salts of higher fatty acids. In industry and household use, soaps (or commercial soaps) are cleansing mixtures of water-soluble salts of fatty acids, often with additives of other substances. The mixtures are usually based on sodium salts of saturated and unsaturated fatty acids with 12–18 carbon atoms (stearic, palmitic, myristic, lauric, and oleic acids). Potassium and ammonium salts are used less frequently. Salts of naphthenic and resin acids, and sometimes of other compounds having cleansing action in solution, are often considered soaps. Salts of fatty acids that are insoluble in water, as well as salts of alkaline earths and polyvalent metals, are called metallic soaps.

Water-soluble soaps are typical micelle-forming surface-active agents. At concentrations higher than a certain critical value, micelles, or colloidal particles formed by the aggregation of molecules into large clusters, appear in addition to individual molecules (ions) of the solute. The presence of micelles and high surface (adsorption) activity give rise to the characteristic properties of soap solutions: the capacity to wash out dirt, to foam, to wet hydrophobic surfaces, and to emulsify oils.

According to Pliny the Elder, the production of soap by treating fats with plant ash, lime and natural bases was known to the ancient Gauls and Germanic peoples. Soap is mentioned in the writings of the Roman physician Galen (second century A.D.). However, the use of soap as a detergent did not begin until much later. Its use as a detergent had apparently become widespread in Europe by the 17th century. The soap industry took shape in the 19th century. Its development was facilitated by advances in the chemistry of fats (in the work of the French chemist M. E. Chevreul, 1813–23) and the establishment of the widespread production of soda by the method of the French chemist N. Leblanc (1820).

The modern soap industry produces various brands and grades of soap. A distinction is made among household, toilet, and industrial soaps, which may be in solid, soft, liquid, or powder form. The fatty raw material in the production of soap is animal fat or fatty vegetable oil, as well as fat substitutes, such as synthetic fatty acids, rosin, naphthenic acids, and tall oil. Hard soaps are produced from hard fats and hydrogenated fats (vegetable oils and liquid fats of marine animals that have been solidified by hydrogenation). Liquid vegetable oils are the main raw material for liquid soaps; fat substitutes are also used. Fat substitutes are not used in the production of liquid toilet soaps.

The industrial production of soap is divided into two stages. The first stage is the boiling of the soap, followed by conversion of the boiled soap into the commercial product. Soap boiling is done in special kettles. The fatty raw material is saponified by heating in a strongly basic solution, usually caustic soda (sodium hydroxide). The fats are thereby converted into a mixture of fatty acid salts and glycerol. Fats that have undergone prior hydrolysis, with the formation of free fatty acids, are sometimes used. The hydrolyzed fats in the kettle are neutralized with sodium carbonate and then saponified to completion using a strong base. Middle soap, which is a homogeneous, viscous liquid that thickens upon cooling, forms as a result of the boiling process, in both cases.

Commercial soap that is obtained directly from middle soap is called filled soap; its content of fatty acids is usually 40–60 percent. Treatment of middle soap with electrolytes (salting-out) leads to its separation into layers. Upon complete salting-out with a caustic alkali or sodium chloride solution, two layers form in the kettle. The upper layer is a concentrated solution of soap containing not less than 60 percent fatty acids; it is called neat soap. It is used in the production of high-quality soaps (neat soap). The lower layer is a solution of the electrolyte with low soap content, called nigre; most of the glycerol (which is extracted as a valuable by-product of soap production), as well as impurities introduced into the middle soap with the raw materials, passes into this layer. The production of middle soap is called the direct method, and the production of neat soap, the indirect method. Both methods are used in the production of household soaps. Toilet soaps are usually prepared by the indirect method, and the neat soap is made from the best fatty raw material and subjected to additional purification.

In the second stage in the production of hard soaps, the soap material produced by the boiling process is cooled, dried, and then mechanically processed in special apparatus to impart plasticity and uniformity. The soap product is then shaped and cut into standard-size bars. Fragrances, dyes, antioxidants, and in some cases disinfectants and medicinal and foaming agents, are added to toilet soaps. Mineral fillers (bentonite clays and purified kaolin) are added to inexpensive brands of soap. Fatty enriched soaps form a special group of toilet soaps; they have no free bases and usually contain cosmetic supplements (higher fatty alcohols and nutrients).

Powdered soaps are produced by aeration drying of soap solutions. They are sold either without additives (soap powders) or in the form of a mixture with significant quantities of basic electrolytes (soda and phosphates), which improve their cleansing power (detergent powders). Continuous-operation automated equipment is used in soap production.

World production of household soaps is gradually diminishing because of the increasing use of synthetic detergents and the shortage of the fatty raw materials. However, in spite of the spread of various synthetic soaplike substances, soaps have not lost their significance as the most important agent of personal hygiene. As in the past, soaps are widely used in the household and in many branches of industry, particularly the textile industry. Along with other surface-active agents, soaps are used as wetting agents, emulsifying agents, and stabilizers of colloidal disperse systems. They are used as a component of cooling lubricants for metalworking machine tools and in the concentration of valuable ores by flotation. In the chemical industry they are used in the production of polymers by the emulsion method, and also in the paint and varnish industry. Metallic soaps are used as thickeners in plastic lubricants and as siccatives in oil varnishes and drying oils.