# Soap

Hortus sanitatis , Mainz 1491. Illustration to the chapter Sapo soap

Soaps (of . Ahd seifa "soap, resin") are sodium - or potassium - salts of fatty acids . As surfactants , they are used as cleaning agents that are mainly used for body cleaning and to a certain extent also for surface cleaning . They have lost their importance as laundry detergents for textiles because they form insoluble calcium and magnesium salts, the so-called lime soaps , in hard water . The fatty acid salts of other metals are called metal soaps ; they are important in various branches of industry and are described there.

In common parlance, soap is understood to mean fine soap or toilet soap . They are solid preparations based on the sodium salts of fatty acids ( curd soap ).

## History of soap

Shop window decoration on the history of soap

The Sumerians gave the first indications of soap production . They recognized that plant ash mixed with oils has special properties and created the basis for a soap recipe. It is believed that they overlooked the cleansing effect of the alkaline mixture and used it as a remedy for injuries. Egyptians and Greeks took over the instructions for chemical production, although the cleaning effect of the soap was first established by the Romans .

In the Old Testament in Jeremiah (Jer. 2, 22) the use of mineral soda ( Heb. נֶתֶר nether , English natron ) and lye from plant ash ( Heb. בֹּרִית borith , German 'lye from plant ash' ) mentioned for washing.

Pliny described an ancient soap made from goat tallow and wood ash, and that the Germans use a soft type of soap. Galen found soap-like substances to be used frequently by the Gauls .

In the Middle East, oil and lye were boiled together for the first time in the 7th century, creating the soap it is known today. With the conquests of the Arabs, this knowledge quickly spread to Europe. France and Spain later belonged to the centers of soap manufacturing worldwide.

Traditional Savon de Marseille , a type of curd soap

In the Middle Ages, visiting the bathhouse was very popular and the body cleansing was better than commonly assumed. It was not until the outbreak of plague and cholera that washing with water was stopped. Since the transmission routes were unknown, it was believed that the bath water opened the body to the pathogens. You couldn't tell that it was because of the dirty streets and rivulets in front of the houses and the rats. Dry cleaning was used. Pathogens , as well as lice and fleas as carriers, could spread unhindered. Up until the 17th century, doctors in Europe were of the opinion that water and air were harmful to the body. Clothing served as protection from these harmful elements . The powdering also served the purpose of closing off the body from the outside. Underwear soaked up body sweat; it was thought that this was how the body was purified.

In the 17th century, the French King Louis XIV helped soap to flourish again by bringing the best soap makers to Versailles . In 1688 he issued the purity law for soap, which is still known today. Accordingly, a soap was considered to be of particularly high quality if it contained at least 72% pure oil. In the middle of the 17th century, large soap factories were built in the French cities of Marseille , Toulon and Lyon . The French Nicolas Leblanc (1742–1806) succeeded for the first time in 1790 in artificially producing large quantities of soda so that the potash previously used could be replaced. In 1829, around 4,000 tons of soap were produced in France. In England and Germany, too, there were already important soap manufacturers. Soaps were also used to clean fabrics and wood, and to steam wash textiles . The disadvantage was the formation of lime soap , so the washing water was first decalcified with soda solution.

In 1865, the Belgian Ernest Solvay developed the Solvay process , which replaced the Leblanc process . So there was enough soda to make soap and soap became an affordable product. The body could now be washed regularly with soap and freed from unpleasant odors.

The traditional production of soaps from olive oil continues to this day in Marseille ( Savon de Marseille ) and many Mediterranean countries.

## Soap making

Soap production from sheep fat
Soap on the display board "The recovery of the bone"

Soaps are usually made from vegetable or animal fats . Inferior fats, which can be obtained by hot pressing or extraction with solvents , are mostly used to make soaps . Mainly vegetable fats such as coconut oil , palm kernel oil , palm oil , olive oil , sunflower oil , corn oil, soybean oil and animal fats such as tallow , lard or fat from bones , which result from animal processing , are used.

