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Surfactants are compounds that lower the
surface tension Surface tension is the tendency of liquid surfaces to shrink into the minimum surface area possible. Surface tension is what allows heavier than water i.e., denser than water objects such as razor blades, insects (e.g. Gerridae, water striders ...

surface tension
(or interfacial tension) between two liquids, between a gas and a liquid, or between a liquid and a solid. Surfactants may act as
detergent A detergent is a surfactant or a mixture of surfactants with cleansing properties in dilute solutions. These substances are usually alkylbenzene sulfonates, a family of compounds that are similar to soap Soap is a salt (chemistry), salt ...
s,
wetting Wetting is the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. The degree of wetting (wettability) is determined by a force balance between adhesiv ...

wetting
agents,
emulsifiers An emulsion is a mixture In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavi ...
,
foaming agentA foaming agent is a material that facilitates the formation of foam foam bubbles Foam is an object formed by trapping pockets of gas in a liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that contin ...
s, or
dispersantA dispersant or a dispersing agent is a substance, typically a surfactant Surfactants are compounds that lower the surface tension (or interfacial tension) between two liquids, between a gas and a liquid, or between a liquid and a solid. Surfac ...
s. The word "surfactant" is a blend of ''surface-active agent'', coined . 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 is
salting out Salting out (also known as salt-induced precipitation, salt fractionation, anti-solvent crystallization, precipitation crystallization, or drowning out) is a purification technique that utilizes the reduced solubility of certain molecules in a sol ...
: by adding an inorganic salt to an aqueous solution of a weakly polar substance, the substance will precipitate. The substance may itself be a surfactant - this is one of the reasons why many surfactants are ineffective in sea water. diagram of a
micelle Image:Micelle scheme-en.svg, 250px, Scheme of a micelle formed by phospholipids in an aqueous solution A micelle () or micella () (plural micelles or micellae, respectively) is an aggregate (or supramolecular assembly) of surfactant phospholipid ...

micelle
 – the
lipophilicLipophilicity (from Greek language, Greek λίπος "fat" and :wikt:φίλος, φίλος "friendly"), refers to the ability of a chemical compound to dissolve in fats, oils, lipids, and non-polar solvents such as hexane or toluene. Such non-polar ...
tails of the surfactant ions remain inside the oil because they interact more strongly with oil than with water. The
polar Polar may refer to: Geography Polar may refer to: * Geographical pole, either of two fixed points on the surface of a rotating body or planet, at 90 degrees from the equator, based on the axis around which a body rotates *Polar climate, the clim ...
"heads" of the surfactant molecules coating the micelle interact more strongly with water, so they form a
hydrophilic A hydrophile is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral group of two or more atoms ...
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 typically
amphiphilic 250px, Cross-section view of the structures that can be formed by phospholipids, biological amphiphiles in aqueous solutions. Unlike this illustration, micelles are usually formed by non-biological, single-chain, amphiphiles, soaps or detergents, ...
in nature, meaning that micelles may be stable either as droplets of
aproticA polar aprotic solvent is a solvent that lacks an acidic proton and is polar. Such solvents lack hydroxyl and amine groups. These solvents do not serve as proton donors in hydrogen bonding, although they can be proton acceptors. Many solvents, inc ...
solvents such as oil in water, or as protic solvents such as water in oil. When the droplet is aprotic it is sometimes known as a reverse micelle.


Composition and structure

Surfactants are usually
organic compound , CH4; is among the simplest organic compounds. In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, ...
s that are
amphiphilic 250px, Cross-section view of the structures that can be formed by phospholipids, biological amphiphiles in aqueous solutions. Unlike this illustration, micelles are usually formed by non-biological, single-chain, amphiphiles, soaps or detergents, ...
, meaning they contain both
hydrophobic In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they ...
groups (their ''tails'') and
hydrophilic A hydrophile is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral group of two or more atoms ...
groups (their ''heads''). Therefore, a surfactant contains both a water-insoluble (or oil-soluble) component and a water-soluble component. Surfactants will diffuse in water and
adsorb of multilayer adsorption is a random distribution of molecules on the material surface. Adsorption is the adhesion Adhesion is the tendency of dissimilar Particle, particles or interface (matter), surfaces to cling to one another (Cohesion ...

