Soap

Soap is a salt of a fatty acid used for cleaning and lubricating products as well as other applications. In a domestic setting, soaps, specifically "toilet soaps", are surfactants usually used for washing, bathing, and other types of housekeeping. In industrial settings, soaps are used as thickeners, components of some lubricants, emulsifiers, and catalysts.
Soaps are often produced by mixing fats and oils with a base. Humans have used soap for millennia; evidence exists for the production of soap-like materials in ancient Babylon around 2800 BC.

Types

Toilet soaps

In a domestic setting, "soap" usually refers to what is technically called a toilet soap, used for household and personal cleaning. Toilet soaps are salts of fatty acids with the general formula M⁺, where M is Na or K.
When used for cleaning, soap solubilizes particles and grime, which can then be separated from the article being cleaned. The insoluble oil/fat "dirt" become associated inside micelles, tiny spheres formed from soap molecules with polar hydrophilic groups on the outside and encasing a lipophilic pocket, which shields the oil/fat molecules from the water, making them soluble. Anything that is soluble will be washed away with the water. In hand washing, as a surfactant, when lathered with a little water, soap kills microorganisms by disorganizing their membrane lipid bilayer and denaturing their proteins. It also emulsifies oils, enabling them to be carried away by running water.
When used in hard water, soap does not lather well but forms soap scum.

Non-toilet soaps

So-called metallic soaps are key components of most lubricating greases and thickeners. A commercially important example is lithium stearate. Greases are usually emulsions of calcium soap or lithium soap and mineral oil. Many other metallic soaps are also useful, including those of aluminium, sodium, and mixtures thereof. Such soaps are also used as thickeners to increase the viscosity of oils. In ancient times, lubricating greases were made by the addition of lime to olive oil, which would produce calcium soaps. Metal soaps are also included in modern artists' oil paints formulations as a rheology modifier. Metal soaps can be prepared by neutralizing fatty acids with metal oxides:
A cation from an organic base such as ammonium can be used instead of a metal; ammonium nonanoate is an ammonium-based soap that is used as an herbicide.
Another class of non-toilet soaps are resin soaps, which are produced in the paper industry by the action of tree rosin with alkaline reagents used to separate cellulose from raw wood. A major component of such soaps is the sodium salt of abietic acid. Resin soaps are used as emulsifiers.

Soapmaking

The production of toilet soaps usually entails saponification of triglycerides, which are vegetable or animal oils and fats. An alkaline solution induces saponification whereby the triglyceride fats first hydrolyze into salts of fatty acids. Glycerol is liberated. The glycerin is sometimes left in the soap product as a softening agent, although it is sometimes separated. The glycerine makes the soap softer. The addition of glycerol and processing of this soap produces glycerin soap.

Additives

or pumice may be added to produce a scouring soap. The scouring agents serve to remove dead cells from the skin surface being cleaned. This process is called exfoliation.
To make antibacterial soap, compounds such as triclosan or triclocarban can be added. There is some concern that use of antibacterial soaps and other products might encourage antimicrobial resistance in microorganisms.
The type of alkali metal used determines the kind of soap product. Sodium soaps, prepared from sodium hydroxide, are firm, whereas potassium soaps, derived from potassium hydroxide, are softer or often liquid. Historically, potassium hydroxide was extracted from the ashes of bracken or other plants. Lithium soaps also tend to be hard. These are used exclusively in greases.
For making toilet soaps, triglycerides are derived from coconut, olive, or palm oils, as well as tallow. Triglyceride is the chemical name for the triesters of fatty acids and glycerin. Tallow, i.e., rendered fat, is the most available triglyceride from animals. Hence, the fat compound in many soap is known as sodium tallowate. Each species offers quite different fatty acid content, resulting in soaps of distinct feel. The seed oils give softer but milder soaps. Soap made from pure olive oil, sometimes called Castile soap or Marseille soap, is reputed for its particular mildness. The term "Castile" is also sometimes applied to soaps from a mixture of oils with a high percentage of olive oil.
Most soaps for body washing are "refatted" or "superfatted". Superfatting agents can achieve multiple goals. First, superfatting ensures the absence of lye in the soap. For cake soaps, superfatting confers plasticity, which prevents cracking. Finally, superfats replenish the skin oils that are washed away during the cleansing process. Typical superfatting agents are lecithins, lanolin, and various alkanolamides. Handmade soap can be refatted using fat or coconut oil beyond that needed to consume the alkali used, and the glycerol left in acts as a moisturizing agent. Superfatted soap is more skin-friendly than one without extra fat, although it can leave a "greasy" feel. Sometimes, an emollient is added, such as jojoba oil or shea butter.

