surface-active agent A substance that, when used in small quantities, modifies the surface properties of liquids or solids. A surface-active agent reduces surface tension in a fluid or the interfacial tension between two immiscible fluids, such as oil and water. Surfactants are particularly useful in accomplishing the wetting or penetration of solids by aqueous liquids and serve in the manner of detergent, emulsifying, or dispersing agents. They are more effective than soap in certain situations and are used by conservators for such purposes as cleaning, wetting, and dispersing
