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 complex [Pd(Et4dien)(H2O)]2+, where Et4dien = Et2N(CH2)2NH(CH2)2NEt2, by thiourea, methylthiourea, and ethylthiourea have been studied at 25.0 °C in heptane−AOT−water microemulsion over a wide range of the molar ratio R (=[H2O]/[AOT]) at the constant surfactant concentration of 0.13 mol dm-3. The reaction rates are significantly higher in microemulsions than in bulk water and decrease rapidly as the parameter R increases. The kinetic data, interpreted quantitatively by applying the pseudophase model to the microemulsion, indicate that there is a weak partitioning of the nucleophiles between the water core and the AOT interface, while the palladium complex is strongly associated with the negatively charged surfactant interface. All the substitution reactions take place only in the AOT interfacial region with rates very close to those in bulk water. The estimated partition coefficients of the nucleophiles increase with increasing their hydrophobic character. These data are discussed and compared with those previously obtained in the presence of cationic and anionic micelles.