A substance used to enhance the cleansing action of water. A detergent is an emulsifier, which penetrates and breaks up the oily film that binds dirt particles, and a wetting agent, which helps them to float off. Emulsifier molecules have an oillike nonpolar portion which is drawn into the oil, and a polar group that is water-soluble; by bridging the oil-water interface, they break the oil into dispersible droplets (emulsion). As a surfactant, a detergent decreases the surface tension of water to help it penetrate soil.