐PADA complex are approximately 50 times greater than those of the Ni 2+ ‐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)