detergent (dētûrˈjənt, dĭ–) [key], substance that aids in the removal of dirt. Detergents act mainly on the oily films that trap dirt particles. The detergent molecules have a hydrocarbon portion, soluble in oil, and an ionic portion, soluble in water. The detergent acts as an emulsifier, i.e., by bridging the water and oil phases, it breaks the oil into tiny droplets suspended in water. The disruption of the oil film allows the dirt particles to become solubilized. Soap, the sodium salt of long-chain fatty acids, is a good detergent although it has some disadvantages, e.g., it forms insoluble compounds with certain salts found in hard water thus diminishing its effectiveness, and in acid solutions, frequently used in industry, it is decomposed (thus precipitating the free fatty acid of the soap). Synthetic detergents were produced experimentally in France before the middle of the 19th cent. and were further developed in Germany during World War I. However, not until the 1930s were chemical processes developed that made production in quantity feasible in any country. Synthetic detergents were first developed for commercial use in the 1950s. Detergents are classified as anionic, or negatively charged, e.g., soaps; cationic, or positively charged, e.g., tetraalkyl ammonium chloride, used as fabric softeners; nonionic, e.g., certain esters made from oil, used as degreasing agents in industry; and zwitterionic, containing both positive and negative ions on the same molecule. Detergents are incorporated in such products as dry-cleaning solutions, toothpastes, antiseptics, and solutions for removing poison sprays from vegetables and fruit. Laundry detergent preparations may contain substances called builders, which enhance cleansing; however, phosphate-containing builders have been found to contribute to eutrophication of waterways and their use has been banned in many areas. Detergents that can be decomposed by microorganisms are termed biodegradable. Detergents are important chemicals used for enhanced petroleum recovery.