Surfactant

Surfactants are compounds that lower the surface tension of a liquid, the interfacial tension between two liquids, or that between a liquid and a solid. Surfactants may act as detergents, wetting agents, emulsifiers, foaming agents, and dispersants.

Etymology
The term surfactant is a blend of surface active agents.

In Index Medicus and the United States National Library of Medicine, surfactant is reserved for the meaning pulmonary surfactant. For the more general meaning, surface active agent is the heading.

Properties


Surfactants are usually organic compounds that are amphiphilic, meaning they contain both hydrophobic groups (their tails) and hydrophilic groups (their heads). Therefore, a surfactant molecule contains both a water insoluble (or oil soluble) component and a water soluble component. Surfactant molecules will migrate to the water surface, where the insoluble hydrophobic group may extend out of the bulk water phase, either into the air or, if water is mixed with an oil, into the oil phase, while the water soluble head group remains in the water phase. This alignment and aggregation of surfactant molecules at the surface acts to alter the surface properties of water at the water/air or water/oil interface. Because air is not hydrophillic, surfactants are also foaming agents to varying degrees. Completely non-polar solvents known as degreasers can also remove hydrophobic contaminants, but lacking polar elements may not dissolve in water.

Surfactants reduce the surface tension of water by adsorbing at the liquid-gas interface. They also reduce the interfacial tension their tails form a core that can encapsulate an oil droplet, and their (ionic/polar) heads form an outer shell that maintains favorable contact with water. When surfactants assemble in oil, the aggregate is referred to as a reverse micelle. In a reverse micelle, the heads are in the core and the tails maintain favorable contact with oil. Surfactants are also often classified into four primary groups; anionic, cationic, non-ionic, and zwitterionic (dual charge).

Thermodynamics of the surfactant systems are of great importance, theoretically and practically. This is because surfactant systems represent systems between ordered and disordered states of matter. Surfactant solutions may contain an ordered phase (micelles) and a disordered phase (free surfactant molecules and/or ions in the solution).

The spectroscopy and analytical chemistry of individual surfactant compounds is given in multiple references, including a multi-volume analysis reference textbook

Applications and sources
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
 * Emulsions
 * Paints
 * Adhesives
 * Inks
 * Anti-fogs
 * Ski waxes, snowboard wax
 * Deinking of recycled papers, in flotation, washing and enzymatic processes
 * Laxatives
 * Agrochemical formulations
 * Herbicides (some)
 * Insecticides
 * Quantum dot coatings
 * Biocides (sanitizers)
 * Cosmetics:
 * Shampoos
 * Hair conditioners (after shampoo)
 * Toothpastes
 * Spermicides (nonoxynol-9)
 * Firefighting
 * Pipelines, liquid drag reducing agent
 * Alkali Surfactant Polymers (used to mobilize oil in oil wells)
 * Ferrofluids
 * Leak Detectors

Detergents in biochemistry and biotechnology
In solution: detergents help solubilize molecules by dissociating aggregates and unfolding proteins. These include SDS, CTAB. Detergents are key reagents to extract protein by lysis of the cells and tissues: they disorganize the membrane's lipidic bilayer (SDS, Triton X-100, X-114, CHAPS, DOC, NP-40), and solubilize proteins. Milder detergents such as (OctylThioGlucosides) are used to solubilize sensible proteins (enzymes, receptors). 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.

According to the composition of their tail
The tail of surfactants can be:
 * A hydrocarbon chain: aromatic hydrocarbons (arenes), alkanes (alkyl), alkenes, cycloalkanes, alkyne-based;
 * An alkyl ether chain:
 * Ethoxylated surfactants: polyethylene oxides are inserted to increase the hydrophilic character of a surfactant;
 * Propoxylated surfactants: polypropylene oxides are inserted to increase the lipophilic character of a surfactant;
 * A fluorocarbon chain: fluorosurfactants;
 * A siloxane chain: siloxane surfactants

A surfactant can have one or two tails, these are called double-chained.

According to the composition of their head


A surfactant can be classified by the presence of formally charged groups in its head. A non-ionic surfactant has no charge groups in its head. The head of an ionic surfactant carries a net 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 zwitterionic.