adsorb
at
interfaces Interface or interfacing may refer to: Academic journals * Interface (journal), ''Interface'' (journal), by the Electrochemical Society * ''Interface, Journal of Applied Linguistics'', now merged with ''ITL International Journal of Applied Lin ...
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 the air or into the oil phase, while the water-soluble head group remains in the water phase. World production of surfactants is estimated at 15 million tons per year, of which about half are
soap 300px, The chemical structure of sodium laureth sulfate, a typical ingredient found in liquid soaps. Soap is a salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of Sa ...

soap
s. Other surfactants produced on a particularly large scale are linear
alkylbenzene sulfonates Alkylbenzene sulfonates are a class of anionic surfactants, consisting of a hydrophilic sulfonate head-group and a hydrophobic alkylbenzene tail-group. Along with sodium laureth sulfate they are one of the oldest and most widely used synthetic dete ...
(1.7 million tons/y),
lignin sulfonateLignosulfonates, or sulfonated lignin are water-soluble anionic polyelectrolyte polymers: they are byproducts from the production of wood pulp using sulfite process, sulfite pulping. Most delignification in sulfite pulping involves acidic cleavage o ...
s (600,000 tons/y),
fatty alcohol Fatty alcohols (or long-chain alcohols) are usually high-molecular-weight, straight-chain primary alcohols, but can also range from as few as 4–6 carbons to as many as 22–26, derived from natural fats and oils. The precise chain length varies w ...
ethoxylate Ethoxylation is a chemical reaction in which ethylene oxide adds to a substrate. It is the most widely practiced alkoxylation, which involves the addition of epoxides to substrates. In the usual application, alcohols and phenols are converted into ...
s (700,000 tons/y), and
alkylphenol Chemical structure of the alkylphenol nonylphenol Alkylphenols are a family of organic compounds obtained by the alkylation of phenol Phenol (also called carbolic acid) is an aromatic organic compound , CH4; is among the simplest organic comp ...
ethoxylate Ethoxylation is a chemical reaction in which ethylene oxide adds to a substrate. It is the most widely practiced alkoxylation, which involves the addition of epoxides to substrates. In the usual application, alcohols and phenols are converted into ...
s (500,000 tons/y).Kurt Kosswig "Surfactants" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, 2005, Weinheim.


Structure of surfactant phases in water

In the bulk aqueous phase, surfactants form aggregates, such as
micelles 250px, Scheme of a micelle formed by phospholipids in an aqueous solution A micelle () or micella () (plural micelles or micellae, respectively) is an aggregate (or supramolecular assembly) of surfactant phospholipid molecules dispersed in a li ...

micelles
, 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 or
lipid bilayer The lipid bilayer (or phospholipid bilayer) is a thin polar membrane A polarized membrane is a lipid bilayer, lipid membrane that has a positive electrical charge on one side and a negative charge on another side, which produces the resting pote ...
s. 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 the
hydrophilic-lipophilic balance The hydrophilic-lipophilic balance of a surfactant is a measure of the degree to which it is hydrophilic or lipophilic, determined by calculating values for the different regions of the molecule, as described by Griffin in 1949 and 1954. Other metho ...
(HLB). Surfactants reduce the
surface tension Surface tension is the tendency of liquid surfaces to shrink into the minimum surface area possible. Surface tension is what allows heavier than water i.e., denser than water objects such as razor blades, insects (e.g. Gerridae, water striders ...

surface tension
of water by
adsorbing
adsorbing
at the liquid-air interface. The relation that links the surface tension and the surface excess is known as the
Gibbs isothermThe Gibbs adsorption isotherm for multicomponent systems is an equation used to relate the changes in concentration of a component in contact with a surface with changes in the surface tension, which results in a corresponding change in surface energ ...
.


