Emulsions are liquid disperse systems consisting of at least two immiscible liquids (or two liquids that are saturated with each other), one of which is dispersed as small globules (internal or dispersed phase) within the other liquid phase (external or continuous phase), generally stabilized by a third substance called emulsifying agent. The process of formation of an emulsion is termed emulsification.
There are two basic types of emulsions
- Oil-in-water (O/W) emulsion
- Water-in-oil (W/O) emulsion
However, depending upon the need, more complex systems (referred to as “double emulsions” or “multiple emulsions”) in which the oil-in-water or water-in-oil emulsions are dispersed in another liquid medium can be formulated.
By considering particle size, pharmaceutical emulsions can be
- Macro emulsions (droplets size usually exceeds 10 mm)
- Mini emulsions (droplets size usually 0.1–10 µm)
- Microemulsions (droplets size usually 100-600 nm)
- Nano Emulsions (droplets size usually below 100 nm)
Emulsions can also be classified based on mode of administration into
- Oral emulsions e.g., castor oil, liquid paraffin
- External emulsions e.g., creams
- Parenteral emulsions e.g., vitamins
- Rectal emulsions e.g., enema.
This article will focus on oil-in-water (o/w) emulsions, water-in-oil emulsions, multiple emulsions, and microemulsions.
Oil-in-water (o/w) emulsions contain oil droplets dispersed as globules throughout an aqueous continuous phase. An oil-in-water emulsion is generally formed if the aqueous phase constitutes more than 45% of the total weight and a water-loving emulsifier, such as sodium lauryl sulfate, triethanolamine stearate, sodium oleate, and glyceryl monostearate is used. The emulsifier is present in the external, continuous phase and helps stabilize the interface with the dispersed phase globules.
Fats or oils for oral administration, either as medicaments in their own right or as vehicles for oil-soluble drugs, are always formulated as o/w emulsions. Oil-in-water emulsions are non-greasy and are easily removable from the skin surface. They are used externally to provide cooling effect and internally to also mask the bitter taste of oil.
Water-soluble drugs are more quickly released from o/w emulsion. O/W emulsions give a positive conductivity test as water, the external phase is a good conductor of electricity.
In a water-in-oil (w/o) emulsion, the aqueous phase is dispersed as globules in the oil continuous phase. A lipophilic emulsifier is used for preparing w/o emulsions. The w/o emulsions are used mainly for external applications and may contain one or several of the following emulsifiers: calcium palmitate, sorbitan esters (Spans), cholesterol, and wool fats. Thus, the use of a lipophilic emulsifier enables the formation of w/o emulsions with the oil phase as the external, continuous phase.
Multiple emulsions are emulsions whose dispersed phase contains droplets of another emulsion. They can be considered as emulsions of emulsions.
Multiple emulsions can be either
- Water-in-oil-in-water (w1/o/w2) emulsion
- Oil-in-water-in-oil (o1/w/o2) emulsion.
Emulsifying a w/o emulsion using water-soluble surfactants (which stabilize an oily dispersed phase) can produce w/o/w emulsions with an external aqueous phase, which generally has a lower viscosity than the primary w/o emulsion. Oil-in-water-in-oil (o/w/o) type multiple emulsions on the other hand consist of very small droplets of oil dispersed in the water globules of a water-in-oil emulsion.
Both water-in-oil-in-water (w/o/w) and oil-in-water-in-oil (o/w/o) multiple emulsions are of interest as delayed- and/or sustained-action drug delivery systems. They also have applications in cosmetics. Multiple emulsions can also be used for the encapsulation of peptides/proteins and hydrophilic drugs.
Microemulsions are visually homogeneous, transparent/isotropic systems of low viscosity. In their simplest form, microemulsions are small droplets (diameter 5–140 nm) of one liquid dispersed throughout another by virtue of the presence of a fairly large amount of surfactant(s) and cosolvent(s). Microemulsions have a very finely subdivided dispersed phase, and often contain a high concentration of the emulsifier(s) and a cosolvent (such as ethanol).
Microemulsions are thermodynamically stable for prolonged periods of time. They can be dispersions of o/w or w/o. The type of microemulsion (w/o or o/w) formed is determined largely by the nature of the surfactants. Microemulsions can be used to increase the bioavailability of poorly water-soluble drugs by incorporating them into the oily phase. Incorporation of etoposide and methotrexate diester derivative into w/o microemulsion has been suggested as a potential carrier for cancer therapy.
- Dash, A., Singh, S. and Tolman, J. (2014). Pharmaceutics: Basic Principles and Application to Pharmacy Practice.USA: Elsevier Inc.
- Mahato, R. and Narang, A. (2018). Pharmaceutical Dosage Forms and Drug Delivery (3rd ).New York: Taylor & Francis Group, LLC.
The article Types of Emulsions provides answers to what is emulsion and its types? What are the classifications of emulsions? What are the types of emulsions?