For production, fats are boiled with a lye (such as caustic soda or potassium hydroxide , formerly also potash or soda ). This process is called soap boiling , the chemical reaction saponification . The fats are broken down into glycerine and the alkali salts of the fatty acids (the actual soaps). Production used to take place in open kettles. Today soaps are produced on a large scale in closed systems in continuous operation .

The viscous emulsion that forms when it boils is called soap glue . To make curd soap , the soap glue is mixed with sodium chloride solution. The emulsion separates by salting out into the floating soap kernel , which mainly contains the sodium salts of fatty acids, and into the lower lye, which mainly contains excess lye, glycerine and dissolved common salt . The soap core is separated from the lower lye by separation , boiled with plenty of water and a little lye in order to remove the remaining impurities. Repeating the salting out leads to an increased purity of the curd soap.

Soap shape

Alternatively, soaps can be made directly from free fatty acids ( lye saponification ) by converting them to their salts with lyes. Suitable fatty acids are, for example, lauric acid , myristic acid , palmitic acid , stearic acid , oleic acid and ricinoleic acid .

The consistency of a soap product depends on the chain length of the fatty acids. Long-chain saturated fatty acids such as stearic acid or palmitic acid tend to have a firm consistency. The decisive factor, however, is whether potassium or sodium salts of the fatty acids were obtained. If the soap core is obtained from the soap glue by adding sodium chloride, a more solid soap tends to form, the curd soap . If, on the other hand, you work with potassium hydroxide solutions and potassium salts, potassium salts of the fatty acids are formed, which are soft to greasy and easily miscible with water. Soft soaps are obtained .

Curd soap is formed in blocks and dried. To make toilet soap bars, the blocks are either cut into cuboids or roughly ground, made into a paste with dyes , fragrances and fillers , calendered on roller mills (to trap air and create a shine) and rolled out, the strips are then extruded or extruded in a hot press and then rolled out The strand is punched into shapes and pressed into bars of soap at the same time.

Artisanal soap making:

Modern soap shop in Tübingen (2019)

In addition to the industrial processes, following the increasing demand for natural cosmetics , soaps are also handcrafted using the cold saponification process. A precisely measured amount of caustic soda is added to the mostly higher quality fats, oils and waxes . The aim is an incomplete saponification of the fats and oils in order to achieve a nourishing effect (called excessive grease). Since the ingredients are subject to natural fluctuations, the necessary amount of caustic soda is calculated using the saponification number, but the excess fat is only roughly indicated, for example "approx. 7% over-greasing ".

Typically these soaps are poured as soap glue in block molds and then cut into pieces or poured into silicone molds. Often fragrances and colors are added to the soaps. These soaps can be found, for example, in organic stores , at craft and Christmas markets and often do not contain any allergenic ingredients (artificial preservatives, fragrances and surfactants), so that they are also suitable for allergy sufferers.

Information on the manufacture of soaps at home and calculation aids to determine the amount of lye required can be found on the Internet. Due to the caustic effect of potassium hydroxide or sodium hydroxide solution and sometimes allergenic additives (e.g. perfume oils), suitable precautionary measures must be taken.

## Washing effect of the soap

Fig. 1: Hydrophobic alkyl radical and hydrophilic carboxylate group
Fig. 2: Micelle of a soap solution
Fig. 3: Soap on the surface of the water
Fig. 4: Soap leads to the formation of stabilized fat droplets in water

Soaps are a mixture of different, longer-chain alkali salts of fatty acids and count among the surfactants , more precisely among the anionic surfactants . The soap molecules owe their properties to the fact that they consist of a long, water-repellent ( hydrophobic ) hydrocarbon chain and a water-attracting ( hydrophilic ) part, the so-called carboxylate group ( - COO - ) (Fig. 1).

Soaps do not completely dissolve in water, but form so-called micelles . In pure water, the micelles (Fig. 2) are very small and cannot be seen. Inside these tiny "droplets" are long, non-polar hydrocarbon chains, while the polar ends protrude into the water. The repelling charges on the ends prevent the micelles from clumping together.

