In this blog series we are looking at soap. The earliest recorded evidence of the production of soap-like materials dates back to around 2800 BC. It is A formula of water, alkali, and cassia oil. In modern context, soap has become more common in our lives. Its main use is to remove bacteria from our body when showering or cleaning. Nowadays there are even customised soaps of different flavour and scent just to suit different types of customers with certain scent-preference.
History of soap
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 alkalis in the ashes left over from a fire to produce saponified compounds such as sodium stearate and the related potassium stearate.
Modern day soaps
Today, soaps are made from fats and oils that react with lye (sodium 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.
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.
How soaps are made
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.
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 benzene sulfonate detergents such as sodium dodecylbenzenesulfonate. Other detergents such as sodium isethionate and sodium cocoyl isethionate are also common.
How Soap Cleans
Figure 2: Illustration of how soap cleans dirt
Soap is an excellent cleanser because of its ability to act as an emulsifying agent. An emulsifier is capable of dispersing one liquid into another immiscible liquid. This means that while oil (which attracts dirt) doesn’t naturally mix with water, soap can suspend oil/dirt in such a way that it can be removed.
The organic part of natural soap is a negatively-charged, polar molecule. Its hydrophilic (water-loving) carboxylate group (-CO2) interacts with water molecules via ion-dipole interactions and hydrogen bonding.
The hydrophobic (water-fearing) part of a soap molecule, its long, nonpolar hydrocarbon chain, does not interact with water molecules. The hydrocarbon chains are attracted to each other by dispersion forces and cluster together, forming structures called micelles. In these micelles, the carboxylate groups form a negatively-charged spherical surface, with the hydrocarbon chains inside the sphere. Because they are negatively charged, soap micelles repel each other and remain dispersed in water.
Grease and oil are nonpolar and insoluble in water. When soap and soiling oils are mixed, the nonpolar hydrocarbon portion of the micelles break up the nonpolar oil molecules. A different type of micelle then forms, with nonpolar soiling molecules in the center. Thus, grease and oil and the ‘dirt’ attached to them are caught inside the micelle and can be rinsed away.