Surfactant

Substance that lowers the surface tension between a liquid and another material

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Schematic diagram of a micelle of oil in aqueous suspension, such as might occur in an emulsion 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 are chemical compounds that decrease the surface tension or interfacial tension between two liquids, a liquid and a gas, or a liquid and a solid. The word "surfactant" is a blend of surface-active agent, coined around 1950. Because they have both hydrophilic and hydrophobic parts, they enable water and oil to mix, form foam, and help in removing dirt.

Surfactants are among the most widely used and commercially significant chemicals. They are employed extensively in households and various industries as detergents and cleaning agents, and also as emulsifiers, wetting agents, foaming agents, antistatic agents, and dispersants.

Surfactants occur naturally in traditional plant-based detergents like horse chestnuts or soap nuts, and even in some caterpillar secretions. Today, the most commonly utilized surfactants, particularly anionic linear alkylbenzene sulfates (LAS), are derived from petroleum products. However, there is a growing trend to produce surfactants from renewable biomass, such as sugar, fatty alcohol from vegetable oils, biofuel by-products, or other biogenic materials.

Classification

Most surfactants are organic compounds with hydrophilic "heads" and hydrophobic "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 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 anionic group. The polyether groups often consist of ethoxylated (polyethylene oxide-like) sequences that enhance the hydrophilic character of a surfactant. In contrast, polypropylene oxides may be inserted to amplify the lipophilic character of a surfactant.

Surfactant molecules can have either one tail or two; those with two tails are known as double-chained.

Surfactant classification according to the composition of their head: non-ionic, anionic, cationic, amphoteric.

Most commonly, surfactants are classified according to their 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 specifically termed 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 or amphoteric. Common examples of each type include:

Anionic: sulfate, sulfonate, and phosphate, carboxylate derivatives

Anionic surfactants contain anionic functional groups at their head, such as sulfate, sulfonate, phosphate, and 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:

• Alkylbenzene sulfonates
• Docusate (dioctyl sodium sulfosuccinate)
• Perfluorooctanesulfonate (PFOS)
• 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 perfluorononanoate, perfluorooctanoate (PFOA or PFO).

Cationic head groups

pH-dependent primary, secondary, or tertiary amines; primary and secondary amines become positively charged at a pH < 10, such as octenidine dihydrochloride.

Permanently charged quaternary ammonium salts: cetrimonium bromide (CTAB), cetylpyridinium chloride (CPC), benzalkonium chloride (BAC), benzethonium chloride (BZT), dimethyldioctadecylammonium chloride, and dioctadecyldimethylammonium bromide (DODAB).

Zwitterionic surfactants

Zwitterionic (ampholytic) 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 sultaines 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 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 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, biodegradability) or permission for use in food.

Ethoxylates

Fatty alcohol ethoxylates

Alkylphenol ethoxylates (APEs or APEOs)

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

Terminally blocked ethoxylates

Fatty acid esters of polyhydroxy compounds

Fatty acid esters of glycerol

Fatty acid esters of sorbitol

Spans:

Tweens:

Fatty acid esters of sucrose

Alkyl polyglucosides

Other classifications

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• Amino acid-based surfactants are surfactants derived from an amino 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.
• Gemini surfactants consist of two surfactant molecules linked together at or near their head groups. Compared to monomeric surfactants, they have much lower critical micelle concentrations.

Composition and structure

Schematic diagram of a micelle – the lipophilic tails of the surfactant ions remain inside the oil because they interact more strongly with oil than with water. The polar "heads" of the surfactant molecules coating the micelle interact more strongly with water, so they form a hydrophilic 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 in nature, meaning that micelles may be stable either as droplets of aprotic 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 usually organic compounds that are akin to amphiphilic, which means that this molecule, being as double-agent, each contains a hydrophilic "water-seeking" group (the head), and a hydrophobic "water-avoiding" group (the tail). As a result, a surfactant contains both a water-soluble component and a water-insoluble component. Surfactants diffuse in water and get adsorbed at interfaces 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 either lipophilic ("oil-seeking") or lipophobic ("oil-avoiding") depending on its chemistry. Hydrocarbon groups are usually lipophilic, for use in soaps and detergents, while fluorocarbon groups are lipophobic, for use in repelling stains or reducing surface tension.

World production of surfactants is estimated at 15 million tons per year, of which about half are soaps. Other surfactants produced on a particularly large scale are linear alkylbenzene sulfonates (1.7 million tons/y), lignin sulfonates (600,000 tons/y), fatty alcohol ethoxylates (700,000 tons/y), and alkylphenol ethoxylates (500,000 tons/y).

Sodium stearate, the most common component of most soap, which comprises about 50% of commercial surfactants.

4-(5-Dodecyl) benzenesulfonate, a linear dodecylbenzenesulfonate, one of the most common surfactants.

Structure of surfactant phases in water

In the bulk aqueous phase, surfactants form aggregates, such as 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 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 the hydrophilic-lipophilic balance (HLB). Surfactants reduce the surface tension of water by adsorbing at the liquid-air interface. The relation that links the surface tension and the surface excess is known as the Gibbs isotherm.

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 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 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 or X-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

Surfactants play an important role as cleaning, wetting, dispersing, emulsifying, foaming and anti-foaming agents in many practical applications and products, including detergents, fabric softeners, motor oils, emulsions, soaps, 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 (to make "wet water" that more quickly soaks into flammable materials) 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. Surfactants enhance remediation via soil washing, bioremediation, and phytoremediation.

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 extract protein by lysis of the cells and tissues: They disorganize the membrane's lipid bilayer (SDS, Triton X-100, X-114, CHAPS, DOC, and NP-40), and solubilize proteins. Milder detergents such as octyl thioglucoside, octyl glucoside or dodecyl maltoside are used to solubilize membrane proteins such as enzymes and receptors without 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 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 alveoli in mammals.

Quantum dot preparation

Surfactants are used with quantum dots in order to manipulate their growth, 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.

Surfactants in droplet-based microfluidics

Surfactants play an important role in