A handmade soap barTwo equivalent images of the chemical structure ofsodium stearate, a typical ingredient found in bar soapsEmulsifying action of soap on oil
Soap is asalt of afatty acid (sometimes other carboxylic acids) used for cleaning and lubricating products as well as other applications.[1] In a domestic setting, soaps, specifically "toilet soaps", aresurfactants usually used forwashing,bathing, and other types ofhousekeeping. In industrial settings, soaps are used asthickeners, components of somelubricants,emulsifiers, andcatalysts.
Soaps are often produced by mixing fats and oils with abase.[2] Humans have used soap for millennia; evidence exists for the production of soap-like materials in ancientBabylon around 2800 BC.[3]
Types
A collection of decorative bar soaps, as often found inhotels
Toilet soaps
Structure of amicelle, a cell-like structure formed by the aggregation of soap subunits (such assodium stearate): The exterior of the micelle is hydrophilic (attracted to water) and the interior is lipophilic (attracted to oils).
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 (RCO2−)M+, where M isNa (sodium) orK (potassium).[4]
When used for cleaning, soapsolubilizes particles and grime, which can then be separated from the article being cleaned. The insoluble oil/fat "dirt" become associated insidemicelles, tiny spheres formed from soap molecules with polarhydrophilic (water-attracting) groups on the outside and encasing alipophilic (fat-attracting) pocket, which shields the oil/fat molecules from the water, making them soluble. Anything that is soluble will be washed away with the water. Inhand washing, as a surfactant, when lathered with a little water, soap killsmicroorganisms by disorganizing their membranelipid bilayer anddenaturing theirproteins.[5][6][7] It alsoemulsifies oils, enabling them to be carried away by running water.[8]
So-calledmetallic soaps are key components of most lubricatinggreases and thickeners.[4] A commercially important example islithium stearate. Greases are usuallyemulsions ofcalcium soap orlithium soap andmineral oil. Many other metallic soaps are also useful, including those ofaluminium,sodium, and mixtures thereof. Such soaps are also used as thickeners to increase theviscosity of oils. In ancient times, lubricating greases were made by the addition oflime toolive oil, which would produce calcium soaps.[10] Metal soaps are also included in modern artists'oil paints formulations as arheology modifier.[11] Metal soaps can be prepared by neutralizing fatty acids with metal oxides:
2 RCO2H + CaO → (RCO2)2Ca + H2O
A cation from anorganic base such asammonium can be used instead of a metal; ammoniumnonanoate is an ammonium-based soap that is used as an herbicide.[12]
Another class of non-toilet soaps areresin soaps, which are produced in thepaper 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 ofabietic acid. Resin soaps are used as emulsifiers.[13]
The production of toilet soaps usually entailssaponification oftriglycerides, which are vegetable or animal oils and fats. An alkaline solution (oftenlye) induces saponification whereby the triglyceride fats firsthydrolyze into salts of fatty acids.Glycerol (glycerin) is liberated. The glycerin is sometimes left in the soap product as a softening agent, although it is sometimes separated.[14][15] Handmade soap can differ from industrially made soap in that an excess of fat or coconut oil beyond that needed to consume thealkali is used (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. The addition of glycerol and processing of this soap producesglycerin soap. Superfatted soap is more skin-friendly than one without extra fat, although it can leave a "greasy" feel. Sometimes, anemollient is added, such asjojoba oil orshea butter.[16]Sand orpumice may be added to produce ascouring soap. The scouring agents serve to remove dead cells from the skin surface being cleaned. This process is calledexfoliation.
The type of alkali metal used determines the kind of soap product.Sodium soaps, prepared fromsodium hydroxide (soda lye), are firm, whereaspotassium soaps, derived frompotassium hydroxide (potash lye), are softer or often liquid. Historically, potassium hydroxide was extracted from the ashes ofbracken or other plants. Lithium soaps also tend to be hard. These are used exclusively ingreases.
