Rate effects of AOT-stabilized microemulsions on reactions of ligand substitution in cationic palladium(II) complexes

Rate data for the substitution reactions of the coordinated ligand X (=2,2'-bipyridine or 4,4'-dimethyl-2,2'-bipyridine) of the palladium(II) complex [Pd(en)X](2+), where en = ethylenediamine, by en or N,N-dimethylethylenediamine in heptane-AOT-water microemulsions have been obtained at 25.0 degrees C as a function of the AOT concentration at the constant R (=[H2O]/[AOT]) values of 3, 8, and 20 or 30. The overall second-order rate constants are higher in microemulsions than in bulk water and decrease significantly as both the AOT concentration (at constant R) and the molar ratio R (at a given [AOT]) increase. The quantitative analysis of the kinetic data, made by applying the pseudophase model, leads to the suggestion that a given palladium complex bound to the negatively charged AOT/water interface reacts simultaneously with the nucleophile partitioned between the aqueous and the interfacial phases. Evidence is obtained that the same rate effects of the incoming and leaving ligands are operative in both bulk water and the AOT interface and that the substitution reactions proceed by the same rates and mechanism in the two different media. The estimated "effective" AOT pseudophase volume (per mole of surfactant) in which reaction occurs has been found to increase as the interfacial AOT layer curvature becomes larger.