Nonionic microemulsions: Effects of the interface on metal-ligand reactions

A kinetic study was carried out into the complexation of Ni²⁺ with the bidentate ligand pyridine-2-azo-p-(N,N-dimethylaniline) (PADA) in Brij30/isooctane/water microemulsions at 25 °C. The results obtained show that the apparent rate constants for the formation of the Ni²⁺-PADA complex decreases as the surfactant concentration and the water content of the system increase. The apparent rate constants are in all cases greater than those observed in an aqueous medium. To interpret the kinetic results a model has been proposed which takes into account the heterogeneity of the system at the microscopic scale and allows us to determine the distribution constants of both reactants and the rate constant at the interface. The balance between two opposite effects, the more efficient hydration of the nickel hexahydrate coordination complex and the lack of hydration, determine the value of the rate constant.