Surfactant

Surfactants are compounds that lower the surface tension of a liquid, allowing easier spreading, and lowering of the interfacial tension between two liquids, or between a liquid and a solid. Surfactants may act as: detergents, wetting agents, emulsifiers, foaming agents, and dispersants.

Contents

Etymology

The term surfactant is a blend of surface active agent.[1] Surfactants are usually organic compounds that are amphiphilic, meaning they contain both hydrophobic groups (their tails) and hydrophilic groups (their heads). Therefore, a surfactant molecule contains both a water insoluble (or oil soluble component) and a water soluble component. Surfactant molecules will migrate to the water surface, where the insoluble hydrophobic group may extend out of the bulk water phase, either into the air or, if water is mixed with an oil, into the oil phase, while the water soluble head group remains in the water phase. This alignment and aggregation of surfactant molecules at the surface, acts to alter the surface properties of water at the water/air or water/oil interface.

In Index Medicus and the United States National Library of Medicine, surfactant is reserved for the meaning pulmonary surfactant. For the more general meaning, surface active agent is the heading.

Properties

A micelle—the lipophilic tails of the surfactant molecules remain on the inside of the micelle due to unfavourable interactions. The polar "heads" of the micelle, due to favourable interactions with water, form a hydrophilic outer layer that in effect protects the hydrophobic core of the micelle. The compounds that make up a micelle are typically amphiphilic in nature, meaning that not only are micelles soluble in protic solvents such as water but also in aprotic solvents as a reverse 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. Examples of such aggregates are vesicles and micelles. The concentration at which surfactants begin to form micelles is known as the critical micelle concentration (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.[2] 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 (many standard laundry detergent powders contain levels of chemicals such as alkali and chelating agents, which can be damaging to plants and 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.

Applications and sources

Surfactants play an important role as cleaning, wetting, dispersing, emulsifying, foaming and anti-foaming agents in many practical applications and products, including:

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

Classification

According to the composition of their tail

The tail of surfactants can be:

Surfactant can have one or two tails (double chained surfactants).

According to the composition of their head

Surfactant classification according to the composition of their head: nonionic, anionic, cationic, amphoteric.

A surfactant can be classified by the presence of formally charged groups in its head. A non-ionic 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:

According to the composition of their counter-ion

In the case of ionic surfactants, the counter-ion can be:

Health and environmental controversy

Some surfactants are known to be toxic to animals, ecosystems and humans, and can increase the diffusion of other environmental contaminants.[3][4][5] Despite this, they are routinely deposited in numerous ways on land and into water systems, whether as part of an intended process or as industrial and household waste. Some surfactants have proposed or voluntary restrictions on their use. For example, PFOS is a persistent organic pollutant as judged by the Stockholm Convention. Additionally, PFOA has been subject to a voluntary agreement by the U.S. Environmental Protection Agency‎ and eight chemical companies to reduce and eliminate emissions of the chemical and its precursors.[6]

See also

Notes

  1. ↑ Rosen MJ (2004). Surfactants and Interfacial Phenomena (3rd ed.). Hoboken, New Jersey: John Wiley & Sons. 
  2. ↑ Baeurle SA, Kroener J (2004). "Modeling Effective Interactions of Micellar Aggregates of Ionic Surfactants with the Gauss-Core Potential". J. Math. Chem. 36: 409–421. doi:10.1023/B:JOMC.0000044526.22457.bb. 
  3. ↑ Metcalfe TL, Dillon PJ, Metcalfe CD (April 2008). "Detecting the transport of toxic pesticides from golf courses into watersheds in the Precambrian Shield region of Ontario, Canada". Environmental Toxicology and Chemistry 27 (4): 811–8. doi:10.1897/07-216.1. PMID 18333674. 
  4. ↑ Emmanuel E, Hanna K, Bazin C, Keck G, Clément B, Perrodin Y (April 2005). "Fate of glutaraldehyde in hospital wastewater and combined effects of glutaraldehyde and surfactants on aquatic organisms". Environment International 31 (3): 399–406. doi:10.1016/j.envint.2004.08.011. PMID 15734192. 
  5. ↑ Murphy MG, Al-Khalidi M, Crocker JF, Lee SH, O'Regan P, Acott PD (April 2005). "Two formulations of the industrial surfactant, Toximul, differentially reduce mouse weight gain and hepatic glycogen in vivo during early development: effects of exposure to Influenza B Virus". Chemosphere 59 (2): 235–46. doi:10.1016/j.chemosphere.2004.11.084. PMID 15722095. 
  6. ↑ USEPA: "2010/15 PFOA Stewardship Program" Accessed October 26, 2008.

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