Understanding soap chemistry begins with a basic knowledge of the washing agent, water. Water has surface tension that can be noticed by simple observation. The reason water beads on a surface, rather than spreading flat, is that the water molecules underneath the surface water molecules pull at the surface molecules.
Unfortunately, this makes it more difficult to thoroughly wet a surface and thus to clean the surface. Soap is, in part, used to reduce the surface tension, allowing the water to spread more effectively. Chemicals that allow this to happen are surface active agents, termed surfactants.
Surfactants are also important because they help loosen soil, distribute the soil throughout the soapy water, and trap the soil until it is washed off. Lastly, surfactants are helpful because they increase the alkalinity of the water, aiding in the removal of acidic soils. Surfactants are what make the soap slippery, bubbly, and slimy.
Genuine soap is made from fats, oils, or fatty acids, combined with an alkali (alkaline substance) such as lye. The combination of the fat with the alkali produces soap with an addition of glycerin, a mild, moisturizing substance that makes true soap bars soft and helps the skin retain moisture after washing.
In natural bars of soap, the ashes of plants produce the alkali used for soap making (lye). But, in commercial production, alkalis used in soapmaking are derivatives of sodium or potassium. The most common are sodium hydroxide (NaOH), often referred to as caustic soda and potassium hydroxide (KOH), caustic potash.
The saponification process involves the heating of the fats and oils (scientifically referred to as triglycerides) and then adding the alkali. This triggers the chemical reaction that creates soap. In commercial production, the moisturizing glycerin is usually removed from the product and sold to lotion making companies, leaving the bar nice and hard, but also very drying. These bars are often sold as beauty bars, body bars, and the like, because they are technically no longer soap.
