The chemistry behind your DIY soap project

*Find defintions for bolded terms in the Glossary tab.

I have been excited to see the growing trend of DIY soap making. It is, of course, important to remember that you are working with dangerous substances, but it is a fun, cost effective way to customize your personal care. An example is the batch I made for the holiday season, charcoal and peppermint bars (pictured above).

Humans have a long and interesting history when it comes to bathing. Uses of a soap like substances started with the Egyptians and Mediterranean’s. Their ancient soap recipe consisted of mixing fats and oils with ashes and used that to clean cotton and wool products. As the practice evolved sodium hydroxide (commonly known as lye) replaced the use of ash and that is still the basic recipe used today.

Castile soap is both an oldie and a goodie. This basic recipe has existed for centuries and consists of only two ingredients: sodium hydroxide and olive oil. A popular castile soap brand is Mr. Bronner’s, but their recipe is a little modified.

I am going to be focusing on the synthesis of solid soap bars, but if you ever want to make liquid soap you would use potassium hydroxide (instead of sodium hydroxide). These compounds are both extremely strong bases which is what is needed to kick start the chemical reaction, but because these are different salts, you get different products.

It is important to understand that when you’re working with strong bases or acids, you are putting yourself at an increased risk. If you plan on making your own soap I advise that you first make sure you are in a well vented environment (I usually work outside), have the proper safety equipment (including gloves and glasses) and cover all exposed skin with gloves or clothes (that you’re okay with potentially ruining). This is because sodium hydroxide is caustic and that means it can burn your bare skin and its fumes can greatly irritate your eyes/nose/mouth.

To start the process, we prep our olive oil. Next, we are going to mix our sodium hydroxide (commonly known as lye) with water. Always pour the base into water, NEVER THE OTHER WAY. This is because this reaction is exothermic (meaning it creates a lot of heat) and pouring water into sodium hydroxide would cause an explosion.

If this seems like this is some real serious business, it’s because it is. But also, don’t be afraid to try it yourself. Just be smart about it.

Mixing the lye into water causes its temperature to shoot to 80ºC, so we’re going to have to let it sit and heat up our olive oil. This technique is actually called “cold process”. There are numerous soap making techniques that the Chem Babes will be experimenting with throughout the semester.

Once our lye solution cools to the temperature of the olive oil (within the 30º-36º range) we are ready to mix the two together. It is important for these two solutions to be at matching temperatures because it effects the reaction environment. Everything from changes in atmospheric pressure to changes in room temperature can affect a chemical reaction. So, it is important that the environment is consistent throughout the reaction.

Mixing by hand is possible, but quite time consuming and not recommended. I personally like to use a stick blender to kick start the reaction. You want to mix until the solution is relatively thick. I judge this by lifting the blender and watching how the drops mix with the rest of the solution. Once the drops are thick enough to leave a traceable pattern, it is ready to pour into a mold.

While the process is mixed it undergoes the following process of saponification. Here, you see this fatty acid in olive oil: Triglyceride.

The group of atoms in red is called an ester and this is where the saponification reaction occurs. The chains bonded to the oxygen 2 and carbon will be referred to as R groups, but they are essentially long bonds of carbon and hydrogen (a characteristic of fatty acids). It’s identified by its shape and the two oxygens covalently bonded to the centre carbon. Oxygen 1 is double bonded to the carbon and oxygen 2 shares one bond with this carbon and one bond to the rest of the molecule.

Lye (sodium hydroxide), however, forms an ionic bond. Ionic bonds are not as strong as covalent bonds and that is why the hydroxide (OH) will seek to form a covalent bond with the ester. This is called a nucleophilic reaction which means a molecule/atom that is partially or fully negatively charged will seek to bond with a partially positively charged atom. Here, the carbon at the centre of the ester holds a partial positive charge.

This is partial positive central carbon is common in nature and follows the rules of electronegativity. As we know, oxygen needs two more electrons to complete its valence shell while carbon needs four, so oxygen is more electronegative and will hog the shared electrons.

So, our hydroxide anion will form a bond to a carbon which starts a chain reaction. In order to make room for this new bond, carbon will break the second bond with oxygen 1 and oxygen 1 will now hold the extra electron and a negative charge. However, because oxygen 1 holds more than 8 valence electrons, it becomes quite unstable and will not hold the extra electron for long.

Oxygen 1 will instead reform its double bond with carbon and this time carbon will break its bond with oxygen 2. Oxygen 2 now carries the negative charge bonded to the R group and our ester is now carboxylic acid. It looks quite similar to the ester group, but you can distinguish it by its hydroxide (OH) tail. This kick starts an Acid-Base reaction. Our oxygen 2 anion attracts this hydrogen with its negative charge. The hydrogen leaves its electron with oxygen 3 which would be less stable except for the resonance structure allowing both oxygen 1 and 3 to share the extra electron and double bond.

Resonance is a hugely important factor in determining the stability of a molecule. Our resonance occurs between both oxygens. As you see, because they are bonded to the same carbon, they can share the negative charge and double bond. It is very difficult to observe resonance, but you can picture it as the oxygens switching back and forth.

Now, oxygen 1 and 3 can balance this negative charge and form an ionic bond with the sodium cation. It’s been present all along, but essentially "observing" this reaction take place. Now that our molecule is bonded with sodium we can call it a salt and that is when we know the saponification reaction is complete.

If we look at the entire molecule we see the charged tip that is ionically bonded to our sodium cation and then covalently bonded to this long tail. This structure is what gives soap its unique characteristics. The tip is described as polar, like water. Therefore, we can say the tip of the chain is hydrophilic which literally translates to “water loving”. The rest of the chain is non-polar and that is because it mainly consists of carbons and hydrogens that are not very electronegative. Common non-polar substances are grease and fats. These non-polar molecules are hydrophobic--water repelling--and cannot be washed using just water. This unique structure is how soap can wash away grease and fats with just the use of water!

Once the solution is thoroughly mixed and the saponification process started, you poor your soup into the mold and wrap it in a towel to insulate. This insulation is important so that the soap can stay at a certain temperature for the rest of the reaction to take place. You are recommended to leave it for roughly 24h before touching it.

The example with olive oil and lye is the most basic, castille soap recipe which is great for children and those with sensitive skin (pictured below). However, there are numerous exciting recipes freely available on the internet that target your own, personal needs.