Surfactants and micelles

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.