In chemistry, a soap is a salt of a fatty acid.
[IUPAC. " IUPAC Gold Book – soap " Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. . . Accessed 2010-08-09] Household uses for soaps include washing, bathing, and other types of housekeeping, where soaps act as , emulsifying oils to enable them to be carried away by water. In industry, they are used as , components of some , and precursors to catalysts.
Kinds of soaps
Since they are salt of fatty acids, soaps have the general formula (RCO2−
(R = alkyl
). The major classification of soaps is determined by the identity of Mn+
. When M = Na or K, the soaps are called toilet soaps, used for handwashing. Many metals dications (Mg2+
, and others give metallic soap
. When M = Li, the result is lithium soap
(e.g., lithium stearate
), which are used in high performance
Soaps are key components of most lubricating greases and thickeners. Greases are usually emulsions of calcium stearate
or lithium stearate
and mineral oil.
[see the main Grease (lubricant) article]
Many other metallic soaps are also useful, including those of aluminium, sodium, and mixtures of them. 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.
Metal soaps are also included in modern artists' oil paints formulations as a rheology modifier.
Production of metallic soaps
Most heavy metal soaps are prepared by neutralization of purified fatty acids:
- 2 RCO2H + CaO → (RCO2)2Ca + H2O
In a domestic setting, "soap" usually refers to what is technically called a toilet soap, used for household and personal cleaning. When used for cleaning, soap solubilizes particles and grime, which can then be separated from the article being cleaned.
The insoluble oil/fat molecules become associated inside
, tiny spheres formed from soap molecules with polar hydrophile
(water-attracting) groups on the outside and encasing a lipophilicity
(fat-attracting) pocket, which shields the oil/fat molecules from the water making it soluble. Anything that is soluble will be washed away with the water.
Production of toilet soaps
Soap making entails the production of metal salts of fatty acids. The production of toilet soaps usually entails saponification of fats (triglycerides). Triglycerides are vegetable or animal oils and fats. An alkaline solution, often called lye
or sodium hydroxide
), induces saponification
whereby the triglyceride fats first Hydrolysis
into salts of fatty acids. Glycerol
(glycerin) is liberated. The glycerin can remain in the soap product as a softening agent, although sometimes separated.
[Cavitch, Susan Miller. The Natural Soap Book. Storey Publishing, 1994 .]
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, (oils and fats) are derived from coconut, olive, or palm oils, as well as tallow.
[David J. Anneken, Sabine Both, Ralf Christoph, Georg Fieg, Udo Steinberner, Alfred Westfechtel "Fatty Acids" in Ullmann's Encyclopedia of Industrial Chemistry 2006, Wiley-VCH, Weinheim. ] Triglyceride is the chemical name for the tri of fatty acids and glycerin. Tallow, i.e., rendered beef fat, is the most available triglyceride from animals. 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, but a high percentage of olive oil.
|+ Fatty acid content of various fats used for soapmaking|
! !! Lauric acid!! Myristic acid!! Palmitic acid!! Stearic acid!! Oleic acid!! Linoleic acid!! Linolenic acid|
History of toilet soaps
The history of soaps for handwashing and related domestic use is described below. Many soaps are not intended for domestic use however.
Ancient Middle East
The earliest recorded evidence of the production of soap-like materials dates back to around 2800 BC in ancient Babylon
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 the bathed regularly and combined animal and vegetable oils with alkaline salts to create a soap-like substance. Egyptian documents mention a soap-like substance was used in the preparation of wool for weaving.
In the reign of Nabonidus (556–539 BC), a recipe for soap consisted of uhulu ashes, cypress oil and sesame seed "for washing the stones for the servant girls".
[Noted in ]
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 the men of the
were more likely to use it than their female counterparts.
[Pliny the Elder, Natural History, XXVIII.191. See also Martial, Epigrammata, VIII, 33, 20. ]
Aretaeus of Cappadocia, writing in the first century AD, observes among "Celts, which are men called Gauls, those alkaline substances that are made into balls ... called soap
[Aretaeus, The Extant Works of Aretaeus, the Cappadocian, ed. and tr. Francis Adams (London) 1856: 238 and 496 , noted in Michael W. Dols, "Leprosy in medieval Arabic medicine" Journal of the History of Medicine 1979:316 note 9; the Gauls with whom the Cappadocian would have been familiar are those of Anatolian Galatia.]