Some commonly encountered surfactants of each type include:


 * Ionic
 * Anionic: based on permanent anions (sulfate, sulfonate, phosphate) or pH-dependent anions (carboxylate):
 * Sulfates:
 * Alkyl sulfates: ammonium lauryl sulfate, sodium lauryl sulfate (SDS, sodium dodecyl sulfate, another name for the compound);
 * Alkyl ether sulfates: sodium laureth sulfate, also known as sodium lauryl ether sulfate (SLES), sodium myreth sulfate;
 * Sulfonates:
 * Docusates: dioctyl sodium sulfosuccinate;
 * Sulfonate fluorosurfactants: perfluorooctanesulfonate (PFOS), perfluorobutanesulfonate;
 * Alkyl benzene sulfonates;
 * Phosphates:
 * Alkyl aryl ether phosphate
 * Alkyl ether phosphate
 * Carboxylates:
 * Alkyl carboxylates: Fatty acid salts (soaps): sodium stearate;
 * Sodium lauroyl sarcosinate;
 * Carboxylate fluorosurfactants: perfluorononanoate, perfluorooctanoate (PFOA or PFO)


 * Cationic: based on:
 * pH-dependent primary, secondary or tertiary amines: primary amines become positively charged at pH < 10, secondary amines become charged at pH < 4:
 * Octenidine dihydrochloride;
 * Permanently charged quaternary ammonium cation:
 * Alkyltrimethylammonium salts: cetyl trimethylammonium bromide (CTAB) a.k.a. hexadecyl trimethyl ammonium bromide, cetyl trimethylammonium chloride (CTAC);
 * Cetylpyridinium chloride (CPC);
 * Polyethoxylated tallow amine (POEA);
 * Benzalkonium chloride (BAC);
 * Benzethonium chloride (BZT);
 * 5-Bromo-5-nitro-1,3-dioxane;
 * Dimethyldioctadecylammonium chloride
 * Dioctadecyldimethylammonium bromide (DODAB)


 * Zwitterionic (amphoteric): based on primary, secondary or tertiary amines or quaternary ammonium cation with:
 * Sulfonates:
 * CHAPS (3-[(3-Cholamidopropyl)dimethylammonio]-1-propanesulfonate);
 * Sultaines: cocamidopropyl hydroxysultaine;
 * Carboxylates:
 * Amino acids
 * Imino acids
 * Betaines: cocamidopropyl betaine;
 * Phosphates: lecithin


 * Nonionic
 * Fatty alcohols:
 * Cetyl alcohol,
 * Stearyl alcohol,
 * Cetostearyl alcohol (consisting predominantly of cetyl and stearyl alcohols),
 * Oleyl alcohol;
 * Polyoxyethylene glycol alkyl ethers (Brij): CH3–(CH2)10–16–(O-C2H4)1–25–OH:
 * Octaethylene glycol monododecyl ether,
 * Pentaethylene glycol monododecyl ether;
 * Polyoxypropylene glycol alkyl ethers: CH3–(CH2)10–16–(O-C3H6)1–25–OH;
 * Glucoside alkyl ethers: CH3–(CH2)10–16–(O-Glucoside)1–3–OH:
 * Decyl glucoside,
 * Lauryl glucoside,
 * Octyl glucoside;
 * Polyoxyethylene glycol octylphenol ethers: C8H17–(C6H4)–(O-C2H4)1–25–OH:
 * Triton X-100;
 * Polyoxyethylene glycol alkylphenol ethers: C9H19–(C6H4)–(O-C2H4)1–25–OH:
 * Nonoxynol-9;
 * Glycerol alkyl esters:
 * Glyceryl laurate
 * Polyoxyethylene glycol sorbitan alkyl esters: Polysorbates;
 * Sorbitan alkyl esters: Spans;
 * Cocamide MEA, cocamide DEA;
 * Dodecyldimethylamine oxide;
 * Block copolymers of polyethylene glycol and polypropylene glycol: Poloxamers

According to the composition of their counter-ion
In the case of ionic surfactants, the counter-ion can be:
 * Monoatomic / Inorganic:
 * Cations: metals : alkali metal, alkaline earth metal, transition metal;
 * Anions: halides: chloride (Cl−), bromide (Br−), iodide (I−);
 * Polyatomic / Organic:
 * Cations: ammonium, pyridinium, triethanolamine (TEA)
 * Anions: tosyls, trifluoromethanesulfonates, methylsulfate

Current market
The annual global production of surfactants was 13 million metric tons in 2008 and the annual turnover reached US$24.33 billion in 2009, nearly 2% up from the previous year. The market is expected to experience quite healthy growth by 2.8% annually to 2012 and by 3.5 - 4% thereafter.