	Lauric acid	Myristic acid	Palmitic acid	Stearic acid	Oleic acid	Linoleic acid	Linolenic acid
fats	C₁₂ saturated	C₁₄ saturated	C₁₆ saturated	C₁₈ saturated	C₁₈ monounsaturated	C₁₈ diunsaturated	C₁₈ triunsaturated
Tallow	0	4	28	23	35	2	1
Coconut oil	48	18	9	3	7	2	0
Palm kernel oil	46	16	8	3	12	2	0
Palm oil	0	1	44	4	37	9	0
Laurel oil	54	0	0	0	15	17	0
Olive oil	0	0	11	2	78	10	0
Canola oil	0	1	3	2	58	9	23

History

Proto-soaps in the Ancient world

Proto-soaps, which mixed fat and alkali and were used for cleansing, are mentioned in Sumerian, Babylonian and Egyptian texts.
The earliest recorded evidence of the production of soap-like materials dates back to around 2800 BC in ancient Babylon. A formula for making a soap-like substance was written on a Sumerian clay tablet around 2500 BC. This was produced by heating a mixture of oil and wood ash, the earliest recorded chemical reaction, and used for washing woolen clothing.
The Ebers papyrus indicates the ancient Egyptians used a soap-like product as a medicine and created this by combining animal fats or vegetable oils with a soda ash substance called trona. Egyptian documents mention a similar substance was used in the preparation of wool for weaving.
In the reign of Nabonidus, a recipe for a soap-like substance consisted of uhulu , cypress and sesame "for washing the stones for the servant girls".

True soaps in the Ancient world

True soaps, which we might recognise as soaps today, were different to proto-soaps. They foamed, were made deliberately, and could be produced in a hard or soft form because of an understanding of lye sources. It is uncertain as to who was the first to invent true soap.
Knowledge of how to produce true soap emerged at some point between early mentions of proto-soaps and the first century AD. Alkali was used to clean textiles such as wool for thousands of years but soap only forms when there is enough fat, and experiments show that washing wool does not create visible quantities of soap. Experiments by Sally Pointer show that the repeated laundering of materials used in perfume-making lead to noticeable amounts of soap forming. This fits with other evidence from Mesopotamian culture.
Pliny the Elder, whose writings chronicle life in the first century AD, describes soap as "an invention of the Gauls". The word sapo, Latin for soap, has connected to a mythical Mount Sapo, a hill near the River Tiber where animals were sacrificed. But in all likelihood, the word was borrowed from an early Germanic language and is cognate with Latin sebum, "tallow". It first appears in Pliny the Elder's account, Historia Naturalis, which discusses the manufacture of soap from tallow and ashes. There he mentions its use in the treatment of scrofulous sores, as well as among the Gauls as a dye to redden hair which the men in Germania were more likely to use than women. The Romans avoided washing with harsh soaps before encountering the milder soaps used by the Gauls around 58 BC. Aretaeus of Cappadocia, writing in the 2nd century AD, observes among "Celts, which are men called Gauls, those alkaline substances that are made into balls called soap". The Romans' preferred method of cleaning the body was to massage oil into the skin and then scrape away both the oil and any dirt with a strigil. The standard design is a curved blade with a handle, all of which is made of metal.
The 2nd-century AD physician Galen describes soap-making using lye and prescribes washing to carry away impurities from the body and clothes. The use of soap for personal cleanliness became increasingly common in this period. According to Galen, the best soaps were Germanic, and soaps from Gaul were second best. Zosimos of Panopolis, circa 300 AD, describes soap and soapmaking.
In the Southern Levant, the ashes from barilla plants, such as species of Salsola, saltwort and Anabasis, were used to make potash. Traditionally, olive oil was used instead of animal lard throughout the Levant, which was boiled in a copper cauldron for several days. As the boiling progresses, alkali ashes and smaller quantities of quicklime are added and constantly stirred. In the case of lard, it required constant stirring while kept lukewarm until it began to trace. Once it began to thicken, the brew was poured into a mold and left to cool and harden for two weeks. After hardening, it was cut into smaller cakes. Aromatic herbs were often added to the rendered soap to impart their fragrance, such as yarrow leaves, lavender, germander, etc.