1. n. [Enhanced Oil Recovery]

A chemical added to a process to enhance the effectiveness of a surfactant. In the oil industry, cosurfactants are often used to increase the oil-solubilizing capacity of microemulsion surfactant systems. An example of such a cosurfactant is a long-chain alcohol. Pure surfactants often organize well at a liquid/liquid boundary, which leads to relatively stiff interfaces or even liquid-crystal phases. To achieve ultralow interfacial tension for enhanced oil recovery applications, a cosurfactant is added to disturb this organization at the liquid/liquid interface. Cosurfactants can also be used to fine-tune the formulation phase behavior, for example, by expanding the temperature or salinity range of microemulsion formation.