Chemistry of Soap: Salt + Fat = Bath Time
Updated: Aug 1
Salt + Fat
Unless you’ve never thought about it before, you might be aware that most soap is created by the chemical reaction of lye with fats and oils. Taking one step backward in time, the more rudimentary version of this was wood or plant ashes heated with rendered animal fat. Wood ashes soaked in water in a pot – “pot ash,” later to become known as potash – created a caustic substance that when heated with the fat produced a soft cleaning product.
As it turned out, the magical “caustic substance” was a makeshift, crude alkali. An “alkali” is a base. More specifically, it is a basic salt compound. You may recall the concept of bases and acids from high school chemistry class, or you are at least familiar with the pH of swimming pools and fish tanks. Or perhaps you’ve even heard buzz about “alkaline” water. (Incidentally, the adjective “alkaline” means the same thing as “basic.”) Essentially, bases neutralize acids, which is indeed the reaction that takes place in soapmaking. Fats and oils are made up of… you got it - fatty acid molecules (and “glycerol,” but we’ll get to that later).
Later, alkalis began to be manufactured in factories rather than mixed over outdoor fires or heated in large vats or pans. No more guessing games. More assured sources were now available, and the quality of products produced could be better controlled.
Specifically, there are two inorganic compounds that are manufactured and used for the alkali needed in soapmaking: potassium hydroxide and sodium hydroxide. They each produce a different kind of soap.
We are familiar with the words “potassium” and “sodium” both relating to salt, right? (Stay with me here. Even though we are using chemistry words, we are still fundamentally talking about salts.)
“Potassium hydroxide” is the closest to original potash. When mixed with fats and oils, soft soaps are produced. (Note: Not all potassium salts have this effect. They must be sufficiently basic, like potassium hydroxide is.[i])
Interestingly, the word “potassium” is even derived from “potash.” In fact, potassium hydroxide literally means “caustic potash,” or “calcinated potash.” When manufacturing first began, white salt (potassium carbonate) was added to a strong solution of calcium hydroxide (slaked lime[ii]) and heated. A series of reactions[iii] produced a solution of “potassium hydroxide.” In the late 19th century, the production method of potassium hydroxide was replaced with electrolysis.[iv]
“Sodium hydroxide” is the other manufactured compound. It is commonly referred to as “lye” or “caustic soda.” Sodium soaps produce hard soaps (commonly called “bar soaps”), which are in greater abundance today than original soft soaps (liquid soaps) produced by potassium hydroxide. Sodium hydroxide was historically produced by the reaction of sodium carbonate (washing soda or soda ash) with calcium hydroxide (slaked lime). The process was called causticizing. Today, electrolysis is also the method used to produce sodium hydroxide.
Both potassium hydroxide and sodium hydroxide are white solids. They are dangerously corrosive. That is why safety precautions are always taken by soapmakers when working with either one. Long sleeves, gloves, safety goggles and good ventilation are some of the measures used to prevent serious harm.
When lye or caustic potash is met with oils and fats, the chemical reaction creates what we know as soap. The amount of lye or caustic potash required for a soap recipe is calculated based on the amount of fats and oils. Many homemade soap recipes today are created using special “lye calculators.”
When the reaction happens, all soap ingredients are chemically changed, including the caustic one. Therefore, there is no more danger. The process is called “saponification.” When oils and fats have been turned into soap, they are described as having been “saponified.” (Berry)
What is happening in saponification at a molecular level is that the alkali has caused the fats or oils molecules to split back into fatty acids and glycerol. The sodium or potassium then joins with the fatty acids to become a fatty acid salt… otherwise known as soap! (Ross)
That’s right. What we know of as soap is known in chemistry as a fatty acid salt. Salt + Fat.
But what happens to the leftover glycerol?
The fats and oils in a soap recipe were made up of triglycerides. (A triglyceride is composed of three fatty acid molecules linked to a glycerol molecule). During saponification, when the alkali compounds split the triglycerides apart and bound to the fatty acids themselves, the glycerol was left hanging out all alone.
For anyone somewhat familiar with glycerin, here might be an Aha! moment. “Glycerol” is the same thing as “glycerin!”
So, glycerin is actually a byproduct of saponification.
Glycerin is just a simple organic compound. It is colorless, odorless and sweet-tasting. Notably, the FDA has approved it as a wound and burn treatment for having antimicrobial and antiviral properties. In skin care, it is a moisturizing powerhouse. In the food industry, it is used as a sweetener.
