Substance that lowers the surface tension between a liquid and another material
Schematic diagram of amicelle of oil in aqueous suspension, such as might occur in anemulsion of oil in water. In this example, the surfactant molecules' oil-soluble tails project into the oil (blue), while the water-soluble ends remain in contact with the water phase (red).
Surfactants arechemical compounds that decrease thesurface tension or interfacial tension between twoliquids, a liquid and agas, or a liquid and asolid. The wordsurfactant is ablend of "surface-active agent",[1] coined in 1950.[2] As they consist of a water-repellent and a water-attracting part, they enable water and oil to mix; they can form foam and facilitate the detachment of dirt.
Surfactants occur naturally in traditional plant-based detergents, e.g.horse chestnuts orsoap nuts; they can also be found in the secretions of some caterpillars. Today one of the most commonly used anionic surfactants, linear alkylbenzene sulfates (LAS), are produced frompetroleum products. However, surfactants are increasingly produced in whole or in part from renewablebiomass, like sugar, fatty alcohol from vegetable oils, by-products of biofuel production, or other biogenic material.[3]
Most surfactants are organic compounds withhydrophilic "heads" andhydrophobic "tails." The "heads" of surfactants are polar and may or may not carry an electrical charge. The "tails" of most surfactants are fairly similar, consisting of ahydrocarbon chain, which can be branched, linear, or aromatic.Fluorosurfactants havefluorocarbon chains.Siloxane surfactants havesiloxane chains.
Many important surfactants include a polyether chain terminating in a highlypolar anionic group. The polyether groups often comprise ethoxylated (polyethylene oxide-like) sequences inserted to increase the hydrophilic character of a surfactant.Polypropylene oxides conversely, may be inserted to increase the lipophilic character of a surfactant.
Surfactant molecules have either one tail or two; those with two tails are said to bedouble-chained.[4]
Surfactant classification according to the composition of their head: non-ionic, anionic, cationic, amphoteric.
Most commonly, surfactants are classified according to polar head group. Anon-ionic surfactant has no charged groups in its head. The head of an ionic surfactant carries a net positive, or negative, charge. If the charge is negative, the surfactant is more specifically calledanionic; if the charge is positive, it is calledcationic. If a surfactant contains a head with two oppositely charged groups, it is termedzwitterionic, oramphoteric. Commonly encountered surfactants of each type include:
Anionic: sulfate, sulfonate, and phosphate, carboxylate derivatives
Non-ionic surfactants have covalently bonded oxygen-containing hydrophilic groups, which are bonded to hydrophobic parent structures. The water-solubility of the oxygen groups is the result ofhydrogen bonding. Hydrogen bonding decreases with increasing temperature, and the water solubility of non-ionic surfactants therefore decreases with increasing temperature.
Non-ionic surfactants are less sensitive to water hardness than anionic surfactants, and they foam less strongly. The differences between the individual types of non-ionic surfactants are slight, and the choice is primarily governed having regard to the costs of special properties (e.g., effectiveness and efficiency, toxicity, dermatological compatibility,biodegradability) or permission for use in food.[6]
Fatty acid ethoxylates are a class of very versatile surfactants, which combine in a single molecule the characteristic of a weakly anionic, pH-responsive head group with the presence of stabilizing and temperature responsive ethyleneoxide units.[7]
Amino acid-based surfactants are surfactants derived from anamino acid. Their properties vary and can be either anionic, cationic, or zwitterionic, depending on the amino acid used and which part of the amino acid is condensed with the alkyl/aryl chain.[8]
Schematic diagram of amicelle – thelipophilic tails of the surfactant ions remain inside the oil because they interact more strongly with oil than with water. Thepolar "heads" of the surfactant molecules coating the micelle interact more strongly with water, so they form ahydrophilic outer layer that forms a barrier between micelles. This inhibits the oil droplets, the hydrophobic cores of micelles, from merging into fewer, larger droplets ("emulsion breaking") of the micelle. The compounds that coat a micelle are typicallyamphiphilic in nature, meaning that micelles may be stable either as droplets ofaprotic solvents such as oil in water, or as protic solvents such as water in oil. When the droplet is aprotic it is sometimes[when?] known as a reverse micelle.
Surfactants are usuallyorganic compounds that are akin toamphiphilic, which means that this molecule, being as double-agent, each contains ahydrophilic "water-seeking" group (thehead), and ahydrophobic "water-avoiding" group (thetail).[9] As a result, a surfactant contains both a water-soluble component and a water-insoluble component. Surfactants diffuse in water and getadsorbed atinterfaces between air and water, or at the interface between oil and water in the case where water is mixed with oil. The water-insoluble hydrophobic group may extend out of the bulk water phase into a non-water phase such as air or oil phase, while the water-soluble head group remains bound in the water phase.
