Soaps And Detergents

Type of physical science: Chemistry

Field of study: Environmental chemistry

The chemical structures of soaps and detergents permit them to interact with soils and to remove them from surfaces. These compounds also suspend the dirt in water. Hence, soaps and detergents provide efficient means of cleaning.

Overview

Soaps for medicinal and cleaning purposes have been known and used for centuries. The process of soap manufacture has remained essentially unchanged. A fat from an animal or plant source is hydrolyzed, or broken apart, by a base to give the salt of a long-chain acid (a soap) and a neutral glycerin compound. This process is called saponification. The soap produced has a polar head, which is water-soluble (hydrophilic), and a long, oily tail, which is insoluble in water (hydrophobic). This dual or amphipatic nature of soap is essential to its usefulness in the cleansing process and in other applications.

Modern soap manufacture began in the nineteenth century. Three significant factors played a part in its development. First, the appropriate steam apparatus was developed, which allowed large-scale production. Second, the chemist Michel Chevreul unraveled the nature of the process that was occurring in soap making and showed that fats and oils obeyed the laws that simpler chemical compounds followed. He determined that fats could be resolved into fatty acids and glycerin, and that palmitic, stearic, and oleic acids are the main constituents of fats and oils. He gave soap manufacture a new level of understanding and precision.

Although fats were easily available, the development of the soap industry was hampered by the lack of availability of the base necessary for the saponification reaction. An alkaline, or basic, solution could be obtained from wood ashes; however, large-scale soap manufacture would have been detrimental to forests. The introduction of solid caustic soda was the third advance in the soap-making process. In 1791, Nicolas Leblanc won a prize offered by the French Academy for developing a successful method for preparing sodium carbonate from available sodium chloride. The overall reaction for the Leblanc process has sodium chloride reacting with sulfuric acid, carbon, and calcium carbonate to give the desired sodium carbonate, carbon dioxide, and two other by-products, calcium sulfide and hydrochloric acid. The first major environmental law, the Alkali Act of 1863, prohibited pollution of streams in Great Britain by the waste hydrochloric acid from this process.

The Leblanc process was used for nearly a hundred years but has since been replaced by the more competitive method of preparing sodium carbonate introduced in the 1860's by Belgian chemist Ernest Solvay. The overall reaction uses sodium chloride and calcium carbonate to prepare the desired sodium carbonate. Calcium chloride is the only by-product. Ammonia is used in the process but is recovered using Solvay's technology. Alternate methods of preparing base from the electrolysis of sodium chloride have been developed. In these processes, electricity is used to prepare chlorine gas and sodium hydroxide, an excellent base for soap manufacture.

Soap, the natural cleansing agent formed from the alkaline hydrolysis of fats, has several important properties. It is relatively nontoxic, it comes from renewable sources (plants and animals), and it is biodegradable. When soap is used in hard water (water that has calcium, iron, magnesium, or other metal ions in it), however, its effectiveness is diminished. The fatty acid part of the soap interacts with these ions to form salts that are not soluble in water. The effects of these interactions are seen in the bathtub ring and in the gray color that is deposited on clothes.

Synthetic detergents (syndets) came onto the commercial market after World War II. They are designed to have the same type of cleansing action as soaps but to work effectively in hard water. They do not react with magnesium, calcium, and iron salts to give unwanted precipitates. There are many different detergents available; sodium lauryl sulfate is a typical example. The compound is amphipatic. It has a polar end, -OSO₃-Na⁺, which allows it to be water-soluble. It also has a long, oily tail of twelve carbons with twenty-five hydrogens. This syndet was prepared by reacting a long-chain alcohol with sulfuric acid and then treating the resulting alkyl sulfate with sodium hydroxide to form the salt. This type of detergent is classified as anionic because the sulfate group has a negative charge.

Cheaper, more easily prepared detergents came on the market in the early 1950's. These were prepared in three steps, starting with the hydrocarbon propylene. This three-carbon monomer was polymerized to give a branched-chain molecule of 12 carbons. This was attached to a benzene ring. This system is treated with sulfuric acid and then sodium carbonate to give an alkylbenzenesulfonate (ABS). ABS detergents became very popular because they could be used in all types of water and were inexpensive.

Within a decade of their large-scale introduction, however, a pollution problem developed from the ABS detergents. Foaming streams and rivers were frequent sights. The detergents were building up in the water supply. Unlike the soaps, these branchedchain molecules were not biodegradable. The enzymes in the sewage treatment plants were unable to convert these to small molecules; hence, the detergents returned essentially unchanged to the water supply. This problem was solved by the development of biodegradable detergents.

The biodegradable detergents are linear alkylsulfonates. They differ from the ABS in that they contain very little branching and thus resemble soaps in the structure of their hydrocarbon tails. They are much more easily degraded by microorganisms and comprise most of the detergents used in home washing.

