Kinetic studies of metal ion complexation in glycerol-in-oil microemulsions

Nanodroplet dispersions containing polar solvents other than water have potential as a novel reaction medium for controlled synthesis. The kinetics of two model reactions, between divalent metal cations (M2+) and either the hydrophilic tridentate ligand murexide (Mu(-)) or the bidentate ligand pyridine-2-azo-p-dimethylaniline (PADA) have been studied in Aerosol-OT (AOT)- or cetyltrimethylammonium bromide (CTAB)-stabilized, nanometer-sized glycerol droplets dispersed in an organic medium. The reactions are readily studied, because the viscosity of the reaction medium is determined by the low-viscosity organic solvent rather than by glycerol. The model systems are extremely well behaved, and useful insights into the significant kinetic factors that control reactivity in these systems are obtained. It is found that the rate of the reaction is very dependent on the metal ion, with Zn2+ > Co2+ > Ni2+. However, reactions are slower in dispersed glycerol as compared with dispersed water. The charge on the surfactant can significantly affect the rate of the forward (complex formation) reaction; reactions in glycerol nanodroplets stabilized by CTAB are considerably faster than those in glycerol nanodroplets stabilized by AOT. This is due to partitioning effects of the reactants after they have located into the same droplet. The determination of activation parameters confirms that diffusion is not determining the rates of complexation.