Our expert says:
Hi Kay, thank you for the question.
Sodium laureth sulfate, or sodium lauryl ether sulfate (SLES), is a detergent and surfactant found in many personal care products (soaps, shampoos, toothpaste etc.). It is an inexpensive and very effective foaming agent.
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.
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