Alkylbenzene sulfonates are a class of
anionic surfactants, consisting of a
hydrophilicsulfonate head-group and a
hydrophobic 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.).[1]
They were 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.[2] 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
soaps.[citation needed]
Branched alkylbenzene sulfonates
An example of a branched alkylbenzene sulfonate (BAS)
Branched alkylbenzene sulfonates (BAS) were introduced in the early 1930s and saw significant growth from the late 1940s onwards,[3] in early literature these synthetic detergents are often abbreviated as syndets. They were prepared by the
Friedel–Crafts alkylation of
benzene with 'propylene tetramer' (also called tetrapropylene) followed by
sulfonation. Propylene tetramer being a broad term for a mixture of compounds formed by the
oligomerization of
propene, its use gave a mixture of highly branched structures.[4]
Compared to traditional soaps, BAS offered superior tolerance to
hard water and better foaming.[5] However, the highly branched tail made it difficult to
biodegrade.[6] 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 foams), as well as foaming problems encountered in sewage treatment[7] and contamination of drinking water.[8] As such, BAS was phased out of most detergent products during the 1960s, being replaced with linear alkylbenzene sulfonates (LAS), which biodegrade much more rapidly. BAS is still important in certain agrochemical and industrial applications, where rapid
biodegradability is of reduced importance. For instance, inhibiting
asphaltene deposition from
crude oil.
Linear alkylbenzene sulfonates
Linear alkylbenzene sulfonates (LAS) are prepared industrially by the
sulfonation of
linear alkylbenzenes (LABs), which can themselves be prepared in several ways.[2] In the most common route
benzene is alkylated by long chain monoalkenes (e.g. dodecene) using
hydrogen fluoride as a catalyst.[9] The purified
dodecylbenzenes (and related derivatives) are then
sulfonated with
sulfur trioxide to give the
sulfonic acid.[10] The sulfonic acid is subsequently neutralized with
sodium hydroxide.[1]
The term "linear" refers to the starting
alkenes 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.[11]
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.[12]
Within LAS itself the detergency of the various isomers are fairly similar,[13][14] however their physical properties (
Krafft point, foaming etc.) are noticeably different.[15][16]
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 is ~15 °C.[17] 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,[6][18][19] and is affected by
isomerization, in this case, branching. The salt of the linear material has an
LD50 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 of approximately 1–3 weeks;[18] oxidative degradation initiates at the alkyl chain.[1] Under anaerobic conditions it degrades very slowly or not at all, causing it to exist in high concentrations in
sewage sludge, but this is not thought to be a cause for concern as it will rapidly degrade once returned to an oxygenated environment.
References
^
abcKurt Kosswig,"Surfactants" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, 2005, Weinheim.
doi:
10.1002/14356007.a25_747
^
abKocal, Joseph A; Vora, Bipin V; Imai, Tamotsu (November 2001). "Production of linear alkylbenzenes". Applied Catalysis A: General. 221 (1–2): 295–301.
doi:
10.1016/S0926-860X(01)00808-0.
^Dee, Foster; Snell, Cornelia T. (August 1958). "50th ANNIVERSARY FEATURE—Fifty Years of Detergent Progress". Industrial & Engineering Chemistry. 50 (8): 48A–51A.
doi:
10.1021/ie50584a005.
^Scheibel, Jeffrey J. (17 December 2015). "The evolution of anionic surfactant technology to meet the requirements of the laundry detergent industry". Journal of Surfactants and Detergents. 7 (4): 319–328.
doi:
10.1007/s11743-004-0317-7.
S2CID93889318.
^Hill, J. A. (22 October 2008). "The Chemistry and Application of Detergents". Journal of the Society of Dyers and Colourists. 63 (10): 319–322.
doi:
10.1111/j.1478-4408.1947.tb02430.x.
^
abHashim, M. A.; Kulandai, J.; Hassan, R. S. (24 April 2007). "Biodegradability of branched alkylbenzene sulphonates". Journal of Chemical Technology & Biotechnology. 54 (3): 207–214.
doi:
10.1002/jctb.280540302.
^McKinney, Ross E. (1957). "Syndets and Waste Disposal". Sewage and Industrial Wastes. 29 (6): 654–666.
JSTOR25033358.
^Cahn, AOCS]. Ed.: Arno (2003). 5th World Conference on Detergents: Reinventing the industry - opportunities and challenges ([Elektronische Ressource] ed.). Champaign, Ill.: AOCS Press.
ISBN1-893997-40-5.
^Roberts, David W. (May 1998). "Sulfonation Technology for Anionic Surfactant Manufacture". Organic Process Research & Development. 2 (3): 194–202.
doi:
10.1021/op9700439.
^Matheson, K. Lee (August 1985). "Detergency performance comparison between LAS and ABS using calcium sulfonate precipitation boundary diagrams". Journal of the American Oil Chemists' Society. 62 (8): 1269–1274.
doi:
10.1007/BF02541841.
S2CID98677989.
^Matheson, K. Lee; Matsoim, Ted P. (September 1983). "Effect of carbon chain and phenyl isomer distribution on use properties of linear alkylbenzene sulfonate: A comparison of 'high' and 'low' 2-phenyl LAS homologs". Journal of the American Oil Chemists' Society. 60 (9): 1693–1698.
doi:
10.1007/BF02662436.
S2CID97659271.
^Baumgartner, F. N. (June 1954). "Relation of Molecular Structure to Detergency of Some Alkylbenzene Sulfonates". Industrial & Engineering Chemistry. 46 (6): 1349–1352.
doi:
10.1021/ie50534a061.
^Drozd, Joseph C.; Gorman, Wilma (March 1988). "Formulating characteristics of high and low 2-phenyl linear alkylbenzene sulfonates in liquid detergents". Journal of the American Oil Chemists' Society. 65 (3): 398–404.
doi:
10.1007/BF02663085.
S2CID93127857.
^Sweeney, W. A.; Olson, A. C. (December 1964). "Performance of straight-chain alkylbenzene sulfonates (LAS) in heavy-duty detergents". Journal of the American Oil Chemists' Society. 41 (12): 815–822.
doi:
10.1007/BF02663964.
S2CID98586085.
^Mackay, Donald; Di Guardo, Antonio; Paterson, Sally; Kicsi, Gabriel; Cowan, Christina E.; Kane, David M. (September 1996). "Assessment of chemical fate in the environment using evaluative, regional and local-scale models: Illustrative application to chlorobenzene and linear alkylbenzene sulfonates". Environmental Toxicology and Chemistry. 15 (9): 1638–1648.
doi:
10.1002/etc.5620150930.