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 Emulsions consist of a hydrophobic or hydrophilic substance (internal phase) dispersed as small droplets within a more abundant hydrophobic or hydrophilic substance(sometimes called the external or continuous phase), usually with the addition of an emulsifying agent. They are used to create a mixture of two liquids that are otherwise insoluble. The internal and external phases have opposite solubility characteristics(oil-in-water or water-in-oil). To add volume to the emulsion, it can be diluted with its external phase, or with a substance similar to it.

Emulsions can be semisolids or liquids. Semisolid emulsions are used topically, and liquids can be used topically, orally, or through the parenteral route.

 Water-in-oil emulsions contain a hydrophilic internal phase, with an oleaginous(hydrophobic or fat soluble) external phase. Water-in-oil emulsions tend to disperse more evenly over the skin because of the sebum on the skin's surface which is more easily compatible with oil. Water-in-oil emulsions remain on the skin for longer and are harder to wash off with water. Water-in-oil emulsions also do not evaporate and have a protective, moisturizing and softening effect on the skin because they lock water onto the top layer of the skin and do not evaporate.

 In contrast, oil-in-water emulsions have a water soluble external phase, and fat soluble internal phase. Oil-in-water emulsions are easily washed off with water, and have an evaporating external phase.

 Emulsifying agents have solubility characteristics of the external and internal phase, and are therefore able to orient themselves between the internal and external phases, stopping the droplets of the internal phase from combining to form larger droplets by decreasing surface tension, while also decreasing the repelling forces between the two phases.

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