Surfactant

This article is about surfactants in general. For the compound produced by alveolar cells, see pulmonary surfactant.

Surfactants, also known as tensides, are wetting agents that lower the surface tension of a liquid, allowing easier spreading, and lower the interfacial tension between two liquids.

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Etymology

The term surfactant is a blend of "surface acting 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 soluble in both organic solvents and water. The term surfactant was coined by Antara Products in 1950.

In Index Medicus and the United States National Library of Medicine, "surfactant" is reserved for the meaning pulmonary surfactant (see "alveoli" link below). For the more general meaning, "surface active agent" is the heading.

The most common, biological example of surfactant is that coating the surfaces of the Alveoli, the small air sacs of the lungs that serve as the site of gas exchange.

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. Some of these aggregates 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 can encapsulate 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).

Thermodynamics of the surfactant systems are of great importance, theoretically and practically. This is because surfactant systems represent systems between ordered and disordered states of matter. Surfactant solutions may contain an ordered phase (micelles) and a disordered phase (free surfactant molecules and/or ions in the solution).

Ordinary washing up (dishwashing) 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, so 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. Some can, however, interfere with the life-cycles of some aquatic organisms, so care should be taken to prevent run-off of these products into streams, and excess product should not be washed down gutters.

Applications and sources

Surfactants play an important role in many practical applications and products, including:

• Detergents
• Fabric softener
• Emulsifiers
• Paints
• Adhesives
• Inks
• Anti-fogging
• Soil remediation
• Wetting
• Ski Wax
• Snowboard Wax
• Foaming
• Defoaming
• Laxatives
• Agrochemical formulations
  • Herbicides
  • Insecticides
• Quantum dot coating
• Biocides (Sanitizers)
• Hair Conditioners (after shampoo)
• Spermicide (Nonoxynol 9)
• Used as an additive in 2.5 gallon fire extinguishers

Surfactants are also naturally secreted by type II cells of the lung alveoli in mammals.

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
  • Cationic (based on quaternary ammonium cations)
    • Cetyl trimethylammonium bromide (CTAB) a.k.a. hexadecyl trimethyl ammonium bromide, and other alkyltrimethylammonium salts
    • Cetylpyridinium chloride (CPC)
    • Polyethoxylated tallow amine (POEA)
    • Benzalkonium chloride (BAC)
    • Benzethonium chloride (BZT)
  • Zwitterionic (amphoteric)
    • Dodecyl betaine
    • Dodecyl dimethylamine oxide
    • Cocamidopropyl betaine
    • Coco ampho glycinate
• Nonionic
  • Alkyl poly(ethylene oxide)
  • Copolymers of poly(ethylene oxide) and poly(propylene oxide) (commercially called Poloxamers or Poloxamines)
  • Alkyl polyglucosides, including:
    • Octyl glucoside
    • Decyl maltoside
  • Fatty alcohols
    • Cetyl alcohol
    • Oleyl alcohol
  • Cocamide MEA, cocamide DEA, cocamide TEA

See also

• Anti-fog