Surfactant is a substance whose interfacial state can be significantly changed by adding a small amount to the solution system. Surfactants have fixed hydrophilic and lipophilic groups that can be oriented on the surface of a solution.

Surfactants reduce the surface tension of water by adsorption at the interface between gas and liquid phases, and can also reduce oil-water interfacial tension by adsorption between liquid interfaces. Many surfactants can also aggregate into aggregates in their native solutions.

Surfactants are classified into ionic surfactants (including cationic surfactants and anionic surfactants), nonionic surfactants, amphoteric surfactants, complex surfactants, other surfactants, etc.