Recent developments in miniemulsions - Formation and stability mechanisms

Polystyrene latexes in a size range of 30 nm to 180 nm can be synthesized b y polymerization in miniemulsions. Since size equilibrium seems to be estab lished by a rate equation of collision induced droplet fusion and ultrasoun d fission, miniemulsions realize the minimal particle size for the distinct amount of surfactant, i.e. they are "critically stabilized" with respect t o collisions (tau(2) -process). Complete stability against Ostwald ripening (tau(1) -process) is obtained by the addition of a hydrophobe, which was v aried over a broad structural range. The efficiency of this hydrophobe agen t is mainly given by its very low water solubility. The growth of the criti cally stabilized miniemulsion droplets is usually slower than the polymeriz ation time, and a 1:1 copy of the particles is obtained, the critically-sta bilized state is frozen. The critical surface coverage of these particles w ith SDS molecules was determined and depends strongly on the particle size: the smaller the particle size, the higher the surface coverage with surfac tant is required in order to realize the critically-stable state.