Cosurfactant

A cosurfactant is a secondary surfactant added to a system that already contains a primary surfactant, with the aim of enhancing interfacial properties in emulsions and microemulsions. By participating at the oil–water interface, a cosurfactant helps lower interfacial tension further and modifies the interfacial film and curvature, which can improve emulsion stability and expand the conditions under which stable microemulsions form.

Cosurfactants influence several aspects of interfacial behavior. They can alter packing density at the interface, adjust the elasticity and thickness of the interfacial film, and change the spontaneous curvature of the surfactant film. These effects can broaden the range of compositions that yield stable emulsions or microemulsions, enable the formation of smaller droplets, and enhance solubilization of otherwise poorly soluble components.

Common cosurfactants include short-chain alcohols and certain glycols, which are often compatible with a variety of surfactants (ionic and nonionic). The choice depends on the primary surfactant system, desired phase behavior, and product requirements. Safety and regulatory considerations are important, as cosurfactants can influence irritation, toxicity, and compatibility with other formulation ingredients.

Applications of cosurfactants span cosmetics, personal care, pharmaceuticals, and other industries that rely on stable emulsions or microemulsions. They are used to improve texture, stability, and solubility of active ingredients, as well as to optimize processing and performance of the final product.