Surface-active cleansing agent. The common detergents are made from fats (hydrocarbons) and sulphuric acid, and their long-chain molecules have a type of structure similar to that of soap molecules: a salt group at one end attached to a long hydrocarbon ‘tail’. They have the advantage over soap in that they do not produce scum by forming insoluble salts with the calcium and magnesium ions present in hard water.
To remove dirt, which is generally attached to materials by means of oil or grease, the hydrocarbon ‘tails’ (soluble in oil or grease) penetrate the oil or grease drops, while the ‘heads’ (soluble in water but insoluble in grease) remain in the water and, being salts, become ionized. Consequently the oil drops become negatively charged and tend to repel one another; thus they remain in suspension and are washed away with the dirt.
Detergents were first developed from coal tar in Germany during World War I, and synthetic organic detergents were increasingly used after World War II.
Domestic powder detergents for use in hot water have alkyl benzene as their main base, and may also include bleaches and fluorescers as whiteners, perborates to free stain-removing oxygen, and water softeners. Environment-friendly detergents contain no phosphates or bleaches. Liquid detergents for washing dishes are based on epoxyethane (ethylene oxide). Cold-water detergents consist of a mixture of various alcohols, plus an ingredient for breaking down the surface tension of the water, so enabling the liquid to penetrate fibres and remove the dirt. When these surface-active agents (surfactants) escape the normal processing of sewage, they cause troublesome foam in rivers; phosphates in some detergents can also cause the excessive enrichment (eutrophication) of rivers and lakes.
There is evidence from Italy that phosphate-free detergents may also be causing pollution. Since 1989 the Adriatic Sea has been plagued by the growth of large mucilaginous mats producing foul-smelling brown foam. Italian scientists suspect this is caused by the zeolites (crystalline minerals) and polycarboxylic acids (PCAs) that are used as substitutes for phosphates. The PCAs coat the zeolites and clay particles in the water keeping them afloat. In addition, micro-organisms and organic molecules bind with the PCAs producing noxious foam.