The hydrophobic tail may be eitherlipophilic ("oil-seeking") orlipophobic ("oil-avoiding") depending on its chemistry.Hydrocarbon groups are usually lipophilic, for use in soaps and detergents, whilefluorocarbon groups are lipophobic, for use inrepelling stains or reducing surface tension.
Sodium stearate, the most common component of most soap, which comprises about 50% of commercial surfactants4-(5-Dodecyl) benzenesulfonate, a linear dodecylbenzenesulfonate, one of the most common surfactants
In the bulk aqueous phase, surfactants form aggregates, such asmicelles, where the hydrophobic tails form the core of the aggregate and the hydrophilic heads are in contact with the surrounding liquid. Other types of aggregates can also be formed, such as spherical or cylindrical micelles orlipid bilayers. The shape of the aggregates depends on the chemical structure of the surfactants, namely the balance in size between the hydrophilic head and hydrophobic tail. A measure of this is thehydrophilic-lipophilic balance (HLB). Surfactants reduce thesurface tension of water byadsorbing at the liquid-air interface. The relation that links the surface tension and the surface excess is known as theGibbs isotherm.
The dynamics of surfactant adsorption is of great importance for practical applications such as in foaming, emulsifying or coating processes, where bubbles or drops are rapidly generated and need to be stabilized. The dynamics of absorption depend on thediffusion coefficient of the surfactant. As the interface is created, the adsorption is limited by the diffusion of the surfactant to the interface. In some cases, there can exist an energetic barrier to adsorption or desorption of the surfactant. If such a barrier limits the adsorption rate, the dynamics are said to be ‘kinetically limited'. Such energy barriers can be due tosteric orelectrostatic repulsions.Thesurface rheology of surfactant layers, including the elasticity and viscosity of the layer, play an important role in the stability of foams and emulsions.
Characterization of interfaces and surfactant layers
Interfacial and surface tension can be characterized by classical methods such as the-pendant orspinning drop method.Dynamic surface tensions, i.e. surface tension as a function of time, can be obtained by themaximum bubble pressure apparatus
Surface rheology can be characterized by the oscillating drop method or shear surface rheometers such as double-cone, double-ring or magnetic rod shear surface rheometer.
Surfactants act to cause the displacement of air from the matrix of cotton pads and bandages so that medicinal solutions can be absorbed for application to various body areas. They also act to displace dirt and debris by the use of detergents in the washing of wounds[13] and via the application of medicinal lotions and sprays to surface of skin and mucous membranes.[14] Surfactants enhance remediation via soil washing, bioremediation, and phytoremediation.[15]
Detergents have also been used to decellularise organs. This process maintains a matrix of proteins that preserves the structure of the organ and often the microvascular network. The process has been successfully used to prepare organs such as the liver and heart for transplant in rats.[16]Pulmonary surfactants are also naturally secreted by type II cells of the lungalveoli inmammals.
Surfactants are used withquantum dots in order to manipulate their growth,[17] assembly, and electrical properties, in addition to mediating reactions on their surfaces. Research is ongoing in how surfactants arrange themselves on the surface of the quantum dots.[18]
Surfactants play an important role indroplet-based microfluidics in the stabilization of the droplets, and the prevention of the fusion of droplets during incubation.[19]
Agents that increase surface tension are "surface active" in the literal sense but are not called surfactants as their effect is opposite to the common meaning. A common example of surface tension increase issalting out: adding an inorganic salt to an aqueous solution of a weakly polar substance will cause the substance to precipitate. The substance may itself be a surfactant, which is one of the reasons why many surfactants are ineffective in sea water.
