(33a) Interfacial Phenomena in Miniemulsions and Applications

Miniemulsions provide an alternate route to preparing a wide array of polymer colloids (latexes) for applications ranging from large volume usage in paints, adhesives and coatings to small volume, high value products such as in medical diagnostics and therapeutics, and drug delivery systems. They are particularly useful for preparing materials that cannot be made by conventional emulsion polymerization processes. Miniemulsions differ from conventional emulsions in their droplet size (50 ? 500 nm vs. 1 ? 10 microns) and their increased stability brought about by the addition of a costabilizer, a low molecular weight and low water solubility compound. Two types of costabilizer, namely a fatty alcohol (cetyl alcohol) or an alkane (hexadecane), have been successfully used in preparing miniemulsions. The former system acts in a complex manner to bring about stability against both diffusional degradation (Ostwald ripening) and aggregation by organizing with the surfactant (sodium dodecyl sulfate) primarily at the oil/water interface and much effort has been made to characterize this interfacial behavior. The latter acts primarily to retard Ostwald ripening owing to its presence within the droplets. Polymerization occurs by direct entry of free radicals into the small monomer droplets and efforts to explain the polymerization kinetics both without (reduced rate) and with a small amount of added polymer in the droplets (increased rate referred to as enhanced nucleation) have been made. Further fundamental efforts are underway to better understand the mechanism of droplet formation; adsorption at the interface is the prime suspect being investigated in terms of the rate of surfactant adsorption, desorption, and energetics. Applications of miniemulsion technology are being advanced in many areas to prepare unconventional polymer-based colloids such as synthetic and hybrid latexes, high solids latexes, polymerization of highly water-insoluble monomers and macromonomers, controlled molecular weight via living free radical polymerization, controlled polymer microstructure and morphology, and encapsulation and entrapment of inorganic particles, metal particles, dyes, drugs, and perfumes. Our studies have shown that interfacial behaviors are key to the success of the encapsulation process of nanoparticles via the miniemulsion process.