substance such as a detergent that, when added to a liquid, reduces its surface tension, thereby increasing its spreading and wetting properties. In the dyeing of textiles (textile), surface-active agents help the dye penetrate the fabric evenly. They are used to disperse aqueous suspensions of insoluble dyes and perfumes.

The surface-active molecule must be partly hydrophilic (water-soluble) and partly lipophilic (soluble in lipids, or oils). It concentrates at the interfaces between bodies or droplets of water and those of oil, or lipids, to act as an emulsifying agent, or foaming agent.

Other surface-active agents that are more lipophilic and less hydrophilic may be used as defoaming agents, or as demulsifiers. Certain surface-active agents are germicides, fungicides, and insecticides.

Surface-active agents are used in corrosion inhibition, in ore flotation, to promote oil flow in porous rocks, and to produce aerosols. See also detergent.