Dictionary:

surfactant

  (sər-făk'tənt, sûr'făk'-)

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1. A surface-active substance.
2. A substance composed of lipoprotein that is secreted by the alveolar cells of the lung and serves to maintain the stability of pulmonary tissue by reducing the surface tension of fluids that coat the lung.

[SURF(ACE)-ACT(IVE) + A(GE)NT.]

A member of the class of materials that, in small quantity, markedly affect the surface characteristics of a system; also known as surface-active agent. In a two-phase system, for example, liquid-liquid or solid-liquid, a surfactant tends to locate at the interface of the two phases, where it introduces a degree of continuity between the two different materials. Soaps and detergents are classic examples of surfactants due to their dual (amphipathic) character. These substances consist of a hydrophobic tail portion, usually a long-chain hydrocarbon, and a hydrophilic polar head group, which is often ionic. A material possessing these characteristics is known as an amphiphile. It tends to dissolve in both aqueous and oil phase and to locate at the oil-water interface. See also Interface of phases; Soap.

Surfactants are employed to increase the contact of two materials, sometimes known as wettability. Surfactants and surface activity are controlling features in many important systems, including emulsification, detergency, foaming, wetting, lubrication, water repellance, waterproofing, spreading and dispersion, and colloid stability. See also Emulsion; Micelle.

In general, surfactants are divided into four classes: amphoteric, with zwitterionic head groups; anionic, with negatively charged head groups; cationic, with positively charged head groups; and nonionic, with uncharged hydrophilic head groups. Those with anionic head groups include long-chain fatty acids, sulfosuccinates, alkyl sulfates, phosphates, and sulfonates. Cationic surfactants may be protonated long-chain amines and long-chain quaternary ammonium compounds. The class of amphoteric surfactants is represented by betaines and certain lecithins, while nonionic surfactants include polyethylene oxide, alcohols, and other polar groups.

Quite different materials, such as polymers and clays, can also exhibit surface activity; many polymeric materials, for example, polyvinyl alcohol and polyacrylamide, are excellent stabilizers for a variety of colloid systems. These entities adsorb at the colloid interface and, by means of steric effects, prevent colloid-colloid adhesion and flocculation. Clays readily adsorb other materials or adsorb onto large particles suspended in solution, so that the particle interface consists of charged clay particles, which increase colloid stability by electrostatic and steric effects. See also Adsorption; Colloid; Ion exchange; Polymer; Surface and interfacial chemistry.

Surfactant is a chemical that sounds like a detergent — which it is. If you could get enough you could try it in a dishwasher, although it would froth too much: the word is from ‘surf’ or sea-froth.

The most important site of surfactant is the lining of the alveoli of the lungs. Here it reduces the force needed to inflate the lungs and allows comfortable, quiet breathing. If you compare blowing up a bubble of a soap film with a party balloon, much more force is needed for the latter. This is because the molecules of the balloon stick together far more tightly than do those of soap solution; they are said to have a higher surface tension. In the 1920s it was shown that something in the alveoli must be reducing the surface tension of of the lining liquid, and this was subsequently shown to be surfactant. It is a mixture of fatty substances linked to proteins, the main ingredient being dipalmitoyl lecithin. It is made in one of the types of cell in the alveolar walls (type II cells), where it can be seen under the electron microscope as onion-like granules. Released into the airspace it spreads out and lines the alveolar surface.

In fetal life, surfactant first appears at about 20 weeks' gestation, and is being fully secreted by 30 weeks, 10 weeks before birth normally takes place. If it is absent the lungs are not only immature, but they can only be inflated with pressures 5-10 times greater than normal. Even if the baby can achieve this, it will rapidly lead to exhaustion. The condition is called Respiratory Distress Syndrome of the Infant (RDSI). Between 20 and 30 weeks' gestation more and more surfactant appears and the premature baby is progressively better able to overcome the defect in its lungs if born during this period. Surfactant production can be encouraged by giving the mother steroids (e.g. cortisol) before delivery, but nowadays these are combined with attempts to put surfactant directly into the infant's lungs. This was first attempted in 1964, but it was twenty to thirty years before the treatment became widespread and successful for premature babies. Either surfactant extracted from animal lungs or a synthetic version is used, and it can be administered directly into the airways or as an aerosol.

Adults can suffer a rather similar condition to RDSI, called ARDS (A=adult). With major traumatic injuries, or in some cases of severe septic shock or tissue destruction, the lining of the alveoli is damaged and the surfactant is ineffective. This leads to serious respiratory difficulties, which can be treated by surfactant replacement.

Surfactants are found in many other sites in the body, as well as in the lungs. For example, in the stomach surfactants may act as a barrier on the surface of the mucosa, which may explain in part why our stomachs are not digested by their own gastric juice. In the airways surfactants probably act as lubricants, allowing mucus and other materials to be cleared easily from the lungs by coughing or by ciliary transport.

— John Widdicombe

See also antenatal development; breathing; infancy; lungs.

Surface active agents; compounds that have an affinity for fats (hydrophobic) and water (hydrophilic) and so act as emulsifiers, e.g. soaps and detergents. Used as wetting agents to assist the reconstitution of powders, including dried foods, to clean and peel fruits and vegetables, also in baked goods and comminuted meat products.

A surface-active agent.

A chemical wetting agent; added to water to improve its penetration into a material; often useful in reducing the amount of water required in removing a material from the surface on which it has been applied.

A surface-active agent, such as soap or a synthetic detergent. In pulmonary physiology, a mixture of phospholipids (mainly dipalmitoylphosphatidylcholine) secreted by the great, or type II, alveolar cells into the alveoli and respiratory air passages, which reduces the surface tension of pulmonary fluids and thus contributes to the elastic properties of pulmonary tissue. See also hyaline membrane disease.

A soaplike compound added to water or some other liquid to increase its wetting properties by reducing the surface tension of the droplets. Also called wetting agent.

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.

Etymology

The term surfactant is a blend of "surface active agent". Surfactants are usually organic compounds that are amphiphilic, 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

A micelle - the lipophilic ends of the surfactant molecules dissolve in the oil, while the hydrophilic charged ends remain outside, shielding the rest of the hydrophobic micelle

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

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