Dynamics of surfactants at interfaces

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 the
diffusion coefficient Diffusivity, mass diffusivity or diffusion coefficient is a proportionality constant between the molar flux due to molecular diffusion and the gradient in the concentration of the species (or the driving force for diffusion). Diffusivity is encou ...
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 to
steric Steric effects are nonbonding interactions that influence the shape (conformational isomerism, conformation) and chemical reaction, reactivity of ions and molecules. Steric effects complement electronic effects, which dictate the shape and reacti ...
or electrostatic repulsions. The surface 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 or spinning drop method. Dynamic surface tensions, i.e. surface tension as a function of time, can be obtained by the maximum bubble pressure apparatus The structure of surfactant layers can be studied by
ellipsometry Ellipsometry is an optical Optics is the branch of physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and ti ...

ellipsometry
or
X-ray reflectivityX-ray reflectivity (sometimes known as X-ray specular reflectivity, X-ray reflectometry, or XRR) is a surface-sensitive analytical technique used in chemistry Chemistry is the scientific discipline involved with Chemical element, elements and c ...
. 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.


In biology

300px, Phosphatidylcholine, found in lecithin, is a pervasive biological surfactant. Shown in – choline and phosphate group; – glycerol; – monounsaturated fatty acid; – saturated fatty acid. The human body produces diverse surfactants. Pulmonary surfactant is produced in the lungs in order to facilitate breathing by increasing total lung capacity, and lung compliance. In Respiratory distress syndrome, adult, respiratory distress syndrome or RDS, Pulmonary surfactant (medication), surfactant replacement therapy helps patients have normal respiration by using pharmaceutical forms of the surfactants. One example of pharmaceutical pulmonary surfactants is Survanta (beractant) or its generic form Beraksurf produced by AbbVie Inc., Abbvie and Tekzima respectively. Bile salts, a surfactant produced in the liver, play an important role in digestion.


Safety and environmental risks

Most anionic and non-ionic surfactants are non-toxic, having LD50 comparable to nacl, table salt. The toxicity of Quaternary ammonium cation, quaternary ammonium compounds, which are antibacterial and antifungal, varies. Dialkyldimethylammonium chlorides (DDAC, DSDMAC) used as fabric softeners have low LD50 (5 g/kg) and are essentially non-toxic, while the disinfectant alkylbenzyldimethylammonium chloride has an LD50 of 0.35 g/kg. Prolonged exposure to surfactants can irritate and damage the skin because surfactants disrupt the lipid membrane that protects skin and other cells. Skin irritancy generally increases in the series non-ionic, amphoteric, anionic, cationic surfactants. 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. Anionic surfactants can be found in soils as the result of sewage 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. In the case of the Deepwater Horizon oil spill, unprecedented amounts of Corexit 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 is dioctyl sodium sulfosuccinate (DOSS), sorbitan monooleate (Span 80), and polyoxyethylenated sorbitan monooleate (Polysorbate 80, Tween-80).


Biodegradation

Because of the volume of surfactants released into the environment, their biodegradation is of great interest. Strategies to enhance degradation include ozone treatment and biodegradation. Two major surfactants, linear alkylbenzene sulfonates (LAS) and the alkyl phenol
ethoxylate Ethoxylation is a chemical reaction in which ethylene oxide adds to a substrate. It is the most widely practiced alkoxylation, which involves the addition of epoxides to substrates. In the usual application, alcohols and phenols are converted into ...
s (APE) break down under wikt:aerobic, aerobic conditions found in sewage treatment plants and in soil to nonylphenol, which is thought to be an endocrine disruptor.Mergel, Maria. "Nonylphenol and Nonylphenol Ethoxylates." Toxipedia.org. N.p., 1 Nov. 2011. Web. 27 Apr. 2014. Interest in biodegradable surfactants has led to much interest in "biosurfactants" such as those derived from amino acids. Attracting much attention is the non-biodegradability of fluorosurfactant, e.g. perfluorooctanoic acid (PFOA).