Soaps lower the surface tension (more generally: interfacial tension ) of water by arranging themselves on the water surface (Fig. 3). This intensifies the wetting of surfaces. The actual cleaning effect of the soaps lies in the dissolving and emulsifying of fats. The long lipophilic hydrocarbon chains of the soap molecules are attracted to the fat (Fig. 4). However, the polar lipophobic ends protrude into the surrounding water. The fat droplet is completely enveloped by the soap molecules and detached from the surface to be cleaned. The division of the fats and oils into many small droplets leads to a so-called emulsion , which can be rinsed off with fresh water.

Tap water contains regionally increased concentrations of calcium and magnesium ions . They make this water " hard " and react with the polar ends of the soap. Lime soaps that are insoluble in the water and have no washing effect are formed, which float on the water surface as a white surface skin , settle on the walls of the vessels and fittings and lead to the formation of fatty lice .

## Types of soap

### Glue soap

Glue soaps (soap glue) are homogeneous masses in which the glycerine is not separated after saponification. Cold-boiled soaps are sometimes offered as glue soap. The fats and lye are saponified at 40 ° C and the mass is poured into a container immediately afterwards. Many homemade glue soaps are offered.

### Curd soap

Two-layer French curd soap

Curd soaps are solid soaps and are usually made from the sodium salts of fatty acids. They are obtained by salting out the soap glue, whereby the glycerine is separated. Curd soaps are most common body soaps, including fine soaps. In the trade, cheaper, unscented soaps are called "curd soaps", which are used for cleaning purposes and for felting .

### Soft soap

Soft soaps are liquid or semi-solid soaps that are made from inexpensive fats or oils by saponifying them with potassium hydroxide . So they are a mixture of potassium - salts of higher fatty acids , so potassium soaps . They are also called liquid soap or historically barrel soap , can easily be dissolved in water and used for cleaning purposes e.g. B. use in the household. Due to the formation of lime soaps , theamountof soap must beincreased significantlyin hard water in order to maintain the washing effect.

Soft soap was already known under this name before 1859 and was often made from linseed oil, rapeseed oil and hemp oil (green soap).

### Fine soap

Fine soaps or toilet soaps are usually preparations based on pure, odorless curd soaps that are mainly used to wash hands. They are often mixed with nourishing additives such as lanolin (wool wax), as well as perfumes and dyes. Sometimes glue soaps are also offered as fine soaps.

Toilet soaps were used as early as 1859, when perfume oils were added to cover the odor of sebum residues in the soap.

### Moisturizing soaps

So-called moisturizing soaps are often offered. These soaps are intended to replace the sebum that is loosened during the normal washing process. To do this, fats are added to the curd soap , or cold-boiled soaps with excess fat are used.

### Glycerine soap

Glycerine soap (transparent soap) is a soap that has a high glycerine content. It is cloudy to glassy and transparent. It is easy to melt (like many waxes ) and is therefore also used as a craft soap. Glycerine soaps are lighter than water.

The transparent soap was already known before 1859, at that time the fatty acid was dissolved in alcohol and the mixture was poured into molds until it solidified.

### Paper soap

Paper soap is fine soap cut very thinly. The pieces are portioned in such a way that they dissolve quickly.

### Shaving soap

Shaving soap is made from stearin and coconut oil with a high proportion of stearic acid so that the foam is creamy and remains stable. It is not only saponified with caustic soda, but often with a portion of caustic potash. This makes the shaving soap smoother and easier to lather. It is available in the form of round bars of soap as well as sticks.

### Gall soap

Another soap is the gall soap , which is created when soap is mixed with beef bile . The bile acids act as additional emulsifiers and help to remove fat and protein stains from textiles.

Gaultier de Claubry pointed out the advantages of this soap for removing stains as early as 1859 . The soap was also marketed under the name stain soap or stain balls .

### Medical soap and antibacterial soap

So-called doctor soaps are soaps with supposedly skin-friendly compositions. “Doctor's soap” is not necessarily a disinfectant . Pure glycerine soaps are also often offered as doctor soaps. Some soaps contain bacteria- inhibiting additives, such as B. Farnesol or Triclosan . Research from the University of Michigan has shown that antibacterial soaps made for home use are no better at removing germs than traditional soaps. As with all antibacterial agents, there is a risk that the germs develop resistance. Soaps for medical use that contain significantly higher concentrations of antibacterial agents were not examined.

