CHEMICAL-REACTIVITY IN AOT MICROEMULSIONS - KINETICS OF WATER REPLACEMENT IN A SQUARE-PLANAR PALLADIUM(II) AQUO COMPLEX BY MONOALKYLTHIOUREAS

The kinetics of water replacement in the cationic palladium(II) aquo c omplex [Pd(Et(4)dien)(H2O)](2+), where Et(4)dien = Et2N(CH2)(2)NH(CH2) (2)NEt2, by thiourea, methylthiourea, and ethylthiourea have been stud ied at 25.0 degrees C in heptane-AOT-water microemulsion over a wide r ange of the molar ratio R (=[H2O]/[AOT]) at the constant surfactant co ncentration of 0.13 mol dm(-3). The reaction rates are significantly h igher in microemulsions than in bulk water and decrease rapidly as the parameter R increases. The kinetic data, interpreted quantitatively b y applying the pseudophase model to the microemulsion, indicate that t here is a weak partitioning of the nucleophiles between the water core and the AOT interface, while the palladium complex is strongly associ ated with the negatively charged surfactant interface. All the substit ution reactions take place only in the AOT interfacial region with rat es very close to those in bulk water. The estimated partition coeffici ents of the nucleophiles increase with increasing their hydrophobic ch aracter. These data are discussed and compared with those previously o btained in the presence of cationic and anionic micelles.