Soap

Soap

Soap is a cleansing agent created by the chemical reaction of a fatty acid with an alkali metal hydroxide. Chemically speaking, it is a salt composed of an alka-limetal, such as sodium or potassium, and a mixture of "fatty" carboxylic acids. The cleansing action of soap comes from its unique ability to surround oil particles, causing them to be dispersed in water and easily rinsed away. Soap has been used for centuries and continues to be widely used as a cleansing agent, mild antiseptic and ingestible antidote to some forms of poisoning.

The history of soap

It is unknown exactly when soap was discovered. Ancient writings suggest it was known to the Phoenicians as early as around 600 B.C., and was used to some extent by the ancient Romans. During these times, soap was made by boiling tallow (animal fat) or vegetable oils with alkali containing wood ashes. This costly method of production coupled with negative social attitudes toward cleanliness made soap a luxury item affordable only to the rich until the late eighteenth century.

Methods of soapmaking improved when two scientific discoveries were made in the late eighteenth and early nineteenth centuries. In 1790, the French chemist Nicholas Leblanc (1742-1806) invented a process for creating caustic soda (sodium hydroxide) from common table salt (sodium chloride). His invention made inexpensive soap manufacture possible by enabling chemists to develop a procedure whereby natural fats and oils can react with caustic soda . The method was further refined when another French chemist, Michel Eugène Chevreul (1786-1889), discovered the nature of fats and oils in 1823. As soap production became less expensive and attitudes toward cleanliness changed, soapmaking became an important industry.

What is soap?

Soap is a salt of an alkali metal, such as sodium or potassium, with a mixture of "fatty" carboxylic acids. It is the result of a chemical reaction, called saponification, between triglycerides and a base such as sodium hydroxide. During this reaction, the triglycerides are broken down into their component fatty acids, and neutralized into salts by the base. In addition to soap, this chemical reaction produces glycerin.

Soap has the general chemical formula RCOOX. The X represents an alkali metal, an element in the first column on the periodic table of elements. The R represents a hydrocarbon chain composed of a line of anywhere from 8-22 carbon atoms bonded together and surrounded by hydrogen atoms. An example of a soap molecule is sodium palmitate (C16).

How is soap made?

Before the end of World War II, soap was manufactured by a "full-boiled" process. This process required mixing fats and oils in large, open kettles, with caustic soda (NaOH) in the presence of steam. With the addition of tons of salt, the soap was made to precipitate out and float to the top. Here, it was skimmed off and made into flakes or bars. This process required large amounts of energy and over six days to complete one batch.

After World War II, a continuous process of soap manufacture became popular. In the continuous process of soap manufacture, fats and oils react directly with caustic soda. The saponification reaction is accelerated by being run at high temperatures (248°F; 120°C) and pressures (2 atm). Glycerin is washed out of the system and soap is obtained after centrifugation and neutralization. This process has several advantages over the "full-boiled" process. It is more energy efficient, time efficient, allows greater control of soap composition and concentration, and the important by-product, glycerin, is readily recovered.

Both manufacturing methods yield pure soap. Certain chemicals can be added to this pure soap to improve its physical characteristics. The foam in soap is enhanced by additives such as fatty acids. Glycerin is added to reduce the harshness of soap on the skin. Other additives include fragrances and dyes.

How does soap work?

Because soap is a salt, it partially separates into its component ions in water. The active ion of the soap molecule is the RCOO^-. The two ends of this ion behave in different fashions. The carboxylate end (-COO^-) is hy-drophilic (water-loving), and is said to be the "head" of the ion. The hydrocarbon portion is lipophilic (oil-loving) and is called the "tail" of the molecule. This unusual molecular structure is responsible for the unique surface and solubility characteristics of soaps and other surfactants (agents affecting the surface of a material).

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