### Petrol soap

Petrol Soap is a petrol-based stain remover for removing organic soiling and for pretreating soiling with lubricating oil and grease on textiles.

### Modern liquid soaps

pH-neutral liquid soaps are used to wash hands, but mainly as shower gels , shampoos and foam baths . They emerged from soft soaps, but have completely different ingredients (surfactants) and properties.

### laundry detergent

As textile detergent soaps have become less important, as by the water hardness insoluble flocculent to tacky calcium and magnesium salts of fatty acids form soaps and washing are active only in the basic range, which textile fibers can strain. In today's detergents, soaps are only added in small amounts, since the lime soaps produced during the washing process reduce the build-up of foam, i.e. H. act as a defoamer .

Soap shop in Berlin-Karlshorst

Soaps are rarely used as detergents in industrialized countries because other surfactants are available. Soaps in modern detergents only regulate foam formation through the formation of lime soaps .

• Soaps not only remove dirt, but also part of the skin's natural grease film. This can lead to cracked, rough skin, especially if you wash too often. Soaps with a high glycerine content (which, for example, remains in the finished product during cold saponification) provide protection.
• Soaps, as alkali salts of fatty acids, have an alkaline reaction in water , which means they increase the pH value . This can not only damage textile fibers , but also impair the protective acid mantle of the skin:
${\ displaystyle \ mathrm {RCOO ^ {-} + H_ {2} O \ longrightarrow RCOOH + OH ^ {-}}}$
${\ displaystyle \ mathrm {2 \ RCOO ^ {-} + Ca ^ {2 +} \ longrightarrow (RCOO) _ {2} Ca}}$

Advantages of soaps over synthetic surfactants are:

• Natural soaps (e.g. olive oil soaps) are suitable for allergy sufferers , as soaps made from natural fats are tolerated by most people. In contrast, synthetic surfactants can act as allergens.
• low energy consumption and use of resources in production

## Physiology of washing with soap

• When washing , soap removes sebum , powder and cream residues from the pores. This normalizes skin breathing .
• Soap attacks the skin's oily layer and more or less loosens it.
• The soap alkali neutralizes the skin's acid mantle. However, this effect is balanced again 30 minutes after washing.
• Soap solution causes the skin to swell. This swelling effect is irrelevant in healthy skin, but can lead to drying out and cracking in the pathological condition.
• Soaps can be irritating if they contain higher amounts of short-chain, saturated fatty acids . However, allergic skin reactions are more likely to be triggered by the perfume oils and additives they contain.

## Soap and education

In raising children, especially of American cultural space the soap was used up in the recent past: to the use of the children Schimpf- wean and Fäkalwörtern whose was mouth applied as a punishment with soap, usually on a rag, washed out. This is intended to clarify the “dirtyness” of certain terms. The nauseating taste should condition the children to avoid using the words.

## Ingestion and inhalation of soaps

Small children occasionally swallow solid or liquid soaps, especially flavored products ( ingestion ). The danger ( toxicity ) of soap is low, but it is irritating to the mucous membranes and can lead to burning in the throat, nausea, gagging, vomiting, flatulence or even abdominal pain. If soap products get into the lungs, they act on the surface proteins in the alveoli and can cause inflammation and tissue changes. Accidental inhalation (accidental aspiration ), v. a. with foaming soaps, often manifests itself in irritation of the throat or shortness of breath. Occasionally, shortness of breath, other lung problems ( pulmonary complaints), fever or vomiting occur after a delay. In order to prevent the soap from foaming up in the stomach and possible inhalation, a “ defoamer ” ( dimethicone ) should be taken as soon as possible after consumption and still water or tea should be drunk.

## Surface treatment soaps

In Morocco and other North African countries, sodium and potassium soaps are applied to lime plaster using the Tadelakt technique to decorate house interior walls. The formation of water-insoluble lime soaps results in a waterproof, shiny surface. Freshly plastered or limed (with lime plaster or lime paint ) surfaces are rubbed with soap and polished with smooth stones. Depending on the design, there are delicately shimmering, partly marbled surfaces.

