Emulsions

Emulsions
Emulsion is a dispersion in which the dispersed phase is composed of small globules of a liquid distributed throughout a vehicle in which it is immiscible.
The dispersed phase is the internal phase and the dispersion medium is the external phase or continuous phase.
A third phase which is essential for emulsion formation is the emulsifying agent.
Emulsions with an oleaginous internal phase and an aqueous external phase are oil in water (o/w) and vice versa, because the external phase of an emulsion is continuous, an o/w emulsion may be diluted or extended with water and w/o with an oleaginous liquid.
Depending on the constituents and the intended application, liquid emulsions may be employed orally, topically or parenterally.

Purpose of emulsions and of emulsification:
1. Facilitate the preparation of relatively stable and homogenous mixtures of two immiscible liquids.
2. Permits administration of a liquid drug in the form of minute globules rather than in bulk.
3. Administration of distasteful oil by dispensing it in a sweetened, flavored aqueous vehicle.
4. Reduced particle size of the oil globules may render the oil more digestible and more readily absorbed for example increased efficacy of mineral oil as a cathartic when emulsified.

Emulsions applied topically may be o/w or w/o depending on:
• Nature of therapeutic agents.
• Desirability for an emollient or tissue softening effect.
• The condition of the skin.
Medicinal agents that irritate the skin are less irritating in the internal phase, on the unbroken skin a w/o emulsion can be applied more evenly because the skin is covered within a thin film of sebum, also w/o emulsion is more softening the skin because it resists drying and removal on contact with water.
On the other hand if we desired to formulate emulsion which is easily washed off with water an o/w emulsion is preferred.

Types of emulsions:



Theories of emulsification:
We have many theories to explain how emulsifying agents promote emulsification.
1. Surface tension theory.
2. Oriented- wedge theory.
3. Interfacial film theory.

Surface tension theory
According to this theory, the use of surface active agents (surfactants) lowers the the interfacial tension of the two immiscible liquids, reducing the repellent force between the liquids and diminishing each liquids attraction for its own molecules.
Surfactants facilitate the breakup of large globules into smaller ones, which then have a lesser tendency to reunite or coalesce.

Oriented-wedge theory