Emulsions

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Chapter: Pharmaceutical Drugs and Dosage: Dosage forms - Emulsions

An emulsion consists of at least two immiscible liquid phases, one of which is dispersed as globules (dispersed phase) and into the other liquid phase (continuous phase).


Emulsions

Introduction

An emulsion consists of at least two immiscible liquid phases, one of which is dispersed as globules (dispersed phase) and into the other liquid phase (continuous phase). Emulsions are thermodynamically unstable and are usually stabilized by the presence of an emulsifying agent. The process of forma-tion of an emulsion is termed emulsification. The diameter of the dispersed phase globules is generally in the range of about 0.1 to 10 μm, although it can be as small as 0.01 μm or as large as 100 μm. Emulsions in which the size of the dispersed phase is so small, in the nanometer range, that all of the dispersed phase is subvisible appear as a homogeneous, isotropic liquid. Such systems are called microemulsions or nanoemulsion. Emulsified sys-tems range from lotions of relatively low viscosity to ointments and creams, which are semisolid in nature. Pharmaceutical emulsions are used for the administration of nutrients, drugs, and diagnostic agents. Topical creams and lotions are popular forms of emulsions for external use.

The main advantages of emulsions as drug delivery systems include the following:

1. Increased drug bioavailability: Many drugs are highly hydrophobic, with high log P values (partition coefficient between oil and water). These drugs are usually poorly soluble in water but readily soluble in oils. Formulation of drug-dosage form as an emulsion allows the administration of a hydrophobic drug in a soluble/dissolved state. This can improve the oral bioavailability of a biopharmaceutics clas-sification system (BCS) class II (low solubility, high permeability) and class IV (low solubility, low permeability) drugs because absorption from an emulsion does not require the dissolution step. In addition, oral administration of a drug as an emulsion involves coadministra-tion of lipid, which increases bile secretion. Higher concentration of bile in the gastrointestinal (GI) fluids further enhance oral absorption of hydrophobic drugs.

2. Increased drug stability: Drugs that are more stable in an oily phase compared to an aqueous medium can show improved stability in an emulsion dosage form.

3. Prolonged drug action: The oily phase can serve as a reservoir of the drug, which slowly partitions into the aqueous phase for absorption. This phenomenon, especially with semisolid emulsions, can help pro-long drug action. For example, intramuscular injection of an emul-sion can prolong drug absorption time.

A key disadvantage of using emulsions as a delivery system is their physical instability and complex, energy-intensive manufacturing process. In addi-tion, solid dosage forms are generally preferred over liquid dosage forms for oral administration.

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