Chemistry of Natural Soap Making
A customer who is a middle school science teacher and also dabbles in soap making asked if I could explain the chemistry of soap making in a detailed but simple way.
Of course I thought what a great idea for a blog.
You do not need to understand the science behind your bar of soap.
However, an in depth look at the soap-making process may help you understand why a cold-processed, handmade bar of soap is an exceptional choice for your skin.
If you dig deep back to your high school chemistry days, you may remember learning about acid-base reactions. When an acid and a base combine they neutralize each other and make a salt.
In simple terms, saponification is the name for a chemical reaction between an acid and a base to form a salt called "soap."
I explained the simple version of saponification in our blog titled, Is There Lye In Natural Soap? Won't It Harm My Skin?
Let's Dig Deeper into the Chemistry of Soap Making
Sodium hydroxide is an alkali (base) and the acids are the fatty acids that make up the triglycerides present in oils and butters. When the fatty acid and the sodium hydroxide (lye) are combined they neutralize each other and make a salt we call SOAP.
Let's begin by discussing the chemical structure of fats (oils).
On the left hand side you see a glycerol molecule. This molecule is always the same in every fat (triglyceride) molecule. Each black “O” in the glycerol molecule represents an oxygen atom. Notice there are 3 of them each of which attached to a fatty acid (the zigzag lines on the right).
While in the drawing I show 3 different looking fatty acids attached to the glycerol, they can be all the same or any combination as long as there are three.
Fats and oils are called triglycerides because there are three things (the fatty acids) joined to a glycerin molecule.
So just imagine the glycerol molecule like a tree trunk and the fatty acids are its three branches. Every fatty acid is made up of a unique chemical composition that adds different characteristics to our final bar or soap.
Fatty Acids and Soap Making
It is difficult to imagine what a fatty acid is because we can not see it. There are two main types of fatty acids, saturated and unsaturated. You have no doubt heard about these relative to nutrition and they are extremely important in determining the qualities of a bar of soap.
In the drawing of a Saturated Fatty Acid, the dash "-" lines in between the carbon atoms (C) represent a single bond. Notice that the carbon chain is straight and that there are only single bonds between the carbon atoms. If there are only single bonds between neighboring carbons in the hydrocarbon chain, a fatty acid is said to be saturated.
Saturated fats also tend tend to be solid at room temperature. This makes sense if you can picture how they would neatly stack together. Fats and oils containing higher levels of saturated fats will add "hardness" properties to a bar of soap. They are usually also the oils responsible for bubbly lather. The four most common saturated fatty acids found in soap making oils are myristic, lauric, stearic, and palmitic.
Notice in the unsaturated fatty acid that the carbon chain is no longer straight and now there are double dash lines "=" between some of the carbon atoms. These double dash lines (that look like equal signs) are called "Double Bonds." When the hydrocarbon chain in a fatty acid has at least one double bond, the fatty acid is no longer saturated because it contains less hydrogen atoms.
Fats containing a single double bond are called monounsaturated, like canola oil and olive oil. Fats containing many double bonds are called polyunsaturated like safflower oil and sunflower oil.
Unsaturated fats usually come from plant sources and since their molecules do not compact neatly they are usually liquid at room temperature. These fatty acids will add a creaminess to lather and conditioning properties to soap. The four most common unsaturated fatty acids found in soap making oils are oleic, linoleic, linolenic, and ricinoleic.
Now Let's Talk About Saponification
The word saponification comes from two Latin words "sapo" meaning "soap" and "facere" meaning "to make."
In the Saponification Reaction graphic you see there is one triglyceride (fat) molecule. That molecule is made up of three fatty acids (indicated by the "R" in the green circle).
Since each fatty acid needs to react with its own sodium hydroxide (NaOH) molecule, you need 3 molecules of NaOH (lye) for each triglyceride. When making liquid soap a different base or alkali called Potassium Hydroxide (KOH) is used instead of NaOH.
Lye comes in small crystals and must be dissolved in a water-based liquid for the saponification reaction to occur. While the typical liquid used in most soaps is water, other liquids like milks or juices can be used. Although not shown in the diagram, there is also a bit of water created along with the glycerin and soap, but as the soap cures, much of the water evaporates.
These molecules are miniscule. It's not like we can use cups, teaspoons or even scales to measure out the number molecules we need. Luckily there are standard equations and even soap making calculators to make the job easier.
