Edited by Pichet Adstamongkonkul
A double emulsion is simply defined as an emulsion in an emulsion. An emulsion is a dispersed, multiphase system consisting of at least two immiscible liquids. The liquid that forms droplets is called dispersed phase, while the liquid in the bulk surrounding the droplets is called continuous phase. The dispersion is usually not stable and phase separation will eventually occur at long time scale. Emulsions are considered as parts of a more general class called colloids, although sometimes the two terms are interchangeably used.
The double emulsion can be viewed as a system, in which two liquids are separated by a third liquid which is not miscible with the first two liquids. In the case of water and oil, there are two possible cases of double emulsions: water-in-oil-in-water (w/o/w) emulsion and oil-in-water-in-oil (o/w/o) emulsion. In the former case, for example, each dispersed droplets of water forms a vesicular structure with single or multiple aqueous compartments separated forms the continuous aqueous phase by a layer of oil phase. In most cases, the double emulsion droplets are very polydispersed, meaning that there are a wide range of sizes and shapes of these droplets.
Multiple emulsions have been prepared in two main processes: one-step emulsification and two-step emulsification, involving two sets of emulsifiers (hydrophobic emulsifier is designed to stabilize the interface of w/o internal emulsion; hydrophilic emulsifier is for the external interface of o/w emulsion). The most spontaneous formation is the w/o emulsion. In the one-step emulsification, the w/o emulsions would be formed first and, with the right balance between hydrophilic-lipophilic emulsifiers and heat, could be inverted, forming multiple emulsions. However, there is less control over the whole process.
An alternative would be the two-step emulsification. The primary w/o emulsions are formed under high shear conditions, typically ultrasonification or homogenization. The secondary emulsification, on the other hand, is normally carried out without the shear, which would disrupt the already-formed primary emulsions. The composition of the double emulsion is crucial, since the different surfactants, the properties and concentration of the oil phase would affect the stability of these emulsions. It was well-established that the inner hydrophobic emulsifiers must be used in excess (about 10-30% by weight) while the hydrophilic emulsifiers are used in low concentration (0.5-5% by weight) in order to maintain stability.
Multiple emulsions were first described by Seifriz in 1925 and since then, more studies have been conducted on them and they have been exploited in many applications, specifically in food sciences, cosmetics, pharmacology, and polymer sciences. The potential pharmaceutical applications include adjuvant vaccines, controlled drug delivery systems, drug reservoirs, taste masking, and immobilization of enzymes.