Worlds of David Darling
Encyclopedia of Science
Home > Encyclopedia of Science

soaps and detergents

Substances which, when dissolved in water, are cleansing agents. Soap has been known since about 600 BC; it was used as a medicine until its use for washing was discovered in the 2nd century AD. Until about 1500 it was made by boiling animal fat with wood ashes (which contain the potassium carbonate). Then caustic soda (sodium hydroxide), a more effective alkali, was used; vegetable fats and oils were also introduced.

Saponification, the chemical reaction in soap-making, is an alkaline hydrolysis of the fat (an ester) to yield glycerol and the sodium salt of a long-chain carboxylic acid. The potassium salt is used for soft soap. In the modern process, the hydrolysis is effected by superheated water with a zinc catalyst, and the free acid produced is then neutralized. Synthetic detergents, introduced in World War I, generally consist of the sodium salts of various long-chain sulfonic acids, derived from oils and petroleum products.

The principle of soaps and detergents is the same: the hydrophobic long-chain hydrocarbon part of the molecule attaches itself to the grease and dirt particles, and the hydrophilic acid group makes the particles soluble in water, so that by agitation they are loosed from the fabric or other surface and dispersed. Detergents do not (unlike soaps) form scum in hard water. Their persistence in rivers, however, causes pollution problems, and biodegradable detergents have been developed. Household detergents may contain several additives: bleaches, brighteners, and enzymes to digest protein stains (egg, blood, etc.).

Related category