These calculations are extremely important. For every soap recipe, there is a precise amount fat (oils and butters) needed to ensure that all of the lye has been transformed into soap. If we add too much sodium hydroxide (or not enough oil) there will not be enough fatty acids available to combine with the NaOH. The result would be a very harsh soap with free lye left in the bar.
On the other hand if we do not add enough sodium hydroxide (or too much oil) there will be too much oil left over at the end of the reaction. It is a very fine a delicate balance.
We always play it safe is by superfatting all of our soap and shampoo bars.
Superfatting involves adding just the right amount of extra fat (plant oils and butters) beyond that which is needed to saponify all the lye. Superfatting ensures that all lye has been used up and creates a mild soap with superior moisturizing and emollient qualities.
The Result of Saponification
When the triglycerides in fats or oils react with NaOH (dissolved in water), they are converted into soap and glycerin. You can see on the right side of the equation (after the arrow) there is one molecule of Glycerin plus three molecules of Soap formed for each triglyceride.
Side note: You will often see the words glycerol, glycerin or glycerine used interchangeably. That is because all three names refer to the same compound and there is no chemical difference between. The common name for glycerol is glycerin.
In every soap making reaction the glycerin is glycerin, it is always the same. Glycerin, a humectant and a moisturizer, is often extracted from commercial soap to make the bars harder, but all the glycerine goodness is left in handmade soap.
But the three soap molecules, with their unique fatty acid compositions are the most interesting part of our new bar of soap.
This reaction is not simply happening to one triglyceride molecule, it is taking place for each and every molecule in a fat/oil mixture.
Millions and millions of triglyceride molecules yield millions and millions of soap molecules, each one with its own distinctive properties that it contributes to our bar of soap.
The saponification reaction seems pretty straightforward if we simply mix a bunch of oils and lye together, but the science understands that each oil is comprised of different types of fatty acids in varying percentage.
Depending on what type of fatty acids are present, the resulting soap will have completely different properties.
A blend of oils with varying fatty acids is needed to create the best soaps.
While more than 300 different fatty acids are known, for soap making we really only need to understand the properties of about eight, four saturated fatty acids and four unsaturated fatty acids.
The four saturated fatty acids come mostly from oils that are solid at room temperature. They include myristic, lauric, palmitic, and stearic acids.
- Myristic acid, again from coconut oil and palm kernel oils, contributes to hardness, cleansing and lather.
- Lauric acid usually derived from coconut oil and palm kernel oil contributes to big bubbles.
- Palmitic acid, found in palm oil and cocoa butter helps create a hard soap bar with lovely stable lather.
- Stearic acid, found in solid butters like shea, mango, cocoa butters, is also in palm oil and beef tallow. This acid also helps create a hard soap bar with stable lather.
The four unsaturated fatty acids come from plant oils that are usually liquid at room temperature. They include oleic, linoleic, linolenic and ricinoleic.
- Oleic acid, best known from olive oil but also present in other oils like canola, adds moisturizing and conditioning properties to soap.
- Linoleic acid, found in sunflower and safflowers oils also adds moisturizing and conditioning properties.
- Linolenic acid, found in pomegranate, rosehip and hempseed oils and again adds moisturizing and conditioning properties.
Ricinoleic, found only in castor oil adds moisturizing qualities to soap. But most importantly it adds amazing dense, creamy and bubbly lather. Due to its unique fatty acid make-up, there is truly no other oil quite like it.
Saponification is the result of an acid/base reaction. When an acid and a base combine they neutralize each other and make a salt. Sodium hydroxide is an alkali (base) and the acids are the fatty acids that make up the triglycerides present in oils and butters.
Once we select the oils and mix them with sodium hydroxide and a liquid (lye), the molecules combine, a chemical reaction occurs, called saponification (pictured below), and a totally different substance is created -- SOAP plus glycerin.
No, you do not need to understand the chemistry behind soap making to use soap or even to make soap. However, a chef will tell you that the ability to create beautiful food is more than simply following a recipe, it is a science and an art and you definitely need both. The magic comes with an understanding of the science that takes place in the kitchen.
I believe the same is true about soap making. While anyone can follow a recipe, making soap involves understanding the science of how the fats and oils react with lye to create a quality bar of handmade soap. The choice of which specific fats and oils to use as well as their proportions is critical.