For making toilet soaps,triglycerides (oils and fats) are derived from coconut, olive, or palm oils, as well astallow.[18] Triglyceride is the chemical name for thetriesters of fatty acids andglycerin. Tallow,i.e.,rendered fat, is the most available triglyceride from animals. Hence, the fat compound in many soap is known assodium 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 pureolive oil, sometimes calledCastile soap orMarseille 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.
Fatty acid content of various fats used for soapmaking
The earliest recorded evidence of the production of soap-like materials dates back to around 2800 BC in ancient Babylon.[21] 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 andwood ash, the earliest recorded chemical reaction, and used for washingwoolen clothing.[22]
TheEbers papyrus (Egypt, 1550 BC) indicates the ancient Egyptians used a soap-like product as a medicine and created this by combining animal fats or vegetable oils with asoda ash substance calledtrona.[22] Egyptian documents mention a similar substance was used in the preparation ofwool for weaving.[citation needed]
In the reign ofNabonidus (556–539 BC), a recipe for a soap-like substance consisted ofuhulu [ashes],cypress [oil] and sesame [seed oil] "for washing the stones for the servant girls".[23]
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.[20] It is uncertain as to who was the first to invent true soap.[19][24]
Knowledge of how to produce true soap emerged at some point between early mentions of proto-soaps and the first century AD.[19][20] Alkali was used to clean textiles such as wool for thousands of years[25] but soap only forms when there is enough fat, and experiments show that washing wool does not create visible quantities of soap.[19] Experiments by Sally Pointer show that the repeated laundering of materials used inperfume-making lead to noticeable amounts of soap forming. This fits with other evidence fromMesopotamian culture.[19]
Pliny the Elder, whose writings chronicle life in the first century AD, describes soap as "an invention of the Gauls".[26] The wordsapo, Latin for soap, has connected to a mythical Mount Sapo, a hill near the River Tiber where animals were sacrificed.[27] But in all likelihood, the word was borrowed from an early Germanic language and iscognate with Latinsebum, "tallow". It first appears inPliny the Elder's account,[28]Historia Naturalis, which discusses the manufacture of soap from tallow and ashes. There he mentions its use in the treatment ofscrofulous sores, as well as among theGauls as a dye to redden hair which the men inGermania were more likely to use than women.[29][30] The Romans avoided washing with harsh soaps before encountering the milder soaps used by the Gauls around 58 BC.[31]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 [...] calledsoap".[32] 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 astrigil.[33] The standard design is a curved blade with a handle, all of which is made of metal.[34]
The 2nd-century AD physicianGalen 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.[35]
In the SouthernLevant, the ashes frombarilla plants, such as species ofSalsola, saltwort (Seidlitzia rosmarinus) andAnabasis, were used to makepotash.[36][37] Traditionally, olive oil was used instead of animal lard throughout the Levant, which was boiled in a copper cauldron for several days.[38] As the boiling progresses, alkali ashes and smaller quantities ofquicklime are added and constantly stirred.[38] 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 asyarrow leaves,lavender,germander, etc.
Ancient China
A detergent similar to soap was manufactured in ancient China from the seeds ofGleditsia sinensis.[39] Another traditional detergent is a mixture of pig pancreas and plant ash calledzhuyizi (simplified Chinese:猪胰子;traditional Chinese:豬胰子;pinyin:zhūyízǐ). Soap made of animal fat did not appear in China until the modern era.[40] Soap-like detergents were not as popular as ointments and creams.[39]
Islamic Golden Age
Hard toilet soap with a pleasant smell was produced in theMiddle East during theIslamic Golden Age, when soap-making became an established industry. Recipes for soap-making are described byMuhammad ibn Zakariya al-Razi (c. 865–925), who also gave a recipe for producingglycerine fromolive oil. In the Middle East, soap was produced from the interaction offatty oils andfats withalkali. InSyria, soap was produced using olive oil together with alkali andlime. Soap was exported from Syria to other parts of theMuslim world and to Europe.[41]
A 12th-century document describes the process of soap production.[42] It mentions the key ingredient,alkali, which later became crucial to modern chemistry, derived fromal-qaly or "ashes".