Health and environmental controversy
Some surfactants are known to be toxic to animals, ecosystems and humans, and can increase the diffusion of other environmental contaminants. Despite this, they 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. Some surfactants have proposed or voluntary restrictions on their use. For example, PFOS is a persistent organic pollutant as judged by the Stockholm Convention. Additionally, PFOA has been subject to a voluntary agreement by the U.S. Environmental Protection Agency‎ and eight chemical companies to reduce and eliminate emissions of the chemical and its precursors.

The two major surfactants used in the year 2000 were linear alkylbenzene sulphonates (LAS) and the alkyl phenol ethoxylates (APE). They break down in the aerobic conditions found in sewage treatment plants and in soil.

Ordinary dishwashing detergent, for example, will promote water penetration in soil, but the effect would only last a few days (many standard laundry detergent powders contain levels of chemicals such as alkali and chelating agents, which can be damaging to plants and should not be applied to soils). Commercial soil wetting agents will continue to work for a considerable period, but they will eventually be degraded by soil micro-organisms. Some can, however, interfere with the life-cycles of some aquatic organisms, so care should be taken to prevent run-off of these products into streams, and excess product should not be washed down.

Anionic surfactants can be found in soils as the result of 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.

Biosurfactants
Biosurfactants are surface-active substances synthesised by living cells; they are generally non-toxic and biodegradable. Interest in microbial surfactants has been steadily increasing in recent years due to their diversity, environmentally friendly nature, possibility of large-scale production, selectivity, performance under extreme conditions and potential applications in environmental protection.

Biosurfactants enhance the emulsification of hydrocarbons, have the potential to solubilise hydrocarbon contaminants and increase their availability for microbial degradation. The use of chemicals for the treatment of a hydrocarbon polluted site may contaminate the environment with their by-products, whereas biological treatment may efficiently destroy pollutants, while being biodegradable themselves. Hence, biosurfactant producing microorganisms may play an important role in the accelerated bioremediation of hydrocarbon contaminated sites. These compounds can also be used in enhanced oil recovery and may be considered for other potential applications in environmental protection. Other applications include herbicides and pesticides formulations, detergents, health care and cosmetics, pulp and paper, coal, textiles, ceramic processing and food industries, uranium ore-processing and mechanical dewatering of peat.

Several microorganisms are known to synthesise surface-active agents, most of them are bacteria and yeasts. When grown on hydrocarbon substrate as the carbon source, these microorganisms synthesise a wide range of chemicals with surface activity, such as glycolipid, phospholipid and others. These chemicals are apparently synthesised to emulsify the hydrocarbon substrate and facilitate its transport into the cells. In some bacterial species such as Pseudomonas aeruginosa, biosurfactants are also involved in a group motility behavior called swarming motility.

Biosurfactants and Deepwater Horizon
The use of biosurfactants as a way to remove petroleum from contaminated sites has been questioned, and criticized as irresponsible and environmentally unsafe. Biosurfactants were not used by BP after the Deepwater Horizon offshore drilling rig went down on April 20, 2010, on the resulting Deepwater Horizon oil spill. However, unprecedented amounts of Corexit, a surfactant solution produced by Nalco Holding Company (whose active ingredient is Tween-80), were sprayed directly into the ocean at the leak and on the sea-water's surface, the theory being that the surfactants would isolate individual molecules of oil making it easier for petroleum consuming microbes to digest the oil. However some scientists say that rather than helping the situation the surfactants have only managed to disperse and sink the oil below the surface and out of sight. Naturally occurring petroleum consuming microbes have evolved on the bottom of the ocean where they have adapted to live in areas where oil seeps naturally from the ocean floor.