Soaps and Detergents
Soaps and detergents are cleaning ingredients that are able to remove oil particles from surfaces because of their unique chemical properties. Soaps are created by the chemical reaction of a fatty acid with an alkali metal hydroxide. In a chemical sense, soap is a salt made up of a carboxylic acid and an alkali like sodium of potassium. Soaps are a specific type of the more general category of compounds called detergents. The cleaning action of soaps and detergents is a result of their ability to surround oil particles on a surface and disperse it in water. Bar soap has been used for centuries and continues to be an important product for bathing and cleaning. It is also a mild antiseptic and ingestible antidote for certain poisons.
The exact origin of soap is not known, but records suggest that it was known as early as 600 B.C. by the Phoenicians. It was also used by the ancient Romans, as is evidenced by the writings of Pliny, who described a method for making soap by boiling goat tallow with alkali wood ashes. During the eighth century, soap making was common in the southern countries of Europe. However, the production methods were costly, and there was a general negative social attitude toward cleanliness. This made soap a luxury item used primarily as a cosmetic, and available only to the rich.
It was not until the late eighteenth century that soap became widely available and affordable. One important development was made in 1790 by the French chemist Nicholas Leblanc (1724-1806). He invented a method for producing sodium hydroxide (caustic soda) from chalk, salt, sulfuric acid, and coal. Since these ingredients were relatively inexpensive, it significantly reduced the cost of a soap-making process that involved the reaction of natural fats and oils with caustic soda. The method was further refined in 1823, when Michel-Eugéne Chevreul (1786-1889) the process by which fates are hydrolized by water to fatty acids and glycerols to produce soap. As the cost of soap production fell and attitudes about cleanliness changed, soap-making was poised to become an important industry.
By the early nineteenth industry, soap making was an established industry. Important companies included Colgate-Palmolive, and Proctor and Gamble. Soap was used for most cleaning needs, including personal hygiene, laundry, and dish washing. During World War II, fats were in short supply, which prompted companies to develop synthetic detergents. They were first introduced as laundry detergent for automatic washing machines in 1946. Steadily, the synthetic detergents replaced soap. By 1953, their production exceeded that of soap for the first time. Because these early detergents were not biodegradable like soap, they became a public nuisance, causing sewage problems. Detergent manufacturers responded by developing biodegradable, linear alkyl sulfonates. By 1965, production of non-biodegradable synthetic detergents was halted.
Chemically speaking, soap is the salt formed by the reaction of an alkali metal, such a potassium or sodium, with carboxylic acids. It is produced through a chemical reaction, known as saponification, between triglycerides and a base, such a sodium hydroxide. In this reaction, the triglycerides are reduced to their component fatty acids. The base then neutralizes them into salts, a byproduct of this method of soap production or glycerin. Detergents are formed similarly. However, the staring material is derived from linear, alkyl compounds. These materials are reacted with sulfuric acid, then neutralized, and converted to a salt.
Soaps and detergents have the general chemical formula RCOOX. The R represents a hydrocarbon chain made up of anywhere from eight to 22 carbon atoms bonded to each other and to hydrogen atom. The X represents an alkali metal--any of the elements found in the first column on the periodic table. An example of a soap molecule is sodium stearate (C18), which is made from steric acid. The detergent sodium sulfate (C12) is made from lauric acid.
Since soap and detergents are salts, they separate into their component ions in a solution of water. The portion of the molecule that has a cleansing effect is RCOO-. The two ends of the ion have different solubility characteristics. The carboxylate end (-COO-), or "head," is hydrophilic, and tends to associate with the aqueous phase. The hydrocarbon portion (R), or "tail," is lipophilic, and associated with the oily particles in the solution. The unusual molecular structure is responsible for the surface action and solubility of soaps and detergents. For this reason they are generally known as surfactants.
In a system composed of soap and water, the surfactant molecules tend to be uniformly dispersed. However, thus mixture is not a true solution, because the hydrocarbon portions of the surfactant ions are attracted to each other and form structures called micelles. Micelles are spherical aggregates of surfactant molecules that have the molecular tails in their interior and the hydrophilic heads on their exterior. This structure reduces the surface tension between the incompatible species. When oil is present in the system, it gets incorporated into these micelles, and can be rinsed away.
Detergents and soaps can be classified by their ionic nature. Soaps and sulfate detergents have a negatively charged ion, and are called anionic. Cationic detergents have a positively charged ion. There are also non-ionic detergents that have no charge when placed in a water solution. Finally, amphoteric detergents have either a positively or negatively charged ion, depending on the pH of the system they are incorporated into.
Soap manufacture before World War II was done by a "full-boiled" process. In this method, fats and oils were mixed in large, open kettles, and caustic soda was added. The system was heated, and tons of salt were added to make the soap precipitate out and float to the top. The soap was then skimmed off and processed into flakes or bars. The disadvantage to this system was that it took an excessive amount of energy and time. In fact, it required a full six days to complete a single batch.
After World War II, a more continuous process was developed. In this method of production, fats and oils are reacted directly with caustic soda. By using higher temperatures--250°F (121°C) and pressures (2 atmospheres) the reaction is accelerated. The glycerin (another name of for glycerol) is removed from the system, and soap is isolated by using centrifugation and neutralization processes. This production method proved to be more desirable than the "full-boiled" process. It was energy- and time-efficient, allowing greater control of the composition and concentration of the soap, and it allowed the recovery of glycerin.
Detergent manufacture is similar to that of soap. The starting material is typically a vegetable oil or petroleum product. Vegetable oils, such as coconut, palm kernel, or canola contain an appropriate fatty acid distribution. The oils are first reacted with sulfuric acid, which converts them into sulfates. These materials are then neutralized and converted into salts. To improve the characteristics of the soap before selling it, various ingredients can be added. For example, the foam of pure soap can be improved by the addition of fatty acids or alkanolamides. Glycerin can also be added to reduce the harshness of the soap on the skin. Antibacterial compounds, such as triclosan, can be incorporated into the final soap or detergent product. Additionally, fragrances, dyes, and preservatives are used to modify esthetic characteristics.
While soaps are excellent, biodegradable cleansing ingredients, they suffer from the drawback of forming hard water deposits. Hard water contains amounts of calcium and magnesium ions. The carboxylate ion in soap reacts with these ions, forming a water-insoluble salt that remains deposited on fabric and other surfaces. These hard water plaques can dull fabric colors and cause undesirable rings on bathtubs and sinks. An additional drawback of soaps is that they do not function properly at acidic pHs. Under these conditions, soap ions do not dissociate into their component ions. As a result, they lose their active characteristics. Since detergents do not form hard water deposits and are effective under a wide range of pH conditions, they are often preferred to soap.
Soaps and detergents are primarily used for their cleansing ability. However, soap has also proven its effectiveness as a mild antiseptic. It is also an ingestible antidote for mineral acid or heavy metal poisoning. Furthermore, specialized metallic soaps are employed as additive in paints, inks, and lubricating oils.
There is considerable environmental concern about the accumulation of detergents in the ecosystem. In 1997, Union Carbide developed a non-ionic detergent that can reduce the pollution caused by wastewater at institutional cleaners. Under normal conditions, this detergent functions just like a regular detergent. However, when acid is added to the system, the detergent splits into twp non-toxic fragments. This was the first chemical to gain approval as environmentally safe under the United States Environmental Protection Agency's Environmental Technology Initiative.
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