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What are anionic detergents?

Among synthetic detergents, commonly referred to as syndets, anionic-active types are the most important. The molecule of an anionic-active synthetic detergent is a long carbon chain to which a sulfo group (?SO3) is attached, forming the negatively charged (anionic) part. This carbon chain must be so structured that a sulfo group can be attached easily by industrial processes (sulfonation), which may employ sulfuric acid, oleum (fuming sulfuric acid), gaseous sulfur trioxide, or chlorosulfonic acid.


Fatty alcohols are important raw materials for anionic synthetic detergents. The development of commercially feasible methods in the 1930s for obtaining these provided a great impetus to synthetic-detergent production. The first fatty alcohol used in the production of synthetic detergents were derived from body oil of the sperm or bottlenose whale (sperm oil). Efforts soon followed to derive these materials from the less expensive triglycerides (coconut and palm kernel oils and tallow). The first such process, the Bouveault-Blanc method of 1903, long used in laboratories, employed metallic sodium; it became commercially feasible in the 1950s when sodium prices fell to acceptable levels. When the chemical processing industry developed high-pressure hydrogenation and oil-hardening processes for natural oils, detergent manufacturers began to adopt these methods for the reduction of coconut oil, palm kernel oil, and other oils into fatty alcohols. Synthetic fatty alcohols have been produced from ethylene; the process, known as the Awful process, employs diethylaluminum hydride.


Soon after World War II, another raw material, alkylbenzene, became available in huge quantities. Today it is the most important raw material for synthetic detergent production; about 50 percent of all synthetic detergents produced in the United States and western Europe are based on it. The alkyl molecular group has in the past usually been C12H24 (tetrapropylene) obtained from the petrochemical gas propylene. This molecular group is attached to benzene by a reaction called alkylation, with various catalysts, to form the alkylbenzene. By sulfonation, alkyl benzene sulfonate is produced; marketed in powder and liquid form, it has excellent detergent and cleaning properties and produces high foam.


An undesirable effect of the alkylbenzene sulfonates, in contrast to the soap and fatty-alcohol-based synthetic detergents, has been that the large quantity of foam they produce is difficult to get rid of. This foam remains on the surface of wastewater as it passes from towns through drains to sewers and sewage systems, then to rivers, and finally to the sea. It has caused difficulties with river navigation; and, because the foam retards biological degradation of organic material in sewage, it caused problems in sewage-water regeneration systems. In countries where sewage water is used for irrigation, the foam was also a problem. Intensive research in the 1960s led to changes in the alkylbenzene sulfonate molecules. The tetrapropylene, which has a branched structure, was replaced by an alkyl group consisting of a straight carbon chain which is more easily broken down by bacteria.


The organic compounds (fatty alcohols or alkylbenzene) are transformed into anionic surface-active detergents by the process called sulfonation. Sulfation is the chemically exact term when a fatty alcohol is used and sulfonation when alkylbenzene is used. The difference between them is that the detergent produced from fatty alcohol has a sulfate molecular group (?OSO3Na) attached and the detergent produced from an alkylbenzene has a sulfonate group (?SO3Na) attached directly to the benzene ring. Both products are similarly hydrophilic (attracted to water).

Recent sulfonation methods have revolutionized the industry; gaseous sulfur trioxide is now widely used to attach the sulfonate or sulfate group. The sulfur trioxide may be obtained either by vaporizing sulfuric acid anhydride (liquid stabilized SO3) or by burning sulfur and thus converting it to sulfur trioxide.

Research on the part of the petrochemical industry has evolved new anionic synthetic detergents, such as directly sulfonated paraffinic compounds—alpha-olefins, for example. Paraffin have been transformed directly into sulfonates by treatment with sulfur dioxide and air using a catalyst of radioactive cobalt.

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