Surface-active agents, or surfactants, are substances that preferentially localize or adsorb to surfaces or interfaces and reduce surface or inter-facial tension.
Surfactants and
micelles
Introduction
Surface-active
agents, or surfactants, are substances that preferentially localize or adsorb
to surfaces or interfaces and reduce surface or inter-facial tension. Common
interfaces of pharmaceutical relevance are those between two insoluble liquids
or the air–water interface. The interfacial tension between two surfaces
results from lower forces of attractive inter-action between the two materials
(~adhesion) than within the two materi-als (~cohesion), which arise from the
differences in the types of molecular interactions in a material. For example,
hydrocarbon/oil molecules pre-dominantly bind by hydrophobic interactions,
whereas water molecules bond by hydrogen bonding and polar/dipole interactions.
Thus, in an oil– water system, the water–water interactions and the oil–oil
interactions are stronger than the oil–water interactions. This leads to a
thermodynamic propensity of the system to minimize the interfacial area, the
extent of which may be expressed in terms of interfacial tension. Surface
tension is a special case of interfacial tension, when one of the materials is
air.
A
surfactant preferentially adsorbs to the interface due to its molecular
characteristics. Adsorption of surfactant at the interface results in changes
in the nature of the interface and reduces interfacial tension between the two
liquids. This phenomenon is of considerable influence in pharmaceu-tical formulations.
For example, the lowering of the interfacial tension between oil and water
phases facilitates emulsion formation. The adsorp-tion of surfactants on
insoluble particles reduces solid–liquid interfacial tension and enables drug
particles to be dispersed in a suspension.
When
a surfactant is added to a liquid in excess of what is needed to completely
cover the surface, the surfactant forms self-associating struc-tures within the
liquid. These structures are called micelles. When formed in water, these micelles
have a hydrophobic core and a hydrophilic shell. The incorporation of insoluble
compounds within micelles of the surfac-tants in an aqueous solution can
solubilize these insoluble drugs. Therefore, surfactants are commonly used as emulsifying agents, solubilizing agents,
detergents, and wetting agents.
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