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.
The term surfactant is a blend of surface active agents.[1]
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.
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 diffuse in water and adsorb at interfaces between air and water or at the interface between oil and water, in the case where water is mixed with oil. The 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. This alignment of surfactant molecules at the surface modifies the surface properties of water at the water/air or water/oil interface.
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 such as spherical or cylindrical micelles or bilayers can be formed. The shape of the aggregates depends on the chemical structure of the surfactants, depending on the balance of the sizes of the hydrophobic tail and hydrophilic tail. This is known as the HLB, Hydrophilic-lipophilic balance.
Surfactants reduce the surface tension of water by adsorbing at the liquid-gas interface. The decrease of the surface tension depends on the number of adsorbed molecules per unit area, called the surface excess. The relation that links the surface tension and the surface excess is known as the Gibbs isotherm.
The dynamics of adsorption of surfactants is of great importance for practical applications such as foaming, emulsifying or coating processes, where bubbles or drops are rapidly generated and need to be stabilized. The dynamics of adsorption depends on the diffusion coefficient of the surfactants. Indeed, as the interface is created, the adsorption is limited by the diffusion of the surfactants to the interface. In some cases, there exists a barrier of energy for the adsorption or the desorption of the surfactants, then the adsorption dynamics is known as 'kinetically-limited'. Such energy barrier can be due to steric or electrostatic repulsions. The surface rheology of surfactant layers, including the elasticity and viscosity of the surfactant layers plays a very important role in foam or emulsion stability
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.
Surfactants play an important role as cleaning, wetting, dispersing, emulsifying, foaming and anti-foaming agents in many practical applications and products, including:
In solution, detergents help solubilize molecules by dissociating aggregates and unfolding proteins, including 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, and 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.[2] Pulmonary surfactants are also naturally secreted by type II cells of the lung alveoli in mammals.
Surfactants can have a cationic, anionic or neutral head. There exists several types of hydrophobic tails.
see classification of surfactants
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.[3][4]
Some surfactants are known to be toxic to animals, ecosystems, and humans, and can increase the diffusion of other environmental contaminants.[5][6][7] 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.[8]
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.[9]
Ordinary dishwashing detergent, for example, will promote water penetration in soil, but the effect would last only a few days (many standard laundry detergent powders contain levels of chemicals such as alkali and chelating agents that 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.[10][11]
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.[12][13]
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.[14][15][16] These compounds can also be used in enhanced oil recovery and may be considered for other potential applications in environmental protection.[16][17] Other applications include herbicides and pesticides formulations, detergents, healthcare and cosmetics, pulp and paper, coal, textiles, ceramic processing and food industries, uranium ore-processing, and mechanical dewatering of peat.[12][13][18]
Several microorganisms are known to synthesise surface-active agents; most of them are bacteria and yeasts.[19][20] 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.[21][22] These chemicals are 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.
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 managed only 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.
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