Clean feels good. It smells good. Clean means fewer microbes are around to hurt us. Clean clothes feel good. Clean dishes make food safer and more attractive.
For thousands of years, soap was the last word in clean.
The first soaps were probably the saps of certain plants, such as the Soap Plant (Chlorogalum pomeridianum), whose roots can be crushed in water to form a lather, and used as a shampoo.
Other plants, such as Soapbark (Quillaja saponaria), Soapberry (Sapindus mukorossi), and Soapwort (Saponaria officinalis) also contain the same main ingredient, a compound called saponin, which forms the foamy lather, and is also a toxin used to stupefy fish in streams to make them easy to catch.
Later, people learned that fats would react with alkalies in the ashes left over from a fire to produce saponified compounds such as sodium stearate and the related potassium stearate.
Today, soaps are made from fats and oils that react with lye (sodium hydroxide). Solid fats like coconut oil, palm oil, tallow (rendered beef fat), or lard (rendered pork fat), are used to form bars of soap that stay hard and resist dissolving in the water left in the soap dish.
Oils such as olive oil, soybean oil, or canola oil make softer soaps. Castile soap is any soap that is made primarily of olive oil, and is known for being mild and soft.
As warm liquid fats react with lye and begin to saponify, they start to thicken like pudding. At this point dyes and perfumes are often added. The hardening liquid is then poured into molds, where it continues to react, generating heat. After a day, the bars can be cut and wrapped, but the saponification process continues for a few weeks, until all of the lye has reacted with the oils.
Soaps are often superfatted, so after all of the lye has reacted with the fats, there are still fats left over. This is important for two reasons. First, the resulting soap is easier to cut, and feels smoother on the skin. Second, the extra fats make sure that all of the lye reacts, so no lye is left to irritate the skin, and the resulting soap is not too alkaline.
The saponification process results in about 75% soap, and 25% glycerine. In homemade soaps, the glycerine is left in, as it acts as an emollient (skin softener) and adds a nice feel to the soap. In commercial soaps, the glycerine is often removed and sold separately, sometimes showing up in skin moisturizers that remedy the damage done by drying soaps.
Commercial bar soaps contain sodium tallowate, sodium cocoate, sodium palmate and similar ingredients, all of which are the results of reacting solid fats (tallow, coconut oil, and palm kernel oil respectively) with lye.
To these ingredients, they add fatty acids such as coconut acid and palm acid (the fats in coconut oil and palm kernel oil) as the extra fats needed to ensure the lye is completely reacted, and the soap has a good feel.
Polyethylene glycols such as PEG-6 methyl ether may be added as either surfactants, detergents, emulsifiers (to make the dyes and perfumes blend evenly), or as thickeners.
Glycerine is added as an emollient and texture enhancer. Sorbitol is another emollient used along with glycerine. It is often added to help make glycerine soaps more transparent. Titanium dioxide is added to make the soap opaque.
Pentasodium pentetate, tetrasodium etidronate and tetrasodium EDTA are added as water softeners, and to protect the dyes and perfumes from the effects of metal ions in the mixtures. These compounds lock up calcium and magnesium in the water, preventing them from reacting with the soap to form insoluble soap scum.
Not all bars that lather contain just soap. Many contain the same detergents that you find in shampoo, along with soap.
In addition to the soaps and fatty acids, some bars will contain cocamidopropyl betaine (a mild amphoteric detergent added to decrease irritation without decreasing suds or cleaning power) and benzine sulfonate detergents such as sodium dodecylbenzinesulfonate. Other detergents such as sodium isethionate and sodium cocoyl isethionate are also common.