Fatty Acids & Glycerol: The Molecular Magic Behind Soap

The process of making soap is a simple one. Mix oils and butters with lye. Get soap. But what is really happening when we mix the oils, butters, and lye together? To understand this, you first must understand the molecular structure of an oil or butter.

There is an amazing diversity in the structures of oils and butters. However at their core, each oil or butter is made up of triglycerides. Triglycerides are made of three fatty acids (tri means three) and a glycerol.

Glycerol backbone attached to three fatty acids. Illustration from Anatomy & Physiology, Connexions Web site. http://cnx.org/content/col11496/1.6/, Jun 19, 2013. Used under the creative commons license.

These structures are produced by plants and animals as a form of energy storage. Gram for gram, triglycerides contain more than twice the energy content of carbohydrates and proteins (9 calories per gram for fats vs 4 calories per gram in a protein). Fatty acids and glycerol are both made from glucose—a type of sugar. In an animal, the sugar is eaten. In plants, though, they produce their own sugar from sunlight and carbon dioxide in the air through the process of photosynthesis.

The process of photosynthesis in which plants take in water through their roots, carbon dioxide through their leaves, and energy from sunlight to produce oxygen gas and glucose sugar. Image used under a creative commons license.

Fatty acids have a relatively simple structure— a long chain of carbon atoms with many hydrogen atoms connected to them. At the end of a fatty acid, there is a group of atoms called a carboxylic acid. This carboxylic acid has a hydrogen atom that can detach thus making it an acid.

To summarize so far: oils and fats are made of triglycerides which are made of glycerol and three fatty acids. Glycerol and fatty acids are made from glucose (sugar). Fatty acids have a carboxylic acid group with a hydrogen at the end which makes it an acid.

The diversity in fats and oils comes from the different fatty acids that are produced by the different plants and animals. Some plants, like coconuts, make fatty acids with no double bonds. These fatty acids stick together easily because of the large number of hydrogen atoms and their relatively flat structure. Kind of like how burrs stick to dog hair, cat hair, or your pants!

Horse with cockleburs stuck in mane. Image used under a creative commons license.

Other plants, like olives, make a lot of unsaturated fatty acids like Oleic Acid and Linoleic Acid. These fatty acids have double bonds in their long carbon chain making it harder for them to stick together to become solids. The double bonds cause them to bend and makes it so they have fewer hydrogen atoms. So oils with a lot of unsaturated fatty acids are liquid at room temperature.

Structures of stearic acid and oleic acid. Image used under a creative commons license.

Soap makers choose the oils and butter they use to make soap largely based on the fatty acids those oils and butters contain. For example, if a soap maker wants a bar of soap that has a lot of fluffy lather, they would use oils and butters that are higher in lauric acid and myristic acid like palm oil and coconut oil. If they are looking for a more conditioning bar of soap, they would use oils that contain ricinoleic acid like castor oil.

Here at The Clean Chemist, we aim for a bar that is very conditioning, moderately cleansing, and has a moderate amount of bubbles. We also are mindful of the impacts our oils make upon the environment. For example, all palm oil used by The Clean Chemist is RSPO Certified meaning that it is more sustainable and contributes to less deforestation. We carefully select oils not only for their skin benefits but also for their ethical sourcing— choosing suppliers who align with our commitment to cruelty-free practices, animal welfare, and environmental responsibility. Because clean chemistry isn’t just about what goes on your skin— its about the impact it has on the world around us.