Most anionic and non-ionic surfactants are non-toxic, havingLD50 comparable totable salt. The toxicity ofquaternary ammonium compounds, which areantibacterial andantifungal, varies. Dialkyldimethylammonium chlorides (DDAC,DSDMAC) used asfabric softeners have high LD50 (5 g/kg) and are essentially non-toxic, while thedisinfectant alkylbenzyldimethylammonium chloride has an LD50 of 0.35 g/kg. Prolonged exposure to surfactants can irritate and damage the skin because surfactants disrupt thelipid membrane that protects skin and other cells. Skin irritancy generally increases in the series non-ionic, amphoteric, anionic, cationic surfactants.[6]
Surfactants 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.[22][23][24]
Anionic surfactants can be found in soils as the result ofsewage sludge application, wastewater irrigation, and remediation processes. Relatively high concentrations of surfactants together with multimetals can represent an environmental risk. At low concentrations, surfactant application is unlikely to have a significant effect on trace metal mobility.[25][26]
In the case of theDeepwater Horizon oil spill, unprecedented amounts ofCorexit were sprayed directly into the ocean at the leak and on the sea-water's surface. The apparent theory was that the surfactants isolate droplets of oil, making it easier for petroleum-consuming microbes to digest the oil. The active ingredient in Corexit isdioctyl sodium sulfosuccinate (DOSS),sorbitan monooleate (Span 80), and polyoxyethylenated sorbitan monooleate (Tween-80).[27][28]
Because of the volume of surfactants released into the environment, for example laundry detergents in waters, their biodegradation is of great interest. Attracting much attention is the non-biodegradability and extreme persistence offluorosurfactant, e.g.perfluorooctanoic acid (PFOA).[29] Strategies to enhance degradation includeozone treatment and biodegradation.[30][31] Two major surfactants,linear alkylbenzene sulfonates (LAS) and the alkyl phenolethoxylates (APE) break down underaerobic conditions found insewage treatment plants and in soil tononylphenol, which is thought to be anendocrine disruptor.[32][33] Interest in biodegradable surfactants has led to much interest in "biosurfactants" such as those derived from amino acids.[34] Biobased surfactants can offer improved biodegradation. However, whether surfactants damage the cells of fish or cause foam mountains on bodies of water depends primarily on their chemical structure and not on whether the carbon originally used came from fossil sources, carbon dioxide or biomass.[3]
^Rosen MJ, Kunjappu JT (2012).Surfactants and Interfacial Phenomena (4th ed.). Hoboken, New Jersey: John Wiley & Sons. p. 1.ISBN978-1-118-22902-6.Archived from the original on 8 January 2017.A surfactant (a contraction ofsurface-activeagent) is a substance that, when present at low concentration in a system, has the property of adsorbing onto the surfaces or interfaces of the system and of altering to a marked degree the surface or interfacial free energies of those surfaces (or interfaces).
^"surfactant".Oxford English Dictionary (Online ed.).Oxford University Press. (Subscription orparticipating institution membership required.) – "A new word, Surfactants, has been coined by Antara Products, General Aniline & Film Corporation, and has been presented to the chemical industry to cover all materials that have surface activity, including wetting agents, dispersants, emulsifiers, detergents and foaming agents."
^abcKurt Kosswig "Surfactants" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, 2005, Weinheim.doi:10.1002/14356007.a25_747
^Chiappisi, Leonardo (December 2017). "Polyoxyethylene alkyl ether carboxylic acids: An overview of a neglected class of surfactants with multiresponsive properties".Advances in Colloid and Interface Science.250:79–94.doi:10.1016/j.cis.2017.10.001.PMID29056232.
^Paria, Santanu (2008). "Surfactant-enhanced remediation of organic contaminated soil and water".Advances in Colloid and Interface Science.138 (1):24–58.doi:10.1016/j.cis.2007.11.001.PMID18154747.
^Percival, S.l.; Mayer, D.; Malone, M.; Swanson, T; Gibson, D.; Schultz, G. (2 November 2017). "Surfactants and their role in wound cleansing and biofilm management".Journal of Wound Care.26 (11):680–690.doi:10.12968/jowc.2017.26.11.680.ISSN0969-0700.PMID29131752.
^Mc Callion, O. N. M.; Taylor, K. M. G.; Thomas, M.; Taylor, A. J. (8 March 1996). "The influence of surface tension on aerosols produced by medical nebulisers".International Journal of Pharmaceutics.129 (1):123–136.doi:10.1016/0378-5173(95)04279-2.ISSN0378-5173.
^Maldonado-Valderrama, Julia; Wilde, Pete; MacIerzanka, Adam; MacKie, Alan (2011). "The role of bile salts in digestion".Advances in Colloid and Interface Science.165 (1):36–46.doi:10.1016/j.cis.2010.12.002.PMID21236400.
^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".Environ. Toxicol. Chem.27 (4):811–8.Bibcode:2008EnvTC..27..811M.doi:10.1897/07-216.1.PMID18333674.S2CID39914076.
^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.Bibcode:2005Chmsp..59..235M.doi:10.1016/j.chemosphere.2004.11.084.PMID15722095.
^Hernández-Soriano Mdel C, Peña A, Dolores Mingorance M (2010). "Release of metals from metal-amended soil treated with a sulfosuccinamate surfactant: effects of surfactant concentration, soil/solution ratio, and pH".J. Environ. Qual.39 (4):1298–305.Bibcode:2010JEnvQ..39.1298H.doi:10.2134/jeq2009.0242.PMID20830918.
