In chemistry, soap is a salt of a fatty acid. Soap is mainly used for washing and cleaning, but soaps are also important components of lubricants. Soaps for cleansing are obtained by treating vegetable or animal oils and fats with a strongly alkaline solution. The alkaline solution, often lye, promotes what is known as saponification. In saponification, fats are broken down (hydrolyzed) yielding crude soap, i.e. impure salts of fatty acids and glycerol.
Soaps are key components of most lubrication greases, which are usually emulsions of calcium and lithium soaps in a mineral oil. Lithium-based greases are particularly important. Many other metal ions are used, including aluminium, sodium, and mixtures of various metal ions. Such soaps are sometimes classified as thickeners, meaning that they elevate the viscosity of the oil. In ancient times, lubricating greases were prepared by the addition of lime to olive oil.
When used for cleaning, soap serves as a surfactant in conjunction with water. The cleaning action of this mixture is attributed to the action of micelles, tiny spheres coated on the outside with polar carboxylate groups, encasing a hydrophobic (lipophilic) pocket that can surround the grease particles, allowing them to dissolve in water. The hydrophobic portion is made up of the long hydrocarbon chain from the fatty acid. In other words, whereas normally oil and water do not mix, the addition of soap allows oils to dissolve in water, allowing them to be rinsed away. Synthetic detergents operate by similar mechanisms to soap.
The nature of the soap depends on the alkali metal. Sodium soaps, prepared from sodium hydroxide are firmer. Potassium soaps, derived from potassium hydroxide are softer or often liquid. Historically, potassium hydroxide was extracted from the ashes of bracken or other wood ashes. Lithium soaps also tend to be harder - these are used exclusively in greases.
Soaps are derivatives of fatty acids. Traditionally soaps are derived from triglycerides (vegetable and animal fats). Triglyceride is the technical name for these triesters of fatty acids. Sodium tallowate, a common ingredient in many soaps, is derived from rendered beef fat. Typical vegetable oils used in soap making are palm oil, where the product is typically softer. If soap is made from pure olive oil it may be called Castile soap or Marseille soap. The term "castile" is also sometimes applied to soaps with a mix of oils, but a high percentage of olive oil.
Aside from olive oil, other saponifiable oils and fats include coconut, palm, cocoa butter, hemp oil, and shea butter to provide different qualities. For example, olive oil provides mildness in soap. Coconut oil provides lots of lather. Coconut and palm oils provide hardness. Sometimes castor oil can also be used as an humectant. Most common, though, is a combination of coconut, palm, and olive oils. Smaller amounts of unsaponifiable oils and fats that do not yield soap are sometimes added for further benefits.
The earliest recorded evidence of the production of soap-like materials dates back to around 2800 BC in Ancient Babylon. In the reign of Nabonidus (556-539 BCE) a recipe for soap consisted of uḥulu [ashes], cypress [oil] and sesame [seed oil] "for washing the stones for the servant girls".[4] A formula for soap consisting of water, alkali, and cassia oil was written on a Babylonian clay tablet around 2200 BC.
The Ebers papyrus (Egypt, 1550 BC) indicates that ancient Egyptians bathed regularly and combined animal and vegetable oils with alkaline salts to create a soap-like substance. Egyptian documents mention that a soap-like substance was used in the preparation of wool for weaving.
The word sapo, Latin for soap, first appears in Pliny the Elder's Historia Naturalis, which discusses the manufacture of soap from tallow and ashes, but the only use he mentions for it is as a pomade for hair; he mentions rather disapprovingly that among the Gauls and Germans men are likelier to use it than women.
A popular belief encountered in some places claims that soap takes its name from a supposed "Mount Sapo" (q.v.); but there is no such place, and no evidence for the apocryphal story. In fact, the Latin word sapo simply means "soap"; it was likely borrowed from an early Germanic language, and is cognate with Latin sebum, "tallow", which appears in Pliny the Elder's account. Roman animal sacrifices usually burned only the bones and inedible entrails of the sacrificed animals; edible meat and fat from the sacrifices were taken by the humans rather than the gods. Animal sacrifices in the ancient world would not have included enough fat to make much soap.
Zosimos of Panopolis c. 300 AD describes soap and soapmaking. Galen describes soap-making using lye and prescribes washing to carry away impurities from the body and clothes. According to Galen, the best were German and ones from Gaul were second best. This is a reference to true soap in antiquity.
Soap-makers in Naples were members of a guild in the late sixth century, and in the 8th century, soap-making was well-known in Italy and Spain. The Carolingian capitulary De Villis, dating to around 800, and sometimes attributed to Charlemagne, mentions soap as being one of the products the stewards of estates are to tally. Soap-making is mentioned both as "women's work" and the produce of "good workmen" alongside other necessities such as the produce of carpenters, blacksmiths and bakers.
Soaps made from vegetable oils (such as olive oil), aromatic oils (such as thyme oil) and lye (al-Soda al-Kawia) were first produced by Muslim chemists. From the beginning of the 7th century, soap was produced in Nablus (West Bank), Kufa (Iraq), and Basra (Iraq). Soap was perfumed and colored, some of the soaps were liquid and others were solid. They also had special soap for shaving. It was sold for 3 Dirhams (0.3 Dinars) apiece in 981 AD. The Persian Muslim chemist Al-Razi wrote a manuscript on recipes for true soap. A recently discovered manuscript from the 13th century details more recipes for soap making; e.g., take some sesame oil, a sprinkle of potash, alkali, and some lime, mix them all together and boil. When cooked, they are poured into molds and left to set, leaving hard soap.[citation needed]This pleasant-to-use hard soap was imported by Europe from the Arab lands of the Mediterranean and was shipped across the Alps to northern Europe via Italy.
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Topic: Soap (Read 5742 times)
