As we learned in Why Goat Milk Soap?, real soap is a mixture of oil and water or milk. But, how does that mixture of oil and water (or goat milk) actually clean?
Mixing oil and water (or goat milk) must be forced using a chemical reaction called “saponification”, which literally means “the action of making soap”. In that action, either sodium hydroxide or potassium hydroxide—both commonly known as “lye”—first reacts with water to produce a caustic soda.
In addition to caustic soda, this reaction also produces a tremendous amount of heat. Just adding lye to water can cause the water to boil. That’s why making goat milk soap is so much more difficult—you have to be careful not to scald the milk (or burn yourself!) in the process. But, I digress a little…
The caustic soda in turn reacts with the oils, a process called hydrolysis (that’s where the water molecule is breaking the chemical bonds holding the oil molecules together). The end result is a fatty acid salt, or soap molecule, and naturally-occuring glycerin. Each soap molecule contains a water-loving (hydrophilic) carboxylate head and a water-hating (hydrophobic) hydrocarbon chain tail. These hydrocarbon chain tails are attracted to each other while the carboxylate heads are not attracted to each other. With the tails attracting, soap molecules form ball-like clusters with the carboxylate heads to the outside. These clusters are called micelles, and they repel each other when they are suspended in water.
When soiling oils and dirt come into contact with soapy water, the hydrophobic hydrocarbon chain tails break up, attach to the oil and dirt molecules, and recluster into new micelles with the soiling oil or dirt suspended inside. Thus, soap is able to clean because it breaks the surface tension of the soiling products (in other words, it acts as a surfactant) and then holds the soil in suspension (that is, it acts as an emulsifier).
Pretty cool, huh? Chemistry is at play every day even in the simplest act of washing our hands!
Want to see how soap can lift dirt and grim off of your hands or face? Try a bar of Angelwood goat milk soap at www.angelwoodsoapcompany.com!
Reference & Resources
Consumer Product Safety Commission. Soap Business Guidance. n.d. Available: https://www.cpsc.gov/Business--Manufacturing/Business-Education/Business-Guidance/Soap.
Helmenstein, Anne Marie. How Soap Works. ThoughtCo., 19 July 2019. Available: https://www.thoughtco.com/how-dos-soap-clean-606146.
McLeod, Jodie. Soap vs. Detergent: What's the Difference? Savvy Natural Cleaning, n.d. Available: https://savvynaturalcleaning.com/soap-vs-detergent/.
Patel, Edison. The Chemistry of Soap and Detergent. StraightSci, 14 Oct 2023. Available: https://straightsci.com/the-chemistry-of-soap-and-detergents/.
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