From building blocks to plastics, fibres and detergents: Detergents: from benzene, paraffins and ethylene oxide

5.1 Detergents displaced soap because they keep working in hard water

Soap is the sodium salt of fatty acids with 8 to 22 carbons, made by boiling fats and oils with caustic soda. It works by having a fatty tail that dissolves in grease and an ionic head that dissolves in water, so soap molecules surround oily dirt in clusters called micelles and lift it into the wash. Synthetic detergents work the same way. The difference is hard water. Calcium and magnesium ions in hard water form insoluble salts with soap, the familiar scum, which wastes soap and kills lather and cleaning. The calcium salts of synthetic detergents either stay dissolved or remain dispersed (Shreve).

Shortage started the change. After the First World War, Germany lacked edible fats, and chemists made short alkyl naphthalene sulphonates, good wetting agents but only fair detergents. Development moved to long-chain alcohol sulphates in the 1920s and 1930s, alkylaryl sulphonates in the 1940s and branched-chain products in the 1950s and 1960s. Kent notes the new compounds proved more surface active, more tolerant of hard water and better cleaners than soap.

The branched products then caused a problem. The alkylbenzene was made from propylene tetramer, a highly branched C12 olefin, and bacteria in sewage works broke it down only slowly, so rivers and treatment plants foamed. Only linear chains degrade easily. By 1965 most of the industry had switched to linear alkylbenzene, whose sulphonate is degraded by more than 95% in sewage treatment, and branched alkylbenzene was banned in the United States and Western Europe. Wittcoff notes the branched kind was still important in some developing countries in 2004. Most surfactants are anionic, with a negatively charged head like LAS; nonionics such as alcohol ethoxylates carry no charge.

• Shreve (1984), Ch 29 Soap and detergents · pdf pp. 547-552
• Kent (2012), Ch 36 Soap, fatty acids and synthetic detergents · pdf pp. 1433, 1451-1452, 1456-1457
• Wittcoff (2004), Ch 7 Alkylbenzenes · pdf pp. 325-327

5.2 Linear alkylbenzene joins kerosene paraffins to benzene, then takes on SO3

Linear alkylbenzene (LAB) is benzene carrying a straight chain of 11 to 14 carbons (Shreve; Kent gives 9 to 14). The chain comes from kerosene. Straight-chain paraffins are separated from branched and ring molecules by molecular sieves, whose pores admit only slim linear molecules, or by urea, which forms a crystalline adduct only with straight chains. The paraffins are then partly dehydrogenated over a platinum catalyst (the Pacol process) to internal olefins; Kent says this is more selective than thermal cracking. The resulting olefin-paraffin mixture alkylates benzene with liquid hydrogen fluoride as catalyst, or, in newer plants, over a solid acid (the DETAL process), which avoids handling HF.

The catalyst affects the product. HF gives LAB with 13-22% of the 2-phenyl isomer, where the benzene ring sits at the second carbon; aluminium chloride and DETAL give 25-35%. High 2-phenyl grades dissolve better and suit liquid detergents.

LAB becomes a surfactant only when sulphonated. Modern plants make sulphur trioxide on site by burning sulphur and converting SO₂ over vanadium catalyst, dilute it to 4-8% in dry air, and contact it with a thin film of LAB running down cooled tubes, the falling-film reactor. The molar ratio of SO₃ to alkylate is held at 1.0-1.03 and the temperature at 110-150 F (43-66 C); the reaction is almost instant and releases about 420 kJ per kg, so heat removal decides colour and quality. After a short digestion and about 2 parts of water per 100 to break down anhydrides, the product is 95-98% sulphonic acid. Oleum sulphonation needs cheaper equipment but gives only about 90%, with 6-10% spent sulphuric acid to dispose of.

• Kent (2012), Ch 36 Intermediates; Anionic surfactants · pdf pp. 1455-1461
• Shreve (1984), Ch 29 Straight-chain alkylbenzenes; sulfonation · pdf pp. 552-553, 559-561
• Wittcoff (2004), Ch 7 Alkylbenzenes · pdf pp. 325-327

5.3 Ethoxylates and the formulation complete the detergent

The second big surfactant family links back to ethylene oxide. A fatty alcohol and a base catalyst such as caustic soda or potash are charged to a pressure reactor and heated to 115-200 C. Ethylene oxide is fed in until the target amount has reacted; the reaction releases a lot of heat, and the rate of heat removal sets the rate of production. EO adds in a statistical spread, so the product is a mixture of chains carrying different numbers of EO units around the average. Shreve notes that nonionics remove soil better than anionics at the lower wash temperatures needed for synthetic fabrics and are low sudsers, suiting drum washing machines.

The fatty alcohols come by three routes (Kent). Natural alcohols are made by turning coconut, palm kernel or tallow oils into methyl esters and hydrogenating them over copper chromite at 200-350 C and 250-300 bar; coconut and palm kernel are rich in C12 and C14, the most useful lengths. The Ziegler process grows ethylene chains on aluminium alkyls and gives linear, even-numbered alcohols much like natural ones. The oxo process adds carbon monoxide and hydrogen to olefins and gives odd and even chains that are more branched, over 50% with cobalt catalyst. Nonylphenol ethoxylates, once common, are declining because of slow biodegradation and endocrine disruption, and are being replaced by alcohol ethoxylates.

Surfactant is only part of the box. Builders soften the water. Sodium tripolyphosphate was the main builder until phosphates were linked to the eutrophication of lakes; in most of the United States soda ash and zeolite A have replaced it. In the P&G process Shreve describes, surfactant slurry, builder and additives are mixed in a crutcher, pumped up and sprayed under pressure into a 24 m tower against hot air.

• Kent (2012), Ch 36 Fatty alcohols; Nonionic surfactants; Builders · pdf pp. 1454-1457, 1462-1463
• Shreve (1984), Ch 29 Detergents · pdf pp. 552, 555-561
• Wittcoff (2004), Ch 3 Ethylene oxide uses; Ziegler oligomerisation · pdf pp. 140, 153

See it in the table

Check your understanding

Why does soap perform badly in hard water while LAS does not?

What forced the move from branched to linear alkylbenzene in the 1960s?

In a falling-film SO3 sulphonation plant, why is SO3 diluted to 4-8% in dry air?