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 Gauls used soap made from animal fat
A popular belief claims soap takes its name from a supposed Mount Sapo, where animal sacrifices were supposed to have taken place; tallow from these sacrifices would then have mixed with ashes from fires associated with these sacrifices and with water to produce soap, but there is no evidence of a Mount Sapo in the Roman world and no evidence for the story. 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.
[ soap . Etymonline.com. Retrieved on 2011-11-20.] Roman animal 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.
Zosimos of Panopolis, circa 300 AD, describes soap and soapmaking.
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 the 2nd century A.D. According to Galen, the best soaps were Germanic, and soaps from Gaul were second best. This is a reference to true soap in antiquity.
A detergent similar to soap was manufactured in ancient China from the seeds of Gleditsia sinensis.
Another traditional detergent is a mixture of pig pancreas and plant ash called "Zhu yi zi". True soap, made of animal fat, did not appear in China until the modern era.
Soap-like detergents were not as popular as ointments and creams.
Islamic Middle East
Hard toilet soap with a pleasant smell was produced in the Middle East
during the Islamic Golden Age, when soap-making became an established industry. Recipes for soap-making are described by Muhammad ibn Zakariya al-Razi (854–925), who also gave a recipe for producing glycerine
from olive oil
. In the Middle East, soap was produced from the interaction of
. In Syria
, soap was produced using olive oil together with alkali and lime. Soap was exported from Syria, to other parts of the Muslim world
and to Europe.
[Ahmad Y. al-Hassan (2001), Science and Technology in Islam: Technology and applied sciences, pages 73-74 , UNESCO]
A 12th-century Islamic document describes the process of soap production.
[BBC Science and Islam Part 2, Jim Al-Khalili. BBC Productions. Accessed 30 January 2012.] It mentions the key ingredient, alkali, which later becomes crucial to modern chemistry, derived from al-qaly or "ashes".
By the 13th century, the manufacture of soap in the Islamic world had become virtually industrialized, with sources in Nablus, Fes, Damascus, and Aleppo.
Soapmakers in Naples
were members of a guild
in the late sixth century (then under the control of the Eastern Roman Empire),
[footnote 48, p. 104, Understanding the Middle Ages: the transformation of ideas and attitudes in the Medieval world, Harald Kleinschmidt, illustrated, revised, reprint edition, Boydell & Brewer, 2000, .]
and in the eighth century, soap-making was well known in Italy and Spain.
[Anionic and Related Lime Soap Dispersants, Raymond G. Bistline Jr., in Anionic Surfactants: Organic Chemistry, Helmut Stache, ed., Volume 56 of Surfactant science series, CRC Press, 1996, chapter 11, p. 632, .]
The Carolingian capitulary De Villis
, dating to around 800, representing the royal will of Charlemagne
, mentions soap as being one of the products the stewards of royal estates are to tally. The lands of Medieval Spain
were a leading soapmaker by 800, and soapmaking began in the Kingdom of England about 1200.
[ www.soap-flakes.com . www.soap-flakes.com. Retrieved on 2015-10-31.]
Soapmaking is mentioned both as "women's work" and as the produce of "good workmen" alongside other necessities, such as the produce of carpenters, blacksmiths, and bakers.
In Europe, soap in the 9th century was produced from animal fats and had an unpleasant smell. Hard toilet soap with a pleasant smell was later imported from the Middle East.
In France, by the second half of the 15th century, the semi-industrialized professional manufacture of soap was concentrated in a few centers of Provence—Toulon, Hyères, and Marseille—which supplied the rest of France.
In Marseilles, by 1525, production was concentrated in at least two factories, and soap production at Marseille tended to eclipse the other Provençal centers. [Barthélemy, L. (1883) "La savonnerie marseillaise", noted by Nef 1936:660 note 99.] English manufacture tended to concentrate in London. [Nef 1936:653, 660.]
Finer soaps were later produced in Europe from the 16th century, using vegetable oils (such as olive oil) as opposed to animal fats. Many of these soaps are still produced, both industrially and by small-scale artisans. Castile soap is a popular example of the vegetable-only soaps derived from the oldest "white soap" of Italy.
In modern times, the use of soap has become commonplace in industrialized nations due to a better understanding of the role of hygiene in reducing the population size of pathogenic microorganisms. Industrially manufactured bar soaps became available in the late 18th century, as advertising campaigns in Europe and America promoted popular awareness of the relationship between cleanliness and health.
