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Alkylbenzene sulfonates are a class of
anionic surfactant Surfactants are chemical compounds that decrease the surface tension between two liquids, between a gas and a liquid, or interfacial tension between a liquid and a solid. Surfactants may act as detergents, wetting agents, emulsifiers, foaming ...
s, consisting of a
hydrophilic A hydrophile is a molecule or other molecular entity that is attracted to water molecules and tends to be dissolved by water.Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon'' Oxford: Clarendon Press. In contrast, hydrophobes are ...
sulfonate In organosulfur chemistry, a sulfonate is a salt or ester of a sulfonic acid. It contains the functional group , where R is an organic group. Sulfonates are the conjugate bases of sulfonic acids. Sulfonates are generally stable in water, non-o ...
head-group and a
hydrophobic In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, t ...
alkylbenzene tail-group. Along with
sodium laureth sulfate Sodium laureth sulfate (SLES), an accepted contraction of sodium lauryl ether sulfate (SLES), also called sodium alkylethersulfate, is an anionic detergent and surfactant found in many personal care products (soaps, shampoos, toothpaste, etc.) an ...
, they are one of the oldest and most widely used synthetic detergents and may be found in numerous personal-care products (soaps, shampoos, toothpaste etc.) and household-care products (laundry detergent, dishwashing liquid, spray cleaner etc.).Kurt Kosswig,"Surfactants" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, 2005, Weinheim. They were first introduced in the 1930s in the form of branched alkylbenzene sulfonates (BAS). However following environmental concerns these were replaced with linear alkylbenzene sulfonates (LAS) during the 1960s. Since then production has increased significantly from about one million tons in 1980, to around 3.5 million tons in 2016, making them most produced anionic surfactant after
soap Soap is a salt of a fatty acid used in a variety of cleansing and lubricating products. In a domestic setting, soaps are surfactants usually used for washing, bathing, and other types of housekeeping. In industrial settings, soaps are use ...
s. __TOC__


Branched alkylbenzene sulfonates

Branched alkylbenzene sulfonates (BAS) were first introduced in the early 1930s and saw significant growth from the late 1940s onwards, in early literature these synthetic detergents are often abbreviated as syndets. They were prepared by the Friedel–Crafts alkylation of
benzene Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar ring with one hydrogen atom attached to each. Because it contains only carbon and hydrogen atoms ...
with 'propylene tetramer' (also called tetrapropylene) followed by
sulfonation Aromatic sulfonation is an organic reaction in which a hydrogen atom on an arene is replaced by a sulfonic acid functional group in an electrophilic aromatic substitution. Aryl sulfonic acids are used as detergents, dye, and drugs. Stoichiometry a ...
. Propylene tetramer being a broad term for a mixture of compounds formed by the
oligomerization In chemistry and biochemistry, an oligomer () is a molecule that consists of a few repeating units which could be derived, actually or conceptually, from smaller molecules, monomers.Quote: ''Oligomer molecule: A molecule of intermediate relativ ...
of
propene Propylene, also known as propene, is an unsaturated organic compound with the chemical formula CH3CH=CH2. It has one double bond, and is the second simplest member of the alkene class of hydrocarbons. It is a colorless gas with a faint petro ...
, its use gave a mixture of highly branched structures. Compared to traditional soaps, BAS offered superior tolerance to
hard water Hard water is water that has high mineral content (in contrast with "soft water"). Hard water is formed when water percolates through deposits of limestone, chalk or gypsum, which are largely made up of calcium and magnesium carbonates, bicarbo ...
and better foaming. However, the highly branched tail made it difficult to
biodegrade Biodegradation is the breakdown of organic matter by microorganisms, such as bacteria and fungi. It is generally assumed to be a natural process, which differentiates it from composting. Composting is a human-driven process in which biodegradati ...
. BAS was widely blamed for the formation of large expanses of stable foam in areas of wastewater discharge such as lakes, rivers and coastal areas (
sea foam Sea foam, ocean foam, beach foam, or spume is a type of foam created by the agitation of seawater, particularly when it contains higher concentrations of dissolved organic matter (including proteins, lignins, and lipids) derived from sources ...
s), as well as foaming problems encountered in sewage treatment and contamination of drinking water. As such, BAS was phased out of most detergent products during the 1960s, being replaced with linear alkylbenzene sulfonates (LAS). It is still important in certain agrochemical and industrial applications, where rapid
biodegradability Biodegradation is the breakdown of organic matter by microorganisms, such as bacteria and fungi. It is generally assumed to be a natural process, which differentiates it from composting. Composting is a human-driven process in which biodegradati ...
is of reduced importance.