In addition to the surfactant molecule, detergent formulations contain materials designed to improve their performance and/or marketability. These additional components include builders, bleaches, foam stabilizers, optical brighteners, and enzymes. Builders are added to the surfactant molecules of detergents to sequester the magnesium and calcium ions of the hard water. Although the detergent molecules themselves remain soluble in hard water, the ions still interfere with the dirt-suspending power of the detergent.

The most important and effective commercial builder is sodium tripolyphosphate. This compound sequesters the metal ions and also prevents the redeposition of dirt onto the surface being cleaned. It also helps keep the solution basic, and thus aids in keeping the surfactant in solution. In the late 1960's, the phosphate builder in detergents began to be seen as a source of environmental problems. Phosphates are unaffected by sewage treatment and pass into streams and rivers. Phosphates are plant nutrients and encourage the production of algae. The process of eutrophication, or death of a lake, is accelerated.

In response to this environmental concern, detergent manufacturers, after 1975, boasted that their products were low in phosphate or contained no phosphate. Alternate compounds such as sodium aluminosilicate and sodium perborate have been added to detergent formulations, but none is as effective in hard water as the tripolyphosphate.

Soaps and detergents are effective cleansing agents because of their amphipatic nature. Because these chemicals have a polar and a nonpolar end, they can orient themselves in particular ways at surfaces. For example, the nonpolar, oily end of a soap can dissolve in a grease spot on the surface of a fabric. The soap begins to decrease the contact of the dirt with the fabric. Agitation of the system can then allow small grease particles to become suspended within a group of soap molecules in the water solution. The soap molecules orient themselves in a sphere with their oily tails pointing inward and their polar heads in contact with the water. This arrangement is called a micelle. The grease can be contained in the micelle and rinsed away. For a soap or detergent to be effective, there must be enough surfactant present to allow micelles to form; there must be a critical micelle concentration. Heat aids in the cleansing process by melting the oil or grease in a dirt.

Soaps and detergents also function by interacting with water molecules and decreasing their attraction for one another. This allows the water to spread out, or wet fabrics and surfaces more easily. The surface tension of the water is diminished.

Soaps and detergents provide a means to the cleanliness that is essential to humankind's health and aesthetic sense.

Applications

The soap with which people now cleanse themselves is essentially no different from that used by ancient peoples. Tallow from beef or mutton, coconut oil, palm oil, and cottonseed oil are the starting materials. These are broken apart to give fatty acids of chain lengths that range from twelve to eighteen carbons. Most of the soap bars are sodium salts of these fatty acid mixtures with dyestuffs, fragrances, and perhaps some skin care additives and deodorants. Soft soap or liquid soap has a potassium ion instead of a sodium.

Detergents supply a diverse array of cleaning products. These include materials for laundering, hard-surface cleaning, and use in automatic dishwashers. Detergents can be classified according to the type of charge they have in solution. The most widely used surfactants are anionic, that is, in solution they have a negative charge. Other types are cationic and have a positive charge. Some fabric softeners and bathroom cleaners are of this type. Cationic detergents have some bactericidal effects. Liquid laundry products often employ nonionic surfactants, which are very effective in cleaning oily soils. Other types of detergents are amphoteric: They have both a positive and a negative charge. These are generally mild and are used in shampoos and personal-care products.

Many cosmetic compounds have soaps as a significant part of their design. Hand creams and lotions often contain potassium stearate. Face and body powders contain calcium stearate, zinc stearate, and magnesium stearate. These aid in lubrication and in adhesion of the powder. Stick deodorants also use stearate soaps.

In addition to their roles in consumer products, soaps and detergents are used in many research areas and applications. For example, detergents are important in protein purification. The ability of the detergents to form micelles and thus to be soluble in water enables them to aid in the solubilization of membrane-bound proteins. A wide range of pure detergents are available to the biochemist for use in isolation procedures.

Industry has many uses for surfactants. Lithium soaps are used as gelling agents with lubricating oils to form the grease that is used in automobile maintenance. The soaps thicken the oils and entrap them. Other industrial greases use sodium, calcium, aluminum, and barium soaps.

Both soaps and synthetic detergents are used in the plastics industry for the formation of emulsion polymers. The micelles of the surfactant provide the individualized locations for the initiation of the polymerization process. Growing polymers are separated from one another and are less likely to interfere and terminate the reaction. The water system also provides a means of heat transfer. The preparation of polystyrene by emulsion methods is commercially very important, as is the formation of styrene-butadiene rubber, latex paints, and other products.

The textile industry is a major user of soaps and detergents. Cotton fabric is boiled in soap solution to remove natural waxes and residues from the processing. Similarly, silk is degummed in a soap solution to prepare soft, pliable fibers.

Detergents aid in the drilling operations of oil companies, and are found in cement and road construction materials. Detergents even help fire fighters battle blazes involving paper or cotton bales because the water with detergent added will penetrate, or wet, better.

