Information on substances
Linear alkyl benzene sulfonates (2003)
Physical properties are difficult to retrieve. Generally, water solubility decreases with increasing alkyl chain length and it is also dependant on the positive ion of the salt. The most common LAS is sodium dodecyl benzenesulfonate, often called sodium lauryl benzenesulfonate. At room temperature this is a white to light yellow solid substance.
LAS is an anionic surfactant i.e. the surface-active part is a negatively charged ion in water. The linear alkyl chain makes the molecule more biodegradable than alkyl benzenesulfonates with branched carbon chains. The length of the alkyl chain is not always very specified for the substances used as surfactants as it is dependant on the alkyl chain raw material.
When the alkyl chain contains hydrocarbons with a chain length of 10 to 13 carbon atoms the alkyl is often called “dodecyl” after the most common chain length of 12 carbons. If the chain is 12 to 15 carbon atoms long it is called “tridecyl”.
LAS is manufactured by addition of a-olefiner (the alkyl chain to be) of required length to benzene. These, in turn, are produced by uniting four, five ethene and/or propene units, by extracting normal paraffines from kerosine by molecular sieve or by cracking petroleum wax. After the alkyl benzenes have been formed they are sulfonated with sulphuric acid to alkylbenzene sulfonic acids. The acid is neutralized with e.g. sodium hydroxide for use in water-based systems or calcium hydroxide for oil based products. Neutralizing with ammonium ion, e.g. triethanolamine, generates tensides with emulsifying properties in both water and oil based systems. LAS are easy to spray dry to powder, which could be a problem at tenside production and then necessitates handling in solutions. Sulfonation is an inexpensive process because of abundant supply of reactive sulphur oxide groups e.g. sulphuric acid, which results in sulfonates being the most produced surfactants. About 80% of all LAS is dodecylbenzene sulfonic acid and its salts. In 1992, production in the US was about 346,000 tons of different linear alkylbenzene sulfonates. Global demand of linear alkylbenzene was 2.5 million tons in 2001.
LAS has good ability to remove and keep particles in dispersion. This is utilized in detergents for textile to remove inorganic dirt like earth. The alkyl chain of the molecule adhere to the solid surface that mostly has a faint negative charge while the likewise negatively charged sulfonic group tries to reach as far out as possible into the water phase and by this keeps the particle in dispersion. LAS are stable against oxidation making them suitable to use in mixtures containing oxidants like e.g. bleaching agents. They do not work well in hard water where water insoluble calcium soaps are precipitated. This is compensated by using them in mixtures together with softening substances and other surfactants. In oil based systems the calcium salts of LAS keep wear and soot particles in dispersion and they are therefore used in motor oil to prevent deposits in the motor.
Use of LAS in detergents in Sweden has decreased from about 10,000-15,000 tons during the eighties to less than 100 tons in 2001 because of substitution to anionic surfactants with less environmental impact as they do not contain any aromatic part of the molecule. Other countries still use large amounts of LAS in detergents. In Sweden the main use of LAS is in allround cleaners and wash-up preparations, then in combination with other tensides. Another area of use is in different oils for motors, metal work and transmission. Industry also uses LAS as a surface active process aid.