Emulsions

Emulsions are two–phase systems in which one liquid is dispersed throughout the other liquid inthe form of small droplets. The former is known as the dispersed, internal, or discontinuous phase, and the latter is known as the dispersion medium, external phase, or continuous phase.

In pharmaceutical emulsions, one phase is usually an aqueous solution. The other phase is usually lipid, or oily. The lipids range from vegetable or hydrocarbon oils to semi–solid hydrocarbons and waxes. Emulsions are usually described in terms of water and oil. Oil is the lipid, or nonaqueous, phase regardless of its composition. If water is the internal phase, the emulsion is classified as water–in–oil (w/o). If water is the external phase, the emulsion is classified as oil–in–water (o/w). Correct proportions of oil and water should be used during preparation of emulsions. The internal phase should represent about 40–60% of the total volume.

The two liquids in an emulsion are immiscible and require the use of an emulsifying agent. The type of emulsion formed is primarily determined by the relative phase volumes and the emulsifying agent used. Most agents preferentially form one type of emulsion if the phase volume permits. The choice of an emulsifying agent is more important in determining the final type of emulsion. The effectiveness of an emulsifying agent depends on its chemical structure, concentration, solubility, pH, physical properties, and electrostatic effect. True emulsifying agents (primary agents) can form and stabilize emulsions by themselves. Stabilizers (auxiliary agents) do not form acceptable emulsions when used alone, they assist primary agents in stabilizing the product (e.g., increase viscosity).

Emulsifying agents are either natural or synthetic. Natural emulsifying agents include gums, such as acacia and tragacanth, agar, pectin, gelatin, methylcellulose, and carboxy–methylcellulose. Acacia and tragacanth are used to form o/w emulsions. They are best suited for emulsions intended for internal administration. Synthetic emulsifying agents can be anionic, cationic, or nonionic. Anionic synthetic agents include sulfuric acid esters, sulfonic acid derivatives, and soaps. They have a high pH and are therefore sensitive to addition of acids and electrolytes. Cationic synthetic agents are incompatible with soaps. Nonionic synthetic agent are resistant to the addition of acids and electrolytes.

The order of mixing of ingredients in an emulsion depends on the type of emulsion being prepared (o/w or w/o) and on the emulsifying agent chosen. Medicinal agents can be incorporated into an emulsion either during or after its formation. It is best to incorporate a drug into a vehicle during emulsion formation, when it can be incorporated in molecular form Addition of a drug to a preformed emulsion can present some difficulty, depending on the type of emulsion and the nature of the emulsifier.

Emulsions are prepared by the following four methods: wet gum (English) method, dry gum (continental) method, bottle method and beaker method. The latter is used in preparing emulsions with synthetic emulsifying agents. In the wet gum method, the primary emulsion of fixed oil, water, and acacia (in a 4:2:1 ratio) is prepared as follows:

(1) Two parts of water are added all at once to one part of acacia. The mixture is triturated (A rapid motion is essential when triturating the ingredients using a mortar and pestle). The mixture is triturated until a smooth musilage is formed.

(2) Oil is added in small increments (1–5 ml) with continuous trituration until the primary emulsion is formed.

(3) The mixture (an o/w emulsion) is triturated for another 5 minutes.

(4) The mixture can be brought to volume with water, and a hand homogenizer or an electric mixer is used to obtain the final product.

Emulsions are unstable, and addition of preservatives is required if the preparation is intended for use longer than a few days. Generally, a combination of methylparaben (0.2%) and propylparaben (0.02%) is used. The product should be protected from light and extreme temperatures. Both freezing and heat may have an adverse effect on the product stability. A «shake well» label should be placed on the final product.