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).
Asurfactant is achemical compound that decreases 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 areemulsifiers, enabling water and oil to mix. They can also 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. Some 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, and other biogenic material.[3]
Surfactants are 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, often consisting of ahydrocarbon chain (linear or branched) and may comprise aromatic units. Most commonly, surfactants are classified according to the polarity of their 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.
Surfactant classification according to the composition of their head: non-ionic, anionic, cationic, amphoteric.
However, surfactants may also be classified based on chemical structure or based on their properties / their application.
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]
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, see alsopoloxamers.
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]
Alkyl polyglycosides (APGs) are a class of non-ionic surfactants made from asugar (likeglucose) and afatty alcohol. They are produced from renewable resources, possess a high biodegradability and mildness. For these reasons, they are widely used in detergents,cosmetics, and other applications.[8][9]
Surfactant molecules have either one tail or two; those with two tails are said to bedouble-chained.[10]
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.[11]
Surfactant-oil droplets in water. The oil is dispersed in water and the surfactant is at the oil-water interface.
Surfactant molecules at the water surface (= air water interface)
Solid and hydrophobic particles (e. g. dirt) are removed from a solid (e. g. a fabric) with the aid of a surfactant (e. g. during washing in the washing machine)
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 areamphiphilic, which means that this molecule each contains ahydrophilic "water-seeking" group (thehead), and ahydrophobic "water-avoiding" group (thetail).[13] 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.
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 are widely used due to their ability to modify surface and interfacial properties, making them relevant in processes involving the interaction of hydrophobic and hydrophilic substances. Their amphiphilic nature—containing both hydrophilic and hydrophobic parts—enables them to bridge these otherwise immiscible components, thereby facilitating mixing and enhancing the efficiency of various physical and chemical transformations. This makes surfactants useful in numerous fields where control over interfacial interactions is relevant.
The alkalization (saponification) of cocoa fat in drinking cocoa powder serves to reduce the surface tension of the milk and to enable faster wetting or suspension of the semi-fat cocoa powder.
Surfactants are used indetergents,washing-up liquids,shampoos,shower gels, and similar products to increase the “solubility” of fat and dirt particles that adhere to laundry or the body in water.
Fabric softeners can consist of cationic surfactants that prevent laundry from becoming stiff when dry.
Emulsifiers are essential for producingwater-in-oil emulsions, e.g. forskin creams. They are also necessary for a wide range of suspensions to maintain liquid drug formulations.
Plant protection products contain surfactants to improve wetting (spreading) on plants. The most common wetting agent is ethoxylated tallow amine. Trisiloxanes or polyoxyethylated fatty alcohols are also used.[14] Agrochemical formulations that use surfactants includeherbicides (some),insecticides,biocides (sanitizers).[15]
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[16] and via the application of medicinal lotions and sprays to surface of skin and mucous membranes.[17] Surfactants enhance remediation via soil washing, bioremediation, and phytoremediation.[18]
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.[19]Pulmonary surfactants are also naturally secreted by type II cells of the lungalveoli inmammals.
Surfactants have a specific application in plastics technology. Aqueous surfactant solutions are used to test the susceptibility of polymer materials to stress cracking. Surfactants are also used to shorten the failure time of long-term tests, particularly in crack growth tests on polyethylene. Wetting agents are employed in the full notch creep test for testing polyethylene pipelines.
Ionic surfactants also function as external antistatic agents to prevent electrostatic charging of plastic surfaces (ESD protection). Both anionic and cationic surfactants are used for this purpose[9].
Perfluorinated surfactants, such as fluorotelomer alcohols (FTOH), are used as coating agents for textiles, carpets, and construction products to impart or enhance water and grease repellency. As members of the PFC group, however, they are subject to criticism because they are persistent and practically non-degradable in nature.
Surfactants are employed in water-mixed cooling lubricants (water-in-oil emulsions) to provide effective cooling and lubrication during metal cutting operations.
Surfactants regulate the consistency of ink in inkjet printers. An insufficient amount of surfactants results in clumping of the color pigments, whereas an excessive amount renders the ink overly fluid during printing.
Inpaper recycling, surfactants facilitate the detachment of ink particles from paper fibers (deinking) and assist in transporting the ink to the surface.
Alkali surfactant polymers are used to mobilize oil inoil wells. Surfactants also play a key role infroth flotation processes for separating copper and other minerals fromores.
Surfactants are used infirefighting (to make "wet water" that more quickly soaks into flammable materials[20][21]) and pipelines (liquid drag reducing agents).
"Wet water" provides the advantage of allowing the extinguishing water to penetrate burning materials such as wood or fabric more effectively, thereby enhancing its cooling capacity. Additionally, extinguishing water mixed with surface-active agents can be sprayed over greater distances at the same pumping capacity due to their flow-improving properties. Special foaming agents (Aqueous Film Forming Foam, AFFF) for combating liquid fires contain perfluorinated surfactants that form a gas-tight liquid film between the burning material and the foam. This simultaneously imparts superior sliding properties to the foam blanket, thereby enabling the effective extinguishment of larger liquid fires.
Surfactants play an important role indroplet-based microfluidics in the stabilization of the droplets, and the prevention of the fusion of droplets during incubation.[22]
Certain caterpillars (of the moth speciesSpodoptera exigua, South East Asia) spit a surfactant-containing secretion at predators. This deters attacking ants, allowing the caterpillars to escape. The surfactants in the caterpillars' oral secretions reduce its surface tension. Instead of rolling off the ants' water-repellent skin like normal water, the secretion soaks the attackers. The affected ants then clean themselves, which gives the caterpillar enough time to escape.[24]
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.[25][26][27]
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.[28][29]
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).[30][31]
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).[32] Strategies to enhance degradation includeozone treatment and biodegradation.[33][34] 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.[35][36] Interest in biodegradable surfactants has led to much interest in "biosurfactants" such as those derived from amino acids.[37] 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, M. J.; Kunjappu, J. T. (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).
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^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, T. L.; Dillon, P. J.; Metcalfe, C. D. (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.Bibcode:2008EnvTC..27..811M.doi:10.1897/07-216.1.PMID18333674.S2CID39914076.
^Murphy, M. G.; Al-Khalidi, M.; Crocker, J. F.; Lee, S. H.; O'Regan, P.; Acott, P. D. (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, Maria del Carmen; Degryse, Fien; Smolders, Erik (March 2011). "Mechanisms of enhanced mobilisation of trace metals by anionic surfactants in soil".Environmental Pollution.159 (3):809–16.Bibcode:2011EPoll.159..809H.doi:10.1016/j.envpol.2010.11.009.PMID21163562.
^Hernández-Soriano, Maria del Carmen; Peña, Aránzazu; Dolores Mingorance, Ma (2010). "Release of metals from metal-amended soil treated with a sulfosuccinamate surfactant: effects of surfactant concentration, soil/solution ratio, and pH".Journal of Environmental Quality.39 (4):1298–305.Bibcode:2010JEnvQ..39.1298H.doi:10.2134/jeq2009.0242.PMID20830918.
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