Until the Industrial Revolution, soapmaking was conducted on a small scale and the product was rough. In 1780 James Keir established a chemical works at Tipton, for the manufacture of alkali from the sulfates of potash and soda, to which he afterwards added a soap manufactory. The method of extraction proceeded on a discovery of Keir's. Andrew Pears started making a high-quality, transparent soap in 1807
in London. His son-in-law, Thomas J. Barratt, opened a factory in Isleworth in 1862.
During the Restoration era (February 1665 – August 1714) a soap tax was introduced in England, which meant that until the mid-1800s, soap was a luxury, used regularly only by the well-to-do. The soap manufacturing process was closely supervised by revenue officials who made sure that soapmakers' equipment was kept under lock and key when not being supervised. Moreover, soap could not be produced by small makers because of a law which stipulated that soap boilers must manufacture a minimum quantity of one imperial ton at each boiling, which placed the process beyond reach of the average person. The soap trade was boosted and deregulated when the tax was repealed in 1853.
William Gossage produced low-priced, good-quality soap from the 1850s. Robert Spear Hudson began manufacturing a soap powder in 1837, initially by grinding the soap with a mortar and pestle. American manufacturer Benjamin T. Babbitt introduced marketing innovations that included sale of bar soap and distribution of . William Hesketh Lever and his brother, James, bought a small soap works in Warrington in 1886 and founded what is still one of the largest soap businesses, formerly called Lever Brothers and now called Unilever. These soap businesses were among the first to employ large-scale advertising campaigns.
Liquid soap was not invented until the nineteenth century; in 1865, William Shepphard patented a liquid version of soap. In 1898, B.J. Johnson developed a soap derived from palm and olive oils; his company, the B.J. Johnson Soap Company, introduced "Palmolive" brand soap that same year. This new brand of soap became popular rapidly, and to such a degree that B.J. Johnson Soap Company changed its name to Palmolive.
In the early 1900s, other companies began to develop their own liquid soaps. Such products as Pine-Sol and Tide appeared on the market, making the process of cleaning things other than skin, such as clothing, floors, and bathrooms, much easier.
Liquid soap also works better for more traditional or non-machine washing methods, such as using a washboard.
Soap-making for hobbyists
A variety of methods are available for hobbyists to make soap.
[Garzena, Patrizia, and Tadiello, Marina (2013). The Natural Soapmaking Handbook. Online information and Table of Contents . /]
Most soapmakers use processes where the glycerol remains in the product, and the saponification continues for many days after the soap is poured into molds. The glycerol is left during the hot-process method, but at the high temperature employed, the reaction is practically completed in the kettle, before the soap is poured into molds. This simple and quick process is employed in small factories all over the world.
Handmade soap from the cold process also differs from industrially made soap in that an excess of fat is used, beyond that needed to consume the alkali (in a cold-pour process, this excess fat is called "superfatting"), and the glycerol left in acts as a moisturizing agent. However, the glycerine also makes the soap softer. Addition of glycerol and processing of this soap produces glycerin soap. Superfatted soap is more skin-friendly than one without extra fat, although it can leave a "greasy" feel. Sometimes, an Moisturizer is added, such as jojoba oil or shea butter. Sand or pumice may be added to produce a 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 antibiotic resistance in microorganisms.
Image:Azul e Branco.JPG|Azul e branco soap – a bar of blue-white soap
Image:Soap P1140887.jpg|Handmade soaps sold at a shop in Hyères, France
Image:Savon de Marseille.jpg|Traditional Marseille soap
Many commercially available soap molds are made of silicone or various types of plastic, although many soapmaking hobbyists may use cardboard boxes lined with a plastic film. Wooden molds, unlined or lined with silicone sleeves, are also readily available to the general public. Soaps can be made in long bars that are cut into individual portions, or cast into individual molds.
Free ebook at Google Books.
Dunn, Kevin M. (2010). Scientific Soapmaking: The Chemistry of Cold Process. Clavicula Press. .
Garzena, Patrizia, and Marina Tadiello (2004). Soap Naturally: Ingredients, methods and recipes for natural handmade soap. Online information and Table of Contents. /
Garzena, Patrizia, and Marina Tadiello (2013). The Natural Soapmaking Handbook. Online information and Table of Contents. /
Mohr, Merilyn (1979). The Art of Soap Making. A Harrowsmith Contemporary Primer. Firefly Books. .
Thomssen, E. G., Ph. D. (1922). Soap-Making Manual. Free ebook at Project Gutenberg.