Surfactant refers to a class of compounds that reduce the surface tension of a liquid, or the interfacial tension of a liquid-liquid, liquid-solid phase. Its English name is the compound of the surface active agent , which means that "the surfactant is a substance that enhances the surface (or interface) activity."
Commonly used in daily life soap is a typical surfactant. Soap can be used to wash clothes, skin surface of the oil, and a single water is not easy to wash off, because the oil is insoluble in water. So why can soap wash the grunge? This is due to the "two-sided" nature of the surfactant. The soap is a fatty acid metal salt, it is soluble in water, we call it "hydrophilic group"; the other end is difficult to dissolve in water and soluble in oil, we call it "hydrophobic group" or "Lipophilic group". Because of this special structure, the soap can head into the water, the other head into the oil. In this way, the water pulled the soap molecules, soap molecules and then pull the oil, put the grease from the clothes pull down!
Surfactant molecules also have hydrophilic groups and lipophilic groups, and this property is also called "amphiphilic". Due to this characteristic of the surfactant, they can form micelle in water at the appropriate concentration : the hydrophilic head is attracted outward by water, and the lipophilic tail is repelled by water. In the process of washing clothes, the oil is pulled by the oleophilic group into the micelle, and the whole micelle is taken away by water. If they are in oily environments, they can form inverse micelle (inverse micelle), that is, the head in the tail outside. These micelles play a pivotal role in cosmetics.