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Surfactant

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Schematicdiagram of amicelleof oil in aqueous suspension, such as might occur in anemulsionof 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).

Surfactantsarechemical compoundsthat decrease thesurface tensionor interfacial tension between twoliquids,a liquid and agas,or a liquid and asolid.The word "surfactant" is ablendofsurface-activeagent,[1]coinedc. 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 are among the most widespread and commercially important chemicals. Private households as well as many industries use them in large quantities asdetergents and cleaning agents,but also for example asemulsifiers,wettingagents,foaming agents,antistaticadditives, ordispersants.

Surfactants occur naturally in traditional plant-based detergents, e.g.horse chestnutsorsoap nuts;they can also be found in the secretions of some caterpillars. Today the most commonly used surfactants, above all anionic linear alkylbenzene sulfates (LAS), are produced frompetroleum products.However, surfactants are (again) 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]

Classification

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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 ahydrocarbonchain, which can be branched, linear, or aromatic.Fluorosurfactantshavefluorocarbonchains.Siloxane surfactantshavesiloxanechains.

Many important surfactants include a polyether chain terminating in a highlypolaranionic group. The polyether groups often comprise ethoxylated (polyethylene oxide-like) sequences inserted to increase the hydrophilic character of a surfactant.Polypropylene oxidesconversely, 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-ionicsurfactant 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

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Anionicsurfactants contain anionic functional groups at their head, such assulfate,sulfonate,phosphate,andcarboxylates. Prominent alkyl sulfates includeammonium lauryl sulfate,sodium lauryl sulfate(sodium dodecyl sulfate, SLS, or SDS), and the related alkyl-ether sulfatessodium laureth sulfate(sodium lauryl ether sulfate or SLES), andsodium myreth sulfate.

Others include:

Carboxylates are the most common surfactants and comprise the carboxylate salts (soaps), such assodium stearate.More specialized species includesodium lauroyl sarcosinateand carboxylate-based fluorosurfactants such asperfluorononanoate,perfluorooctanoate(PFOA or PFO).

Cationic head groups

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pH-dependent primary, secondary, or tertiaryamines;primary and secondary amines become positively charged at pH < 10:[5]octenidine dihydrochloride.

Permanently chargedquaternary ammonium salts:cetrimonium bromide(CTAB),cetylpyridinium chloride(CPC),benzalkonium chloride(BAC),benzethonium chloride(BZT),dimethyldioctadecylammonium chloride,anddioctadecyldimethylammonium bromide(DODAB).

Zwitterionic surfactants

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Zwitterionic(ampholytic) surfactants have both cationic and anionic centers attached to the same molecule. The cationic part is based on primary, secondary, or tertiaryaminesor quaternary ammonium cations. The anionic part can be more variable and include sulfonates, as in thesultainesCHAPS(3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate) andcocamidopropyl hydroxysultaine.Betainessuch ascocamidopropyl betainehave a carboxylate with the ammonium. The most common biological zwitterionic surfactants have a phosphate anion with an amine or ammonium, such as thephospholipidsphosphatidylserine,phosphatidylethanolamine,phosphatidylcholine,andsphingomyelins.

Lauryldimethylamine oxideandmyristamine oxideare two commonly used zwitterionic surfactants of the tertiaryamine oxidesstructural type.

Non-ionic

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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]

Ethoxylates

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Fatty alcohol ethoxylates
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Alkylphenol ethoxylates (APEs or APEOs)
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Fatty acid ethoxylates
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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]

Special ethoxylated fatty esters and oils
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Ethoxylated amines and/or fatty acid amides
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Terminally blocked ethoxylates
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Fatty acid esters of polyhydroxy compounds

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Fatty acid esters of glycerol
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Fatty acid esters of sorbitol
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Spans:

Tweens:

Fatty acid esters of sucrose
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Alkyl polyglucosides
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Main article:Alkyl polyglycoside

Other classifications

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Gemini amino acid-based surfactant (based oncysteine)

Composition and structure

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Schematicdiagram of amicelle– thelipophilictails 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 ahydrophilicouter 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 typicallyamphiphilicin nature, meaning that micelles may be stable either as droplets ofaproticsolvents 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 compoundsthat 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 getadsorbedatinterfacesbetween 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.Hydrocarbongroups are usually lipophilic, for use in soaps and detergents, whilefluorocarbongroups are lipophobic, for use inrepelling stainsor reducing surface tension.

World production of surfactants is estimated at 15 million tons per year, of which about half aresoaps.Other surfactants produced on a particularly large scale are linearalkylbenzene sulfonates(1.7 million tons/y),lignin sulfonates(600,000 tons/y),fatty alcoholethoxylates(700,000 tons/y), andalkylphenolethoxylates(500,000 tons/y).[6]

Sodium stearate, the most common component of most soap, which comprises about 50% of commercial surfactants
Sodium dodecylbenzenesulfonate
4-(5-Dodecyl) benzenesulfonate, a linear dodecylbenzenesulfonate, one of the most common surfactants

Structure of surfactant phases in water

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Main article:Wetting solution

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 tensionof water byadsorbingat the liquid-air interface. The relation that links the surface tension and the surface excess is known as theGibbs isotherm.

Dynamics of surfactants at interfaces

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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 coefficientof 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 tostericorelectrostatic repulsions. Thesurface rheologyof 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

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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

The structure of surfactant layers can be studied byellipsometryorX-ray reflectivity.