Context

The oldest known record of a chemical reaction is given in the Sumerian Logos, discovered in 1957 by Martin Levey. This tablet tells of soap preparation by ancient Sumerians in 2500 B.C. In addition, it reveals that they used the soap for washing woolen clothes.

The Egyptian Papyrus Ebers (600 B.C.) evidences the use of soap as a medicine. Pliny the Elder (A.D. 60) discussed a soap made from goat fat and beechwood ash which was used by the Germans as a hair pomade. Galen in the second century A.D. wrote again about the cleansing effects of soap.

The Arabs brought soap-making techniques to Spain and from there to the Mediterranean countries. The availability of olive trees and soda ash from sea plants provided the needed raw materials. In the Middle Ages, important soap-making centers arose in Valencia, Seville, Venice, and Genoa. The eighteenth-century developments in soap-making technology made soap a readily available household commodity.

Although a sulfonated oil had been used in the textile industry since the early nineteenth century, it was the shortage of fatty acids in Germany during World War I that became the impetus for the development of synthetic detergents for cleaning purposes. Shell Chemical Company marketed the first widely used synthetic detergent (Teepol) in the 1930's. It was a good wetting agent but not very effective as a cleaning product. Wartime shortages of fats and oils, as well as the Navy's need for cleaning agents that would work in hard seawater, aided in detergent development. After World War II, the availability of materials from the petroleum industry spurred the growth of synthetic detergents. By 1953, more detergent than soap was sold in the United States. Soap is used for personal hygiene, but most other laundering and cleaning operations use detergents.

Consumer interest in the environment has moved the detergent industry toward linear, biodegradable formulations with little or no phosphate. Newer detergents will exhibit even greater degrees of biodegradability and will be better formulated to reduce surfactant levels entering rivers and streams. In addition, higher prices for petrochemicals will shift the detergent industry to more long-chain, sulfonated molecules derived from renewable animal and plant sources.

Variations in the textile composition of clothing have led to and will continue to promote changes in laundry detergent formulations. Synthetic fibers require lower washing temperatures, and this necessitates higher concentrations of surfactants. Greater use of bleaches to ensure hygienic washing will accompany the trend to lower wash temperatures. Enzymes capable of destroying proteins, fats, and cellulose are also becoming important auxiliaries in detergents.

Soaps and detergents are among those synthetic chemicals with which people have the greatest interaction. Research must continue so that they become even safer for people, materials, and the environment.

Principal terms:

EMULSION: a system in which one type of liquid is dispersed throughout another liquid

MICELLE: an aggregate of molecules that forms when the concentration of a soap or detergent reaches a critical level; the micelles are suspended in water with their oily parts interacting

SURFACTANT: a surface-acting chemical that reduces the attraction of water molecules to one another and thus enhances the wetting ability of the water

WETTING: the tendency of a liquid to spread out evenly on a surface and get between a soil and a surface

Bibliography

Durham, K., ed. SURFACE ACTIVITY AND DETERGENCY. London: Macmillan, 1961. This monograph provides a lucid introduction to the physical chemistry of detergents. The three steps in the cleaning process, wetting, removal of dirt, and prevention of redeposition, are explained in understandable terms.

Falbe, Juergen, ed. SURFACTANTS IN CONSUMER PRODUCTS: THEORY, TECHNOLOGY AND APPLICATION. New York: Springer-Verlag, 1987. This book summarizes the changes and trends in surfactant use. It begins with a historical overview and considers applications, synthesis, physical properties, and performance of detergents and personal care products. The contributors are industrial scientists; hence, some attention is paid to the manufacture of surfactants and their environmental impact. This is an excellent, well-organized overview for the serious reader.

Gibbs, F. W. "The History of the Manufacture of Soap." ANNALS OF SCIENCE 4 (1939): 169-190. This excellent article provides the general reader with an overview of the manufacture and use of soap, from early Egypt to the twentieth century.

Haber, L. F. THE CHEMICAL INDUSTRY DURING THE NINETEENTH CENTURY. London: Oxford University Press, 1958. This historical account of the development of the chemical industry in Great Britain and America discusses the effect of the Leblanc soda process on the growth of chemical manufacture.

Joesten, Melvin, David Johnston, John Netterville, and James Wood. WORLD OF CHEMISTRY. Philadelphia: Saunders College Publishing, 1991. This text, designed to accompany the television series of the same name, endeavors to consider chemical applications that affect the quality of human life. In chapter 22, on consumer chemistry, there is a section on cleansing agents that clearly illustrates the structures and modes of action of soaps and detergents, and also discusses the other ingredients in detergent formulations.

Witcoff, Harold, and Bryan Reuben. INDUSTRIAL ORGANIC CHEMICALS IN PERSPECTIVE. Part 2, TECHNOLOGY, FORMULATION, AND USE. New York: John Wiley & Sons, 1980. This book provides a well-focused introduction to the types of organic compounds that are important in the marketplace. Chapter 7, on surface active agents, discusses the various types of soaps and detergents and their wide applicability.

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