Surfactant
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- This article is about surfactants in general. For the compound produced by alveolar cells, see pulmonary surfactant.
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 air-water 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) 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.
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 mollecules and/or ions in the solution).
Applications
Surfactants play an important role in many practical applications and products, including:
- Detergents
- Emulsifiers
- Paints
- Adhesives
- Inks
- Alveoli
- Wetting
- Ski Wax
- Snowboard Wax
- Foaming
- Defoaming
- Laxatives
- Agrochemical formulations
- *Herbicides
- *Insecticides
- Quantum dot coating
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:
- Ionic
- * Anionic (based on sulfate, sulfonate or carboxylate anions)
- ** Sodium dodecyl sulfate (SDS), ammonium lauryl sulfate, and other alkyl sulfate salts
- ** Sodium laureth sulfate, also known as sodium lauryl ether sulfate (SLES)
- ** Alkyl benzene sulfonate
- ** Soaps, or fatty acid salts (see acid salts)
- * Cationic (based on quaternary ammonium cations)
- ** Cetyl trimethylammonium bromide (CTAB) a.k.a. hexadecyl trimethyl ammonium bromide, and other alkyltrimethylammonium salts
- ** Cetyl pyridinium chloride
- ** Polyethoxylated tallow amine (POEA)
- ** Benzalkonium chloride
- Zwitterionic (amphoteric)
- * Dodecyl betaine
- * Dodecyl dimethylamine oxide
- * Cocamidopropyl betaine
- * Coco ampho glycinate
- Nonionic
- * Alkyl poly(ethylene oxide)
- * Alkyl polyglucosides, including:
- ** Octyl glucoside
- ** Decyl maltoside
- * Fatty alcohols
- ** Cetyl alcohol
- ** Oleyl alcohol
- * Cocamide MEA, cocamide DEA, cocamide TEA
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