Alkylbenzene sulfonates are a class of
anionic surfactants, 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 n ...
sulfonate 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, ...
alkylbenzene tail-group. Along with
sodium laureth sulfate, 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 us ...
s.
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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 relat ...
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 petrol ...
, 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.
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 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 alkylbenzenes (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
Hydrogen fluoride (fluorane) is an inorganic compound with the chemical formula . This colorless gas or liquid is the principal industrial source of fluorine, often as an aqueous solution called hydrofluoric acid. It is an important feedstock ...
as a catalyst. The purified
dodecylbenzenes (and related derivatives) are then
sulfonated 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 an ...
with
sulfur trioxide
Sulfur trioxide (alternative spelling sulphur trioxide, also known as ''nisso sulfan'') is the chemical compound with the formula SO3. It has been described as "unquestionably the most important economically" sulfur oxide. It is prepared on an ind ...
to give the
sulfonic acid
In organic chemistry, sulfonic acid (or sulphonic acid) refers to a member of the class of organosulfur compounds with the general formula , where R is an organic alkyl or aryl group and the group a sulfonyl hydroxide. As a substituent, it is k ...
. The sulfonic acid is subsequently neutralized with
sodium hydroxide
Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions .
Sodium hydroxide is a highly caustic base and al ...
.
[
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 ...
s rather than the final product, perfectly linear addition products are not seen, in-line with Markovnikov's rule
In organic chemistry, Markovnikov's rule or Markownikoff's rule describes the outcome of some addition reactions. The rule was formulated by Russian chemist Vladimir Markovnikov in 1870.
Explanation
The rule states that with the addition of a ...
. 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, 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 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 ...
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