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.

Applications

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Surfactants play an important role as cleaning,wetting,dispersing,emulsifying,foamingandanti-foamingagents in many practical applications and products, includingdetergents,fabric softeners,motor oils,emulsions,soaps,paints,adhesives,inks,anti-fogs,ski waxes,snowboard wax,deinkingofrecycled papers,in flotation, washing and enzymatic processes, andlaxatives.Also agrochemical formulations such asherbicides(some),insecticides,biocides(sanitizers), andspermicides(nonoxynol-9).[10]Personal care products such ascosmetics,shampoos,shower gel,hair conditioners,andtoothpastes.Surfactants are used infirefighting(to make "wet water" that more quickly soaks into flammable materials[11][12]) and pipelines (liquid drag reducing agents). Alkali surfactant polymers are used to mobilize oil inoil wells.

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 in biochemistry and biotechnology

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In solution, detergents help solubilize a variety of chemical species by dissociating aggregates and unfolding proteins. Popular surfactants in the biochemistry laboratory aresodium lauryl sulfate(SDS) andcetyl trimethylammonium bromide(CTAB). Detergents are key reagents toextractprotein by lysis of the cells and tissues: They disorganize the membrane'slipid bilayer(SDS,Triton X-100,X-114,CHAPS,DOC,andNP-40), and solubilize proteins. Milder detergents such asoctyl thioglucoside,octyl glucosideordodecyl maltosideare used to solubilize membrane proteins such asenzymesandreceptorswithoutdenaturingthem. Non-solubilized material is harvested by centrifugation or other means. Forelectrophoresis,for example, proteins are classically treated with SDS to denature the nativetertiary and quaternary structures,allowing the separation of proteins according to theirmolecular weight.

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 surfactantsare also naturally secreted by type II cells of the lungalveoliinmammals.

Quantum dot preparation

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Surfactants are used withquantum dotsin 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 in droplet-based microfluidics

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Surfactants play an important role indroplet-based microfluidicsin the stabilization of the droplets, and the prevention of the fusion of droplets during incubation.[19]

Heterogeneous catalysis

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Janus-type material is used as a surfactant-like heterogeneous catalyst for the synthesis of adipic acid.[20]

Increased surface tension

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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.

In biology

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Further information:Pulmonary surfactant
Phosphatidylcholine,found in lecithin, is a pervasive biological surfactant. Shown inred–cholineandphosphategroup;black–glycerol;green–monounsaturated fatty acid;blue–saturated fatty acid.

The human body produces diverse surfactants.Pulmonary surfactantis produced in thelungsin order to facilitate breathing by increasingtotal lung capacity,andlung compliance.Inrespiratory distress syndromeor RDS,surfactant replacementtherapy helps patients have normal respiration by using pharmaceutical forms of the surfactants. One example of a pharmaceutical pulmonary surfactant is Survanta (beractant) or its generic form Beraksurf, produced byAbbvieandTekzimarespectively.Bile salts,a surfactant produced in the liver, play an important role in digestion.[21]

Safety and environmental risks

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Most anionic and non-ionic surfactants are non-toxic, havingLD50comparable totable salt.The toxicity ofquaternary ammonium compounds,which areantibacterialandantifungal,varies. Dialkyldimethylammonium chlorides (DDAC,DSDMAC) used asfabric softenershave high LD50 (5 g/kg) and are essentially non-toxic, while thedisinfectantalkylbenzyldimethylammonium chloride has an LD50 of 0.35 g/kg. Prolonged exposure to surfactants can irritate and damage the skin because surfactants disrupt thelipid membranethat 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 sludgeapplication, 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 ofCorexitwere 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]

Biodegradation

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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 includeozonetreatment and biodegradation.[30][31]Two major surfactants,linear alkylbenzene sulfonates(LAS) and the alkyl phenolethoxylates(APE) break down underaerobicconditions found insewage treatmentplants 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]

See also

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Look upsurfactantin Wiktionary, the free dictionary.
  • Anti-fog– Chemicals that prevent the condensation of water as small droplets on a surface
  • Cleavable detergent– class of chemical compounds
  • Disodium cocoamphodiacetate– mixture of chemicals used as a surfactant
  • Emulsion– Mixture of two or more immiscible liquids
  • Hydrotrope– chemical substance
  • MBAS assay– Scientific testing method, anassaythat indicatesanionicsurfactants in water with a bluing reaction.
  • Niosome– Non-ionic surfactant-based vesicle
  • Oil dispersants– Mixture of emulsifiers and solvents used to treat oil spills
  • Surfactants in paint
  • Surfactant leaching

References

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  20. ^Vafaeezadeh, Majid; Wilhelm, Christian; Breuninger, Paul; Ernst, Stefan; Antonyuk, Sergiy; Thiel, Werner R. (20 May 2020)."A Janus-type Heterogeneous Surfactant for Adipic Acid Synthesis".ChemCatChem.12(10): 2695–2701.doi:10.1002/cctc.202000140.ISSN1867-3880.
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  25. ^Hernández-Soriano Mdel C, Degryse F, Smolders E (March 2011). "Mechanisms of enhanced mobilisation of trace metals by anionic surfactants in soil".Environ. Pollut.159(3): 809–16.doi:10.1016/j.envpol.2010.11.009.PMID21163562.
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