Cosurfactants

Cosurfactants are secondary surfactants added to a primary surfactant system to improve the formation and stabilization of emulsions, foams, and microemulsions. They are typically small, amphiphilic molecules that migrate to the oil–water interface and reduce interfacial tension, enabling easier spreading and creation of new interfacial area. By modulating the interfacial film, cosurfactants can increase fluidity and adjust curvature, expanding the range of compositions that produce stable microemulsions. They also allow lower total surfactant concentrations.

Common cosurfactants include short-chain alcohols such as ethanol and propanol, and polyols such as propylene glycol and glycerol. These agents insert between surfactant molecules at the interface and modify the system’s hydrophilic–lipophilic balance. Selection depends on desired phase behavior, solubilization goals, and safety or regulatory factors. Cosurfactants can affect drug solubility, release, and permeability in pharmaceutical and cosmetic products, as well as viscosity and rheology.

Applications span pharmaceutical and topical formulations, where cosurfactants enhance solubilization of poorly water-soluble drugs and stabilize nanoemulsions; cosmetics, where they improve stability and sensory properties; and industrial formulations, including cleaners and enhanced oil recovery, where interfacial properties are critical. Safety and compatibility are important, because some cosurfactants can irritate or be toxic at higher levels.