Soap Soap is a natural cleansing agent produced by the reaction of an alkali, such as sodium hydroxide, with animal fat or vegetable oil. Soap classifications include toilet soap, which is manufactured as a cleansing agent for the body, and soaps for household use, such as bars, flakes, and granules. A detergent is a cleansing preparation synthesized from a number of readily available raw materials--hydrocarbons from crude oil are most frequently used. All detergents contain a surface-active agent (surfactant) or, more often, a combination of surfactants. The surfactant lowers the surface tension of water and is able to dislodge dirt from surfaces, emulsify it, and suspend it in water. A second important ingredient is a builder, often used in detergents but infrequently used in soap. A builder is incorporated in detergents for such purposes as controlling minerals in hard water, providing alkalinity and buffering so that alkalinity is maintained at an efficient level, suspending soil particles and controlling the redeposition of soil, emulsifying oily soil, and enhancing the surfactant's wetting action. Other detergent ingredients may include antiredeposition agents, brighteners, bleaches, corrosion inhibitors, suds-control agents, perfumes, and colorants. Detergents are formulated for a full range of household, industrial, and institutional uses. The Cleansing Process A number of physical and chemical processes occur simultaneously during washing. The basic explanation of how soaps and detergents work is that their surfactant molecules have polar, or hydrophilic, ends, which attract water molecules; and nonpolar, or hydrophobic, ends, which lack attraction to water. As these molecules lower the water's surface tension, they improve the water's ability to penetrate and loosen soil. The opposite attractions of the molecule loosen the dirt, often aided by hand or machine agitation. The molecules then surround and suspend dirt particles in the water until they are rinsed away. Soap, however, presents problems in hard water, because the water-seeking portion of the molecule unites with some minerals in water to form an insoluble curd, which leaves visible deposits on clothing, bathtubs ("bathtub ring"), and washing machines. Most detergent surfactants do not form curds, and builders in detergents control those properties in the wash water which reduce the surfactant's effectiveness. History of Soaps and Detergents Soap has been used for millennia, whereas detergents have been used for only a relatively short time. The earliest literary reference to soap was found on clay tablets dating from the 3d millennium BC in Mesopotamia. They contained a soap recipe calling for a mixture of potash and oil to be used in the manufacture of cloth. Another recipe contained the ingredients of a medicated soap prescription. The first authentic reference to soap as a cleansing agent as well as a medicinal product appears in the writings of Galen, the 2d-century AD Greek physician. Galen also noted that cleanliness helped cure skin diseases. The ancient Romans spread their knowledge of soapmaking, and in the Middle Ages important centers of soapmaking developed in Spain, France, and England. Manufacturing remained on a crude level, however, until discoveries by Nicolas Leblanc in the 18th century and Michel Eugene Chevreul in the 19th century made it possible to manufacture soap by a formula leading to exact and predictable results. Inventions such as the steam engine and improved equipment and techniques enabled soapmaking to become competitive industry. The formation of curd when soap is used in hard water provided the principal impetus for the development of detergents. The first light-duty detergents, introduced in the United States in the early 1930s, were unbuilt granules, which performed better than soaps did in hard water but which had limited cleaning ability. The next breakthrough was the introduction, after World War II, of the first built synthetic detergent, which provided the benefits of the early detergents when used in hard water as well as all-around cleaning power. A key factor in producing these products was the development of complex phosphates, primarily sodium tripolyphosphate. Some early detergent surfactants were slow to biodegrade; that is, bacteria could not break them down rapidly. Concentrations occasionally built up to the extent that foaming resulted in some sewage-treatment plants and in streams. In 1965, however, the U.S. detergent industry voluntarily changed to new biodegradable materials that are quickly broken down and eliminated from the environment. Now all household laundry products manufactured for use in the United States are readily biodegradable. Another detergent ingredient that has been involved in environmental controversy is the phosphate builder, which contains the element phosphorus. Phosphorus is one of many nutrients essential to water plants and algae, and contributes to accelerated eutrophication--the too-rapid growth of aquatic plant life in water bodies. Several local and state governments have banned the sale of detergents with phosphates. Detergent manufacturers have reduced phosphate use; to add cleaning power to phosphate-free detergents, some manufacturers have added extra surfactants and such digestive ENZYMES as protease and amylase.