Soap is a salt of a fatty acid that is used in a variety of cleansing and lubricating products. Soaps are chemically classified as salts of fatty acids because of the presence of an ionic, or polar head, and a nonpolar glyceride tail. The head is composed of positively charged sodium ions and negatively charged oxygen ions, while the tail is a fatty chain. The major classification of soaps is determined by the identity of the metal cation. When the metal is sodium or potassium, the soaps are called toilet soaps, used for handwashing. Many metal dications (Mg2+, Ca2+, and others) give metallic soap. When the metal is lithium, the result is lithium soap (e.g., lithium stearate), which is used in high-performance greases.
Soaps and salts are different types of chemical compounds. Salts are made up of an equal number of anions (negative ions) and cations (positive ions) . In contrast, soaps are surfactants that are usually used for washing, bathing, and other types of housekeeping. They are effective in removing dirt and stains from fabric without excessive rubbing. Soluble salts of an alkali metal, such as potassium or sodium, are good grease removers. They form an emulsion of the oily or solid particles that are held in suspension in wash water to be rinsed away.
Soap ingredients can vary depending on the type of soap. However, all soaps are made using a caustic chemical called lye, also known as sodium hydroxide or caustic soda. Other common ingredients in soap include:
- Fatty acids
- Alkalis (such as sodium hydroxide or potassium hydroxide)
- Fragrances (either from essential oils of flowers or from synthetic sources)
- pH adjusters
- Sodium benzoate and benzoic acid
- Sodium laureth sulfate
- Methylisothiazolinone and methylchloroisothiazolinone
- Cocamidopropyl betaine
Some harmful chemicals that can be found in soap include SLS (Sodium Laureth Sulfate), which has been linked to skin irritations and allergic reactions such as eczematous dermatitis. Another harmful chemical is 1,4 Dioxane, which is a common contaminant found in many cosmetics and products meant for personal care.
