How Does Soap Work?

What Is a Surfectant and What Does Soap Do?

How Does Soap Work?

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How does soap work? It is a question people have explored since ancient times.

As early as 2800 B.C., a soap-like substance was found in cylindrical clay containers during the excavation of Babylon. People had discovered by then that mixing leach water from wood ashes with leftover fats from slaughter or cooking yielded a curious substance with excellent cleaning properties. They understood the question of what does soap do fairly quickly. Inscriptions on the Babylonian clay cylinders say that fats were boiled with ashes, a primitive method for soap making – therefore, they also understood to some extent how soap was made. But the question of how soap works remained a mystery until the second century, A.D., which is when Greek physician Galen wrote that he recommended soap for cleansing and medicinal purposes. Unfortunately, it appears the use of soap declined severely during the Middle Ages. It was not until the seventh century A.D. that we begin hearing about soap makers in Spain and Italy making soap with goat fat and beech tree ashes. In the twelfth century, A.D. the English began making soap, and records show that wealthy ladies of this period used a scented castile soap made from olive oil—which was a very expensive import.

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In modern times, we understand the answer to the question of how does soap work. Soap acts as a surfactant—that means it works by lowering the surface tension between oils and grime and water, and emulsifying oils to allow them to be washed away. In other words, soap breaks up oil into smaller droplets which can be easily suspended in water. It works because soap is made up of molecules with a water-loving end and a water-hating end—a hydrophilic and a hydrophobic end. The hydrophilic end bonds to water and the hydrophobic end repels water, bonding instead to the oils. As water is rinsed away, so is emulsified oil and soap, along with excess skin flora, dead skin cells and other grime.

Raw soap is, itself, an emulsion of oils, water and caustic lye. Photo by Melanie Teegarden.

Soap is a salt of a fatty acid. In household use, soaps perform a wide range of cleansing and grease-cutting chores, from cleaning floors to cleaning bodies. Certain soaps and soap ingredients are also used in industry as lubricants and thickeners. Metal soaps, such as lithium soap, are used in the production of artists’ oil paints.

Toilet soaps, or soaps used on the skin, are usually saponified fats or oils. An alkaline solution, either sodium hydroxide or potassium hydroxide, starts the saponification process of oils once combined with water, heat and mixing. The fats are hydrolyzed into salts of fatty acids. Hydrolysis is the process of a molecule losing water. In soap, this water slowly evaporates out of the soap, leaving a hard, mild bar. During the process of saponification, glycerol is also freed from the oil molecule. This glycerin often remains in handmade soap, making the soap more humectant. Humectancy is the ability of a substance to draw atmospheric water to itself. In commercial soaps, this glycerin is often removed and used as an industrial product of its own. This makes commercial soaps less humectant.

The type of alkali used will affect what kind of soap is produced. Sodium soaps, made from sodium hydroxide, are hard when cured. Potassium soaps, made from potassium hydroxide, are usually liquid.

Oil and water

Toilet soaps are often made using fats from palm, coconut, or olive plant oils, or from tallow or lard. Usually, a blend of oils yields a better soap than a soap made with just one kind of oil. Exceptions to this include olive oil soap, coconut oil soap and lard soap. Most soaps for toiletry use are superfatted to make them more gentle on the skin. This means that extra oil is added to the soap recipe beyond what is used up in the chemical reaction with the alkali. This free oil decreases the ph of the soap slightly and forms a light, breathable, humectant barrier on the skin. Soap recipes can contain any number of luxury oils, such as shea butter. (Learn how to make shea butter soap here.) Other additives, such as salt, sugar, honey, and sodium lactate in soap can improve the longevity of the bar and the qualities of the lather.

Soaps and soap-like substances have been made and used by humans for millennia. In modern times, we understand the process of how soap works and we are able to produce both natural and synthetic soap-like substances for specific purposes. Commercial toilet soaps tend to be harder and longer-lasting than homemade soaps, whereas homemade soaps are more humectant and emollient. We use soaps for a wide range of tasks, from cleansing to lubricating and disinfecting.

Do you prefer handmade or commercial soaps? Have you tried making soap at home? Share your experiences with us!

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