Applications

The annual global production of surfactants was 13 million tonnes in 2008. In 2014, the world market for surfactants reached a volume of more than US $33 billion. Market researchers expect annual revenues to increase by 2.5% per year to around $40.4 billion until 2022. The commercially most significant type of surfactants is currently the anionic surfactant LAS, which is widely used in cleaners and detergents. Surfactants play an important role as cleaning,
wetting Wetting is the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. The degree of wetting (wettability) is determined by a force balance between adhesiv ...

wetting
, Dispersant, dispersing, Emulsifier, emulsifying, foaming agent, foaming and Defoamer, anti-foaming agents in many practical applications and products, including
detergent A detergent is a surfactant or a mixture of surfactants with cleansing properties in dilute solutions. These substances are usually alkylbenzene sulfonates, a family of compounds that are similar to soap Soap is a salt (chemistry), salt ...
s, fabric softeners, motor oils, emulsions,
soap 300px, The chemical structure of sodium laureth sulfate, a typical ingredient found in liquid soaps. Soap is a salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of Sa ...

soap
s, paints, adhesives, inks, anti-fogs, ski waxes, snowboard wax, deinking of recycled papers, in flotation, washing and enzymatic processes, and laxatives. Also agrochemical formulations such as herbicides (some), insecticides, biocides (sanitizers), and spermicides (nonoxynol-9). Personal care products such as cosmetics, shampoos, shower gel, hair conditioners, and toothpastes. Surfactants are used in firefighting and pipelines (liquid drag reducing agents). Alkali surfactant polymers are used to mobilize oil in oil 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 and via the application of medicinal lotions and sprays to surface of skin and mucous membranes.


Detergents in biochemistry and biotechnology

In solution, detergents help solubilize a variety of chemical species by dissociating aggregates and unfolding proteins. Popular surfactants in the biochemistry laboratory are sodium lauryl sulfate (SDS) and cetyl trimethylammonium bromide (CTAB). Detergents are key reagents to Liquid-liquid extraction, extract protein by lysis of the cells and tissues: They disorganize the membrane's
lipid bilayer The lipid bilayer (or phospholipid bilayer) is a thin polar membrane A polarized membrane is a lipid bilayer, lipid membrane that has a positive electrical charge on one side and a negative charge on another side, which produces the resting pote ...
(SDS, Triton X-100, Triton X-114, X-114, CHAPS detergent, CHAPS, Cholate, DOC, and NP-40), and solubilize proteins. Milder detergents such as n-Octyl beta-D-thioglucopyranoside, octyl thioglucoside, octyl glucoside or maltosides, dodecyl maltoside are used to solubilize membrane proteins such as enzymes and Receptor (biochemistry), receptors without Denaturation (biochemistry), denaturing them. Non-solubilized material is harvested by centrifugation or other means. For electrophoresis, for example, proteins are classically treated with SDS to denature the native protein structure, tertiary and quaternary structures, allowing the separation of proteins according to their molecular 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. Pulmonary surfactants are also naturally secreted by type II cells of the lung Pulmonary alveolus, alveoli in mammals.


Quantum dot preparation

Surfactants are used with quantum dots in order to manipulate the growth, assembly, and electrical properties of quantum dots, in addition to mediating reactions on their surfaces. Research is ongoing in how surfactants arrange on the surface of the quantum dots.


Surfactants in droplet-based microfluidics

Surfactants play an important role in droplet-based microfluidics in the stabilization of the droplets, and the prevention of the fusion of droplets during incubation.


Classification

The "tails" of most surfactants are fairly similar, consisting of a hydrocarbon chain, which can be branched, linear, or aromatic. Fluorosurfactants have fluorocarbon chains. Siloxane surfactants have siloxane chains. Many important surfactants include a polyether chain terminating in a highly
polar Polar may refer to: Geography Polar may refer to: * Geographical pole, either of two fixed points on the surface of a rotating body or planet, at 90 degrees from the equator, based on the axis around which a body rotates *Polar climate, the clim ...
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 be ''double-chained''. Most commonly, surfactants are classified according to polar head group. A non-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 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 Zwitterion, zwitterionic. Commonly encountered surfactants of each type include:


Anionic: sulfate, sulfonate, and phosphate, carboxylate derivatives

Anionic surfactants contain anionic functional groups at their head, such as organosulfate, sulfate, sulfonate, phosphate, and carboxylic acid, carboxylates. Prominent alkyl sulfates include ammonium lauryl sulfate, sodium lauryl sulfate (sodium dodecyl sulfate, SLS, or SDS), and the related alkyl-ether sulfates sodium laureth sulfate (sodium lauryl ether sulfate or SLES), and sodium myreth sulfate. Others include: *Docusate (dioctyl sodium sulfosuccinate) *perfluorooctanesulfonic acid, Perfluorooctanesulfonate (PFOS) *Perfluorobutanesulfonic acid, Perfluorobutanesulfonate * Alkyl-aryl ether phosphates * Alkyl ether phosphates Carboxylates are the most common surfactants and comprise the carboxylate salts (soaps), such as sodium stearate. More specialized species include sodium lauroyl sarcosinate and carboxylate-based fluorosurfactants such as perfluorononanoic acid, perfluorononanoate, Perfluorooctanoic acid, perfluorooctanoate (PFOA or PFO).


Cationic head groups

pH-dependent primary, secondary, or tertiary amines; primary and secondary amines become positively charged at pH < 10: octenidine dihydrochloride. Permanently charged quaternary ammonium salts: cetrimonium bromide (CTAB), cetylpyridinium chloride (CPC), benzalkonium chloride (BAC), benzethonium chloride (BZT), dimethyldioctadecylammonium chloride, and DODAB, dioctadecyldimethylammonium bromide (DODAB).


Zwitterionic surfactants

Zwitterionic (amphoteric) surfactants have both cationic and anionic centers attached to the same molecule. The cationic part is based on primary, secondary, or tertiary amines or quaternary ammonium cations. The anionic part can be more variable and include sulfonates, as in the hydroxysultaine, sultaines CHAPS detergent, CHAPS (3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate) and cocamidopropyl hydroxysultaine. Betaines such as cocamidopropyl betaine have a carboxylate with the ammonium. The most common biological zwitterionic surfactants have a phosphate anion with an amine or ammonium, such as the phospholipids phosphatidylcholine, phosphatidylserine, phosphatidylethanolamine, phosphatidylcholine, and sphingomyelins. Lauryldimethylamine oxide and myristamine oxide are two commonly used zwitterionic surfactants of the tertiary amine oxides structural type.


Non-ionic

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 of hydrogen 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, biodegradation, biodegradability) or permission for use in food.


Ethoxylates


= Fatty alcohol ethoxylates

= * Narrow-range ethoxylate * Octaethylene glycol monododecyl ether * Pentaethylene glycol monododecyl ether


= Alkylphenol ethoxylates (APEs or APEOs)

= * Nonoxynols * Triton X-100


= Fatty acid ethoxylates

= 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.


= Special ethoxylated fatty esters and oils

=


= Ethoxylated amines and/or fatty acid amides

= * Polyethoxylated tallow amine * Cocamide monoethanolamine * Cocamide diethanolamine


= Terminally blocked ethoxylates

= * Poloxamers


Fatty acid esters of polyhydroxy compounds


= Fatty acid esters of glycerol

= * Glycerol monostearate * Glycerol monolaurate


= Fatty acid esters of sorbitol

= Sorbitan#Esters, Spans: * Sorbitan monolaurate * Sorbitan monostearate * Sorbitan tristearate Polysorbate, Tweens: * Tween 20 * Tween 40 * Tween 60 * Tween 80


= Fatty acid esters of sucrose

=


= Alkyl polyglucosides

= * Decyl glucoside * Lauryl glucoside * Octyl glucoside


See also

* * * * * , an assay that indicates Ion#Anions and cations, anionic surfactants in water with a bluing reaction. * * * *Surfactant leaching


References


External links

* {{authority control Surfactants, Bioremediation Biotechnology Cleaning product components Colloidal chemistry Environmental terminology Underwater diving physics