Types of emulsions

Types of emulsions

Emulsions typically consist of a polar (e.g., aqueous) and a relatively nonpo-lar (e.g., an oil) liquid phase. Based on the nature of the internal and/external phase, emulsions can be classified into different types (Figure 17.1).

Figure 17.1 Types of emulsions: (a) o/w emulsions, (b) w/o emulsion, and (c) w/o/w multiple emulsion.

1. Oil-in-water emulsion

When the oil phase is dispersed as globules throughout an aqueous con-tinuous phase, the system is referred to as an oil-in-water (o/w) emulsion. An o/w emulsion is generally formed if the aqueous phase constitutes more than 45% of the total weight and a hydrophilic 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.

2. Water-in-oil emulsion

When the aqueous phase is dispersed, and the oil phase is the continuous phase, the emulsion is termed as water-in-oil (w/o) emulsion. 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, con-tinuous phase.

3. Multiple emulsions

Multiple emulsions are emulsions whose dispersed phase contains droplets of another 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. Emulsifying a w/o emulsion using water-soluble surfactants (which stabilize an oily dis-persed phase) can produce w/o/w emulsions with an external aqueous phase, which generally has a lower viscosity than the primary w/o emulsion. Multiple emulsions can also be used for the encapsulation of peptides/proteins and hydrophilic drugs.

4. Microemulsions

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.

Self-emulsifying drug delivery systems and self-microemulsifying drug delivery systems

A solution of drug in the oil–surfactant–cosolvent mixture can sponta-neously form an emulsion or microemulsion with minimal agitation at room temperature. Whether this mixture forms an emulsion or a micro-emulsion depends on the composition of this mixture and the amount of water added. A higher proportion of oil and a lower proportion of cosolvent lead to the formation of an emulsion. Self-microemulsifying mixtures typically contain a higher proportion of the cosolvent and the surfactant, whereas the proportion of oil is lower. These mixtures are termed as self-emulsifying drug delivery system (SEDDS) or self-microemulsifying drug delivery systems (SMEDDS). The SEDDS and SMEDDS can be administered orally for in vivo emulsion or micro-emulsion formation in the patient’s GI tract. For example, cyclospo-rine is available as a self-microemulsifying preconcentrate (Neoral®), which is more rapidly and consistently absorbed than the original self-emulsifying formulation of cyclosporine (Sandimmune®). Both these show greater and more consistent bioavailability than unformulated cyclosporine.