Coatings with soaps made from unrefined, natural, yellow-red palm oil smell like violets, as the breakdown of the carotene contained in the oil creates the fragrance ionone .

Soap is traditionally used to treat the surface of wood, terracotta and other porous building materials. Color tones are lightened, structures are standardized. In contrast to oil, soap solutions or waxes remain on the surface. Oil penetrates deep into the material, darkening it and highlighting structures and grains. A soap application can follow an oil impregnation to close the pores. The oil is saponified on the surface. The cheering of the surface is softened, but not reversed.

The application of a soap solution leaves a dirt-repellent film, which, however, cannot prevent the penetration of grease and coloring liquids such as red wine. Light stone surfaces usually hardly change their appearance. A haze can form on dark surfaces that is easy to remove. Larger amounts of soap can be polished to a dull shine.

Regular washing with soapy water will bleach stains and wooden surfaces. Grease stains are clearly visible at first, but fade over the course of a few months. Certain wood constituents are washed out, the darkening of the wood is evened out, and softwood is given a very light, leached surface. In Cologne and other regions, tavern tables are made from untreated maple wood, which are given a permanently light, dirt-resistant surface through occasional scrubbing with soap (and possibly fine sand).

## Film contributions

• Annette Frei Berthoud: Foam and fragrance. Documentation from NZZ format , 30 min. ( Information ; online at YouTube ).
• Oriental soap. In Aleppo, Tripoli and Beirut. Documentation, 45 min., A film by Birgitta Ashoff, production: SR , first broadcast: January 17, 2007

## literature

• Eberhard Schmauderer: Soap-like products in the ancient Orient. In: History of Technology. Volume 34, 1967, pp. 300-310.
• Eberhard Schmauderer: Soap and soap-like products in classical antiquity. In: History of Technology. Volume 35, 1968, pp. 205-222.
• Bernhard Dietrich Haage: On German soap recipes of the late Middle Ages. In: Sudhoff's archive. Volume 54, 1970, pp. 294-298.
• Peter Donkor: SMALL-SCALE SOAPMAKING - A handbook . Ebook in English online at SlideShare , 1986, ISBN 0-946688-37-0 .
• Bob Spencer / Practical Action: SOAPMAKING . PDF 2005, English online.

Wiktionary: Soap  - explanations of meanings, word origins, synonyms, translations
Commons : soaps  - collection of pictures, videos and audio files
Wikibooks: Making Soap  - Learning and Teaching Materials
Wikiquote: Soap  - Quotes
Wikisource: Perfumes, Soaps, and Cosmetics  - Sources and Full Texts

## Individual evidence

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2. a b The Brockhaus in text and images , FA Brockhaus, Mannheim, 2000.
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4. ^ Günter Wagner: detergents: chemistry, environment, sustainability . Wiley-VCH, Weinheim 2011, ISBN 978-3-527-64366-0 , pp. 163 ( limited preview in Google Book search).
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6. Jer. 2, 22. In: Blue Letter Bible. Retrieved August 6, 2019 (English / Hebrew).
7. ↑ Concise dictionary of pure and applied chemistry, Friedrich Vieweg and Son Verlag, Braunschweig 1859, p. 776.
8. Soap calculator at seifensiedpunkte.de. 2020, accessed January 24, 2020 .
9. A. von Segesser: Soap, detergents and syndets. Article in the Neue Zürcher Zeitung from June 11, 1952
10. Werner Städtler in: Author collective: Das Grundwissen des Ingenieurs , VEB Fachbuchverlag, Leipzig 1968, pp. 732–892, there pp. 876–877.
11. ^ Günter Vollmer , Manfred Franz: Chemical products in everyday life. Georg Thieme Verlag, Stuttgart 1985, p. 150, ISBN 3-13-670201-8 .
12. ^ Umbach: Cosmetics and Hygiene. 3. Edition. Wiley-VCH Verlag, Weinheim 2004, ISBN 3-527-30996-9 . P. 113 ff.