substances that can accumulate or condense on the contact surface between two bodies, called the interface, or the interphase surface. Surfactants form on the interface a layer of increased concentration known as an adsorption layer.

Any substance that is a component of a liquid solution or of a gas (vapor) may under suitable conditions display surface activity; that is, it may be adsorbed by the action of intermolecular forces on a surface, thus decreasing the substance’s free energy. However, only those substances are ordinarily called surfactants whose adsorption from solutions at even very low concentrations (tenths and hundredths of 1 percent) leads to a sharp decrease in the surface tension.

Typical surfactants are organic compounds with diphilic structure, that is, with atomic groups in their molecular structure differing strongly in the intensity of their interaction with the environment; the environment with the greatest practical importance is water. Thus, surfactant molecules have one or several hydrocarbon radicals that constitute the molecule’s oleophilic or lipophilic part (the hydrophobic part of the molecule) and one or several polar groups constituting the hydrophilic part. Hydrophobic groups, which interact weakly with water, determine the molecule’s tendency to transfer from the aqueous (polar) medium to a hydrocarbon (nonpolar) medium. The hydrophilic groups, on the other hand, retain the molecule in the polar medium, or, if the surfactant molecule is in a hydrocarbon liquid, they determine its tendency to transfer to a polar medium. Thus, the surface activity of surfactants dissolved in nonpolar liquids is caused by the hydrophilic groups, and the surface activity of surfactants dissolved in water is caused by the hydrophobic radicals.

Surfactants may be ionic or nonionic, depending on the type of hydrophilic group. Ionic surfactants dissociate in water into ions, one of which has adsorption (surface) activity, while the other ions (the counterions) are adsorptively nonactive. When the anions are adsorptively active, the surfactant is anionic, and in the opposite case the surfactant is cationic. Organic acids and their salts are anionic surfactants, while bases—amines of different degrees of replacement— and their salts are cationic surfactants. Some surfactants contain both acid and basic groups. These surfactants may be anionic or cationic and are therefore called amphoteric, or ampholytic.

All surfactants may be divided into two categories according to the system they form when they interact with the solvent medium. One category consists of micelle-forming surfactants and the other of surfactants that do not form micelles. In solutions of micelle-forming surfactants, colloidal particles, known as micelles, consisting of dozens or hundreds of molecules or ions appear above the critical micelle concentration (CMC). The micelles reversibly decompose into separate molecules or ions upon dilution of the solution (or, more correctly, of the colloidal dispersion) to a concentration below the CMC. Thus, solutions of micelle-forming surfactants occupy an intermediate position between true (molecular) and colloidal solutions (sols) and hence are often called semicolloidal systems. Micelle-forming surfactants include all detergents, emulsifiers, and wetting and dispersing agents.

Anionic surfactants constitute the greatest part of the world’s production of surfactants. They include carboxylic acids and their salts, alkyl sulfates (sulfoesters), alkylsulfonates, and al-kylarylsulfonates. The most common anionic surfactants are the sodium and potassium soaps of fatty and resin acids, and the neutralized products of the sulfonation of the higher fatty acids, olefins, and alkylbenzenes. Second place in volume of industrial production is occupied by nonionic surfactants (polyethylene glycol ethers and esters). Most nonionic surfactants are produced by adding ethylene oxide to aliphatic alcohols, alkyl-phenols, carboxylic acids, amines, and other compounds with reactive hydrogen atoms. The variety of surfactants is extremely wide. The data presented in Table 1 permit evaluation of the relative volumes of the production of different types of surfactants in 1971.

The world’s production of surfactants is steadily increasing, and the proportion of nonionic and cationic surfactants in the total output is constantly growing. Depending on use and chemical composition, surfactants are produced in solid form (bars, flakes, granules, and powders) and as liquids and semiliquids (pastes and gels). Increasing attention is now devoted to the production of surfactants with linear molecular structures; such surfactants are easily biodegradable under natural conditions and do not pollute the environment.

Surfactants are widely used in industry, agriculture, medicine, and the home. They are used principally in the manufacture of soaps and detergents for technological purposes and the maintenance of cleanliness, in the production of substances used to prepare raw materials for textile manufacture and to finish textiles, and in the manufacture of paints and varnishes. Surfactants are used in many technological processes of the chemical, petrochemical, pharmaceutical, and food-processing industries. They are used as additives to improve the quality of petroleum products, as flotaton agents in the flotation concentration of useful ores, and as components of waterproofing and anti-corrosion coatings.

Surfactants facilitate the processing of metals and other materials and aid in the dispersion of liquids and solids. They are indispensable as stabilizers of such highly concentrated disperse systems as suspensions, pastes, emulsions, and foams. They also play an important role in biological processes and are produced by living organisms for internal needs. Thus, surface activity characterizes components of the gastrointestinal juices and the blood of animals, as well as the juices and extracts of plants.