Water Based System
Emulsions are a class of disperse systems consisting of two immiscible liquids. The liquid droplets (the disperse phase) are dispersed in a liquid medium (the continuous phase). Several classes may be distinguished: oil-in-water (O/W), water-in-oil (W/O) and oil-in-oil (O/O).
The latter class may be exemplified by an emulsion consisting of a polar oil ( e .g. propylene glycol) dispersed in a non-polar oil (paraffinic oil), and vice versa.
To disperse two immiscible liquids one needs a third component, namely the emulsifier. The choice of the emulsifier is crucial in formati on of the emulsion and its long term stability.
Emulsions and dispersions
O/W and W/O macro/emulsions: they usually have a size range of 0.1-5 μm with an average of 1-2 μm. These systems are generally opaque or milky due to the large size of the droplets and the significant difference in the refractive index between the oil and water phases.
Nano-emulsions: usually have a range of sizes 20-100 nm. Like macroemulsions, they are only kinetically stable. They can be transparent, translucent or opaque, depending on the size of the drop, the difference in the refractive index between the two phases and the volume fraction of the dispersed phase.
Double and multiple emulsions: these are emulsions of emulsions, W/ O/W and O/W/O systems. They are usually prepared using a two-step process. For example, a multiple W/O/W emulsion is prepared by forming a W/O emulsion, which is then emulsified in water to form the final multiple emulsion.
Mixed emulsions: these are systems composed of two dispersed droplets that do not mix in a continuous medium.
Micellar emulsions: they usually have sizes ranging from 5 to 50 nm. They are thermodynamically stable and strictly speaking should not be described as emulsions. A better description is “swollen micelles” or “micellar systems”.