On the nanoparticle synthesis in microemulsions: detailed characterization of an applied reaction mixture

In this study we present well-characterized non-ionic (w/o)-microemulsions as suitable reaction media for the synthesis of nanoparticles. For oil-rich three component systems of water, cyclohexane and Marlipal surfactants, we determined the phase behavior, the viscosity, the droplet size and the interdroplet exchange rates. The general influence of the particular microemulsion properties on the mechanism of nanoparticle formation is discussed on the basis of the LaMer model for particle formation. As the nucleation, particle growth and aggregation are controlled by the droplet exchange process, all these steps are retarded in comparison to a homogeneous solution. This retardation causes a particle growth that is not accompanied by aggregation of nuclei. Therefore, a higher number of nuclei is maintained during the precipitation that grows to smaller particles than have been expected for a precipitation in a homogeneous solution. Therefore, we assume that for all precipitation reactions, where the growth step is diffusion controlled and aggregation of nuclei reduces the number of particles, the application of a microemulsion as precipitation medium yields much smaller particles than obtained from homogeneous solutions.