New trends in double emulsions for controlled release

Double emulsions have significant potential in many applications since, at least in theory, they can serve as an entrapping reservoir for active ingredients that can be released by a controlled and sustained transport mechanism. Many of the potential applications are in pharmaceuticals, cosmetics and food. In practice, double emulsions are thermodynamically unstable systems with a strong tendency for coalescence, flocculation and creaming. During the last decade much work has been carried out in order to improve the stability and to control the release rates from double emulsions. The review will mention some of the more interesting studies making use of almost any possible combination and blend of monomeric emulsifiers, oils and stabilizers, polymerizable emulsifiers, macromolecular surfactants both natural occurring and synthetic, increase viscosity of each of the phases, microspheres and microemulsions in the internal emulsions, etc. The presentation will stress also the most recent achievements in this area including: (1) the use of specially tailor-made polymeric emulsifiers to improve interface coverage and to better anchor into the dispersed phases; (2) droplets' size reduction by forming microemulsions or vesicles in the internal phase; (3) an improvement in the understanding of the release mechanisms; (4) the use of different filtration techniques in order to improve the monodispersibility of the droplets; and (5) use of various additives (carriers, complexing agents) to control the release via the reverse micellar mechanism.