An emulsion is a dispersion of at least two immiscible liquids, one of which is dispersed as droplets in the other liquid, and stabilized by an emulsifying agent. Emulsions have been widely used in many areas of application: in industries, agriculture, food technologies, pharmaceutics, and cosmetics.
Very frequently emulsions are used in cosmetic products as topical vehicle for dermal application since they have high patient/consumer acceptance. Pharmaceutical emulsions are currently used internally for the administration of nutrients, drugs, and diagnostic agents.
This article focuses on the differences between oil-in-water emulsion and water-in-oil emulsion.
1. In an oil-in-water (o/w) emulsions, the dispersed phase (discontinuous or internal phase) phase is oil, and the dispersion medium (continuous or external phase) is water while in a water-in-oil (w/o) emulsions the water is the dispersed phase, and oil the dispersion medium.
2. Oil-in-water emulsions are non-greasy and easily removable from the skin surface while water-in-oil emulsions are greasy and not water washable.
3. Oil-in-water emulsions are used externally to provide cooling effect e.g. vanishing cream while water-in-oil emulsions are used externally to prevent evaporation of moisture from the surface of skin e.g. cold cream.
4. Water-soluble drugs are more quickly released from oil-in-water emulsions while oil-soluble drugs are more quickly released from water-in-oil emulsions.
5. Oil-in-water emulsions are preferred for formulations meant for internal use as bitter taste of oils can be masked while water-in-oil emulsions are preferred for formulations meant for external use like creams.
6. Oil-in-water emulsions give a positive conductivity test as water is the external phase which is a good conductor of electricity while water-in-oil emulsions do not give a positive conductivity test as oil (which is a poor conductor of electricity) is the external phase.
7. Because the external phase of an emulsion is continuous, an o/w emulsion may be diluted or extended with water or an aqueous preparation while a w/o emulsion may be diluted or extended with an oleaginous or oil-miscible liquid.