Linear alkylbenzene sulfonates

Linear alkylbenzene sulfonates (LAS) are prepared industrially by the
sulfonation Aromatic sulfonation is an organic reaction in which a hydrogen atom on an arene is replaced by a sulfonic acid functional group in an electrophilic aromatic substitution. Aryl sulfonic acids are used as detergents, dye, and drugs. Stoichiometry a ...
of
linear alkylbenzene Linear alkylbenzenes (sometimes also known as LABs) are a family of organic compounds with the formula C6H5CnH2n+1. Typically, ''n'' lies between 10 and 16, although generally supplied as a tighter cut, such as C12-C15, C12-C13 and C10-C13, for de ...
s (LABs), which can themselves be prepared in several ways. In the most common route
benzene Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar ring with one hydrogen atom attached to each. Because it contains only carbon and hydrogen atoms ...
is alkylated by long chain monoalkenes (e.g. dodecene) using hydrogen fluoride as a catalyst. The purified
dodecylbenzene Dodecylbenzene is an organic compound with the formula . Dodecylbenzene is a colorless liquid with a weak oily odor that floats on water. This colourless waxy solid consists of a dodecyl group () attached to a phenyl group (). Dodecylbenzene is a ...
s (and related derivatives) are then sulfonated with sulfur trioxide to give the sulfonic acid. The sulfonic acid is subsequently neutralized with sodium hydroxide. The term "linear" refers to the starting
alkene In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond. Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, an ...
s rather than the final product, perfectly linear addition products are not seen, in-line with Markovnikov's rule. Thus, the alkylation of linear alkenes, even 1-alkenes such as 1-dodecene, gives several isomers of phenyldodecane.


Structure property relationships

Under ideal conditions the cleaning power of BAS and LAS is very similar, however LAS performs slightly better in normal use conditions, due to it being less affected by hard water. Within LAS itself the detergency of the various isomers are fairly similar, however their physical properties (
Krafft point Krafft temperature is defined as the minimum temperature from which the micelle formation takes place. It is named after German chemist Friedrich Krafft. It has been found that solubility at the Krafft point is nearly equal to critical micelle conce ...
, foaming etc.) are noticeably different. In particular the Krafft point of the high 2-phenyl product (i.e. the least branched isomer) remains below 0 °C up to 25% LAS whereas the low 2-phenyl
cloud point In liquids, the cloud point is the temperature below which a transparent solution undergoes either a liquid-liquid phase separation to form an emulsion or a liquid-solid phase transition to form either a stable sol or a suspension that settles a ...
is ~15 °C. This behavior is often exploited by producers to create either clear or cloudy products.


Environmental fate

The biodegradability of alkylbenzene sulfonates has been well studied, and is affected by
isomerization In chemistry, isomerization or isomerisation is the process in which a molecule, polyatomic ion or molecular fragment is transformed into an isomer with a different chemical structure. Enolization is an example of isomerization, as is tautomeriz ...
, in this case, branching. The salt of the linear material has an of 2.3 mg/liter for fish, about four times more toxic than the branched compound; however the linear compound biodegrades far more quickly, making it the safer choice over time. It is biodegraded rapidly under aerobic conditions with a
half-life Half-life (symbol ) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable at ...
of approximately 1–3 weeks; oxidative degradation initiates at the alkyl chain. Under anaerobic conditions it degrades very slowly or not at all, causing it to exist in high concentrations in
sewage sludge Sewage sludge is the residual, semi-solid material that is produced as a by-product during sewage treatment of industrial or municipal wastewater. The term " septage" also refers to sludge from simple wastewater treatment but is connected to s ...
, but this is not thought to be a cause for concern as it will rapidly degrade once returned to an oxygenated environment.


References

{{Xenobiotic-sensing receptor modulators Organic sodium salts Cleaning product components Anionic surfactants Sulfonates Glycine receptor agonists Alkyl-substituted benzenes