By the 13th century, the manufacture of soap in the Middle East had become a major cottage industry, with sources inNablus,Fes,Damascus, andAleppo.[citation needed]
Medieval Europe
Soapmakers inNaples were members of aguild in the late sixth century (then under the control of theEastern Roman Empire),[43] and in the eighth century, soap-making was well known in Italy and Spain.[44] TheCarolingiancapitularyDe Villis, dating to around 800, representing the royal will ofCharlemagne, mentions soap as being one of the products the stewards of royal estates are to tally. The lands ofMedieval Spain were a leading soapmaker by 800, and soapmaking began in theKingdom of England about 1200.[45] 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.[46]
In Europe, soap in the 9th century was produced from animal fats and had an unpleasant smell. This changed when olive oil began to be used in soap formulas instead, after which much of Europe's soap production moved to the Mediterranean olive-growing regions.[47] Hard toilet soap was introduced to Europe by Arabs and gradually spread as a luxury item. It was often perfumed.[41][47]
In France, by the second half of the 16th century, the semi-industrialized professional manufacture of soap was concentrated in a few centers ofProvence—Toulon,Hyères, andMarseille—which supplied the rest of France.[48] In Marseilles, by 1525, production was concentrated in at least two factories, and soap production at Marseille tended to eclipse the other Provençal centers.[49]
English manufacture tended to concentrate in London.[50] The demand for high-quality hard soap was significant enough during the Tudor period that barrels of ashes were imported for the manufacture of soap.[20]
Finer soaps were later produced in Europe from the 17th century, using vegetable oils (such asolive 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 1634 Charles I granted the newly formed Society of Soapmakers a monopoly in soap production who produced certificates from 'foure Countesses, and five Viscountesses, and divers other Ladies and Gentlewomen of great credite and quality, besides common Laundresses and others', testifying that 'the New White Soap washeth whiter and sweeter than the Old Soap'.[51]
During theRestoration 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 that stipulated that soap boilers must manufacture a minimum quantity of one imperial ton at each boiling, which placed the process beyond the reach of the average person. The soap trade was boosted and deregulated when the tax was repealed in 1853.[52][53][54]
Modern period
Industrially manufacturedbar 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.[55] In modern times, the use of soap has become commonplace in industrialized nations due to a better understanding of the role ofhygiene in reducing the population size ofpathogenicmicroorganisms.[56]
Caricature ofLillie Langtry, fromPunch, Christmas 1890: The soap box on which she sits reflects her endorsements of cosmetics and soaps.
Until theIndustrial Revolution, soapmaking was conducted on a small scale and the product was rough. In 1780,James Keir established a chemical works atTipton, for the manufacture of alkali from the sulfates ofpotash and soda, to which he afterwards added a soap manufactory. The method of extraction proceeded on a discovery of Keir's. In 1790,Nicolas Leblanc discovered how to make alkali fromcommon salt.[31]Andrew Pears started making a high-quality, transparent soap,Pears soap, in 1807 in London.[57] His son-in-law,Thomas J. Barratt, became the brand manager (the first of its kind) for Pears in 1865.[58] In 1882, Barratt recruited English actress and socialiteLillie Langtry to become the poster-girl for Pears soap, making her the first celebrity to endorse a commercial product.[59][60]
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 amortar and pestle. American manufacturerBenjamin T. Babbitt introduced marketing innovations that included the sale of bar soap and distribution ofproduct samples.William Hesketh Lever and his brother,James, bought a small soap works inWarrington in 1886 and founded what is still one of the largest soap businesses, formerly called Lever Brothers and now calledUnilever. These soap businesses were among the first to employ large-scaleadvertising campaigns.