Basically, glycerin will simply remain in a batch of soap that is homemade. It will be distributed throughout the soap, and skin is better for it. However, commercial soap-producing companies separate out the glycerin to be sold as a manufacturing commodity. In fact, saponification is the main industry method for producing glycerin. That is why commercial soaps can seem less gentle to sensitive skin and produce drying effects. (Berry)
Why Does Soap Make Us Clean?
So, if the lye and other soap ingredients are chemically changed during saponification, what has been made instead? How does the newly created fatty acid salt, soap, clean our skin?
The molecules of soap are long and thin. One end of each molecule is “hydrophilic,” and the other is “hydrophobic.” If you look at those words long enough, you will realize what they mean even if you don’t recall them from high school chemistry class. Hydrophilic means the molecule is attracted to water. Hydrophobic repels water. However, hydrophobic molecules are attracted to dirt and oil. Can you guess what happens?
Dirt and oil particles are taken captive by the hydrophobic ends of soap molecules, while the hydrophilic ends are sticking out waiting to attract water. When the water comes, the hydrophilic ends attach to it, and the dirt and oil particles get washed away, too. (Ross)
Actually, another helpful chemical process is also happening at the same time. Soap also happens to be a “surfactant” (surface active agent). This means it reduces surface tension of water. Put simply, where there is great surface tension of water, you have many water droplets. (Think about wiping the kitchen counter and the many little beads of water left behind after the swipe of the wet dish cloth.) Where there is reduced surface tension of water, the water can spread out over surfaces. In essence, reduced surface tension of water causes surfaces to become “wetter” faster. Some say that soap makes “water wetter.” It spreads over the skin, and quickly. As a result, the cleaning process is sped up. (Ross)
(How and why soap molecules came to behave in these ways is taking it too deep for this discussion. If you want to understand that, you’re on your own, brave chemist.)
Whew! How was that for a chemistry lesson from someone with zero credentials to write it? Although it took me three days to write this post and lots of staring at definitions of chemistry words until dots connected, have no fear. My uncle is an Associate Professor of Chemistry at Tennessee State University, where he has been teaching for 15 years, and he approved this post. (Shout out to Dr. Josh Moore!)
Though the chemical process of soapmaking has never fundamentally changed through the years, soapmaking itself has evolved. In ancient days, “soap” as we know it did not even exist at all. What “on earth” did they use? How did we get from there to here?
Check back for the rest of the story!
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Meg Grimm writes biblical studies and research articles that help set women free from impractical expectations of the world. Her goal is to unveil true beauty and provide sensible body care principles from a godly perspective.
Berry, Jan. Simple & Natural Soapmaking. Page Street Publishing Co.: Salem, MA. 2017. (pages 9-10)
Hunt, John A. “A Short History of Soap.” The Pharmaceutical Journal. Dec 1999, online | URI: 20066753. Accessed 10 Oct 2019 through http://www.pharmaceutical-journal.com/opinion/comment/a-short-history-of-soap/20066753.article
Ross, Rachel. “Getting Clean: The Science of Soap.” Live Science. 3 Nov 2016. Accessed 10 Oct 2019 through https://www.livescience.com/57044-science-of-soap.html
[i] Not all potassium salts will produce soap when mixed with a triglyceride. The salts should be basic or basic in nature. As an example, potassium chloride, a common potassium salt, does not initiate saponification. Salts such as potassium hydroxide or potassium carbonate are sufficiently basic to initiate the reaction.
[ii] After limestone is extracted from quarries or mines, appropriate stones are taken to a limestone kiln, where the lime is “calcinated,” a decomposition process with high heat, to produce “quicklime,” or “burnt lime.” The name in chemistry is “calcium oxide.” Calcium oxide is a solid white powder. Before use, calcium oxide is hydrated with water, and this is called “slaking.” Therefore, hydrated lime is “slaked lime.” Dry or wet slaking refers to the amount of water added to produce various consistencies. Since lime is used in masonry, putties and powders and the like have varied uses.
[iii] Calcium carbonate became a solid, which when heated gives off carbon dioxide, producing potassium oxide, which when reacted with water produces potassium hydroxide.
[iv] A direct electric current was passed through a potassium chloride solution to produce the chemical reactions needed to create potassium hydroxide. “Electrolysis” refers to a direct electric current passing through substances to produce chemical reactions. (Beyond this definition, I couldn’t fully grasp how or why, so if you want to know more, you’re on your own from here.)