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This article is about surfactants in general. For the compound produced by alveolar cells, see pulmonary surfactant. Surfactants are wetting agents that lower the surface tension of a liquid, allowing easier spreading, and lower the interfacial tension between two liquids.
Origin of term The term surfactant is a blend of "Surface active agent". Surfactants are usually organic compounds that are amphipathic, meaning they contain both hydrophobic groups (their "tails") and hydrophilic groups (their "heads"). Therefore, they are typically sparingly soluble in both organic solvents and water. Operation and effects Surfactants reduce the surface tension of water by adsorbing at the liquid-gas interface. They also reduce the interfacial tension between oil and water by adsorbing at the liquid-liquid interface. Many surfactants can also assemble in the bulk solution into aggregates that are known as micelles. The concentration at which surfactants begin to form micelles is known as the critical micelle concentration or CMC. When micelles form in water, their tails form a core that is like an oil droplet, and their (ionic/polar) heads form an outer shell that maintains favorable contact with water. When surfactants assemble in oil, the aggregate is referred to as a reverse micelle. In a reverse micelle, the heads are in the core and the tails maintain favorable contact with oil. Surfactants are also often classified into four primary groups; anionic, cationic, non-ionic, and zwitterionic (dual charge). In Index Medicus and the National Library of Medicine (NLM, USA Dept. of Health and Human Services), "surfactant" is reserved for the meaning pulmonary surfactant (see "alveoli" link below). For the more general meaning, "surface active agent" is the heading. Thermodynamics of the surfactant systems are of great importance, theoretical and practical, because surfactant systems represent a systems between ordered and disordered state of matter-surfactant solutions may contain ordered phase (micelles) and disordered phase (free surfactant molecules and/or ions in the solution). Ordinary washing up detergent for example will promote water penetration in soil but the effect would only last a few days (although many standard laundry detergent powders contain levels of chemicals such as sodium and boron which can be damaging to plants and that these should not be applied to soils). Commercial soil wetting agents will continue to work for a considerable period, but they will eventually be degraded by soil micro-organisms. However detergents can interfere with the life-cycles of some aquatic organisms, and care should be taken to prevent run off of these products into streams, and excess product should not be washed down gutters. Applications Surfactants play an important role in many practical applications and products, including: Classification A surfactant can be classified by the presence of formally charged groups in its head. A nonionic surfactant has no charge groups in its head. The head of an ionic surfactant carries a net charge. If the charge is negative, the surfactant is more specifically called anionic; if the charge is positive, it is called cationic. If a surfactant contains a head with two oppositely charged groups, it is termed zwitterionic. Some commonly encountered surfactants of each type include: | ||||||||
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