Soap
From Chempedia
The Chemistry of Detergents
In today’s cutthroat detergent business, it is hard to conceptualize that such a modern, indispensable product has its roots in ancient soap formulations. Current technology brings American consumers new and better detergents that range from “baby soft” to “tough on stains,” but in 2800 BC, Hebrew men and women mixed ashes, limestone, and oil to create the world’s first soap. During the Roman Empire, baths became a luxurious activity that was reserved for the elite. By 1200 AD, baths had finally become commonplace and soap had planted itself into a virtually universal human culture. (Soap and Detergent Association-A, 2005). Detergents play an essential role in personal and public health. They not only clean and remove dirt, but also reduce the presence of germs that can cause diseases.
There are many different ways that detergents can be manufactured depending on the type. There are generally two types of detergents—powder detergents and liquid detergents. Both types of detergents contain similar liquid and dry ingredients, but they differ in the way that they are blended together. (Soap and Detergent Association-C. 2005).
Detergents improve the efficiency of cleaning over plain water due to detergent’s ability to reduce water’s surface tension. Surface tension is the strong attraction of water molecules at the interface between water and air. This is due to the adjacently ordered, hydrogen-bonded, water molecules that are bonded to one another, and to the water molecules below them. Surface tension makes water behave as though it is coated with an invisible film. (Campbell and Reece, 2005). The cleaning process occurs when the surface tension of water is lowered by molecules found in detergent called surfactants or surface active agents. Surfactants are organic chemicals that gather at the water’s surface and force water molecules to expand and spread out, causing the surface tension of water to decrease. When surface tension is decreased, water molecules are able to penetrate soiled material more thoroughly. The decreased surface tension also causes surfactants to form bubbles and suds. (Soap and Detergent Association-B, 2005).
Detergents are salts composed of an alkali metal, such as sodium or potassium, and a fatty acid chain with a carboxylic acid attached to it.
Figure 1. Fatty acid (Soap and Detergent Association-B, 2005).
These molecules are ampipathic; they have a hydrophilic (water-loving) side, and a hydrophobic (water-hating) side. The long fatty acid chain of hydrocarbons called the carboxylate group (-COO-), is hydrophilic and attaches its polar, ionic head to water through ion-dipole forces and hydrogen bonds. The hydrophobic area, which consists of the alkali metal, attaches to dirt molecules through dispersion forces, while the mechanical agitation from either rubbing hands or the washing machine help break the dirt particles up. The agitation also helps the hydrophilic parts of the detergent molecules pull the dirt particles from the material and into the water. (World Book Online, 2005). When the dirt particles enter the water, they are surrounded by water causing them to stay there until they are washed away. (Soap and Detergent Association-B, 2005).
Detergent is an improvement over older soap formulations because of the former’s ability to function in hard water. Mineral salts such as calcium, magnesium, and iron form a scummy precipitate when mixed with soap. (Soap and Detergent Association-B, 2005). In contrast, detergent does not react with these mineral salts and therefore, no soap film or scum is formed.
Considering that we routinely use detergents, we sometimes forget that detergents may have deleterious effects on the environment. Detergents frequently contain sodium tripolyphosphate (Na5O10P3) or STPP, an inorganic source of phosphates. In general terms, STPP softens the water and prevents dirt from settling onto the clothes being washed. (Glennie et al, 2002). After the completion of the wash cycle, waste water bearing detergents are discharged into the sewer system. STPP is then released into the environment and makes its way into lakes and rivers.
In aquatic ecosystems, phosphorus may be the limiting factor in the growth of algae. (Brady and Weil, 2004). When phosphates from detergents or other sources are added to these environments, a process called eutrophication may result. Eutrophication is the degradation of waterways as manifested by the formation of dense masses of algal blooms. (Scott, 2002). After the algal populations collapse and die, they decompose and deplete the waterway of oxygen, causing fish to die. (Brady and Weil, 2004).
Figure 2. Algal bloom (MPCA, 2005).
Intense debate about our aquatic ecosystems led to environmental regulation of STPP in detergents. Approximately half of all of the states have passed either a partial or full ban on STPP in laundry detergents. (Glennie et al, 2002). The State of Minnesota passed rules limiting the phosphorus content of laundry detergents to 0.5% by weight and 11% for dishwashing detergents. (Minn. Rule §7100.0210, 2005). Due to the higher threshold for dishwashing detergents in Minnesota, 8.9% of all point source contributions of phosphorus come directly from dishwashing detergents. (MPCA, 2005).
Considering that we use detergents on a daily basis in our laundry soaps and in our dishwashing detergents, it is easy to take them for granted. However, the production of detergents is a very complex chemical process. While improvements can be made in a detergent’s “cleaning power,” these improvements must be balanced against environmental concerns.
Literature Cited
Brady, Nyle C.; Weil, Ray R. Elements of the Nature and Properties of Soil, 2nd ed.; Pearson/Prentice Hall: Upper Saddle River, MJ, 2004, p. 475
Britannia Soap Machinery Company. Three Main Stages in the Soap Making Process. http://www.britsoap.co.uk/soap-making.htm (accessed September, 2005).
Campbell, A. N; Reece, B. J. Biology; Benjamin Cummings: San Francisco, CA, 2005.
Glennie, E.B.; Littlejohn, C.; Gendebien, A.; Hayes, A. Palfrey, R.; Sivil, D.; Wright, K., EU Environment Directorate: Phosphates and Alternative Detergent Builders—Final Report. http://europa.eu.int/comm/environment/water/phosphates.pdf (accessed September, 2005).
Kitchen Doctor. Soap 101. http://www.kitchendoctor.com/articles/soap.html (accessed September, 2005).
Minnesota Pollution Control Agency. Phosphorus Sources and Estimated Amounts Contributed to Surface Waters. http://www.pca.state.mn.us/publications/reports/pstudy-section3-4.pdf (accessed September, 2005).
Minn. Rules §7100.0210 (West, 2005).
Scott, Alex. Study Provides Economic Case for Ban on STPP in Detergents. Chemical Week. October 9, 2002, 15.
Soap and Detergent Association-A. History. http://www.sdahq.org/sdalatest/html/soaphistory1.htm. (accessed September, 2005).
Soap and Detergent Association-B. Chemistry. http://www.cleaning101.com/cleaning/chemistry/-http://www.cleaning101.com/cleaning/chemistry/ (accessed September, 2005).
Soap and Detergent Association-C. Manufacturing. http://www.sdahq.org/sdalatest/html/soapmanuf1.htm. (accessed October, 2005)
World Book Online. Detergent and Soap. http://www.worldbookonline.com/wb/Article?id=ar156160&st=Soap&sc=1-http://www.worldbookonline.com/wb/Article?id=ar156160&st=Soap&sc=1 (accessed September, 2005).