Liquid soap
Liquid soap was invented in the nineteenth century; in 1865, William Sheppard patented a liquid version of soap.[61] In 1898, B.J. Johnson developed a soap derived from palm and olive oils; his company, theB.J. Johnson Soap Company, introduced "Palmolive" brand soap that same year.[62] This new brand of soap became popular rapidly, and to such a degree that B.J. Johnson Soap Company changed its name toPalmolive.[63]
In the early 1900s, other companies began to develop their own liquid soaps. Such products asPine-Sol andTide appeared on the market, making the process of cleaning things other than skin, such as clothing, floors, and bathrooms, much easier.[64][65]
Liquid soap also works better for more traditional or non-machine washing methods, such as using awashboard.[66]
^Willcox, Michael (2000)."Soap". In Hilda Butler (ed.).Poucher's Perfumes, Cosmetics and Soaps (10th ed.). Dordrecht: Kluwer Academic Publishers. p. 453.ISBN978-0-7514-0479-1.Archived from the original on 2016-08-20.The earliest recorded evidence of the production of soap-like materials dates back to around 2800 BCE in ancient Babylon.
^David J. Anneken, Sabine Both, Ralf Christoph, Georg Fieg, Udo Steinberner, Alfred Westfechtel "Fatty Acids" in Ullmann'sEncyclopedia of Industrial Chemistry 2006, Wiley-VCH, Weinheim.doi:10.1002/14356007.a10_245.pub2.
^Willcox, Michael (2000)."Soap". In Hilda Butler (ed.).Poucher's Perfumes, Cosmetics and Soaps (10th ed.). Dordrecht: Kluwer Academic Publishers. p. 453.ISBN978-0-7514-0479-1.Archived from the original on 2016-08-20.The earliest recorded evidence of the production of soap-like materials dates back to around 2800 BCE in ancient Babylon.
^Noted inLevey, Martin (1958). "Gypsum, salt and soda in ancient Mesopotamian chemical technology".Isis.49 (3): 336–342 (341).doi:10.1086/348678.JSTOR226942.S2CID143632451.
^Aretaeus,The Extant Works of Aretaeus, the Cappadocian, ed. and tr. Francis Adams (London) 1856:238 and 496Archived 2016-06-09 at theWayback Machine, 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 AnatolianGalatia.
^De Puma, Richard. "A Third-Century B.C.E. Etruscan Tomb Group from Bolsena in the Metropolitan Museum of Art".American Journal of Archaeology:429–40.
^Padgett, J. Michael (2002).Objects of Desire: Greek Vases from the John B. Elliot Collection. Record of the Art Museum, Princeton University. pp. 36–48.
^BBCScience and Islam Part 2, Jim Al-Khalili. BBC Productions. Accessed 30 January 2012.
^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,ISBN0-85115-770-X.
^abAnionic and Related Lime Soap Dispersants, Raymond G. Bistline Jr., inAnionic Surfactants: Organic Chemistry, Helmut Stache, ed., Volume 56 of Surfactant science series, CRC Press, 1996, chapter 11, p. 632,ISBN0-8247-9394-3.
^abCharles Springer, ed. (1954).A History of Technology, Volume 2. Clarendon Press. pp. 355–356.ISBN9780198581062.{{cite book}}:ISBN / Date incompatibility (help)
^"When Celebrity Endorsers Go Bad".The Washington Post.Archived from the original on 16 November 2022. Retrieved2 March 2022.British actress Lillie Langtry became the world's first celebrity endorser when her likeness appeared on packages of Pears Soap.
^Richards, Jef I. (2022).A History of Advertising: The First 300,000 Years. Rowman & Littlefield. p. 286.
"Soap".Workshop Receipts, for Manufacturers and Scientific Amateurs. Vol. IVRain Water to Wire Ropes. London: E. & F. N. Spon. 1909. pp. 143–179.OCLC1159761115.
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