What is Soap?!

Have you ever wondered what soap is, or been curious about the science behind the bubbles?

The magical kettle of soapy stew holds lessons in science for me and you!

You don’t have to be a chemistry major to be interested in or understand the basic science of soap. My goal in this post is to make this subject a little more digestible. If you’d like to learn more about One Earth soap, you can check it out here.

To understand soap, we first need a handle on fats and oils!

Fats and oils are composed of fatty acids and glycerides. What are those?

FATTY ACIDS:

An acid is any molecule that can donate a hydrogen atom (H+). A “fatty” acid has a carboxyl group (COOH) attached to a hydrocarbon chain (a chain of carbon and hydrogen atoms). The carboxyl group can release its H+, which makes the molecule an acid. Stearic acid (C17H35COOH) is a common fatty acid in many oils and fats. Here’s a cartoon of stearic acid (black balls are carbon atoms, white balls are hydrogen atoms, red balls are oxygen atoms):

what is soap, soap chemistry, triglyceride, glycerine, fatty acid

Its simple line structure is:

what is soap, soap chemistry, triglyceride, glycerine, fatty acid

The particular fatty acids in soap determine the qualities of the soap, such as cleansing, lather, hardness, conditioning. Each oil or fat has a particular mixture of fatty acids. The major fatty acids in soapmaking oils are:

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Lauric acid, C12H24O2

what is soap, soap chemistry, triglyceride, glycerine, fatty acid

Myristic acid, C14H28O2

what is soap, soap chemistry, triglyceride, glycerine, fatty acid

Palmitic acid, C16H32O2

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Stearic acid, C18H36O2

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Oleic acid, C18H34O2

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Linoleic acid, C18H32O2

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Alpha Linolenic acid, C18H30O2

(Images from Wikimedia Commons)

As you can see, the fatty acids have different “chain lengths” and shapes because they have different numbers and configurations of carbon atoms. The straight-chain fatty acids are dominant in saturated fats and butters, where every carbon has single bonds with other carbons and hydrogens (except for bonds with oxygen at the end of the chain). The bent fatty acids have double bonds between one or more carbon atoms and so are unsaturated. The molecules have a kinked configuration of C and H atoms around the double bonds, compared to the even zig-zag pattern where each C is attached to 2 other carbons and 2 hydrogen atoms. Unsaturated fatty acids are dominant in oils that are liquid at room temperature (around 70°F or 21°C).

But why does the shape of the fatty acid affect whether a fat or oil is solid or liquid at room temperature?

Straight-chain fatty acids can pack closely together, which leads to increased strength of intermolecular attraction between the fatty acid molecules, thus raising the melting temperature. In contrast, unsaturated fatty acids can’t pack closely together, so the intermolecular attraction between fatty acid molecules is weaker and they melt at lower temperatures.

GLYCERINE:

Glycerine (C3H8O3, below) is a molecule with 3 carbon atoms attached to 5 hydrogen atoms and 3 OH groups.

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what is soap, soap chemistry, triglyceride, glycerine, fatty acid

Glycerine, C3H8O3. Images from Wikimedia Commons.

The OH groups of glycerin react with the COOH groups of fatty acids, releasing water and forming glycerides. When 3 fatty acids react with one glycerine molecule, the result is a triglyceride.

Below is an example of a triglyceride. The left part of the molecule shows the glycerine-based backbone; the leftmost O atoms (attached to C atoms) are at the location where OH groups of the glycerine molecule reacted with COOH groups of the fatty acids, releasing H2O and joining the molecules:

what is soap, soap chemistry, triglyceride, glycerine, fatty acid

Fats are made of triglycerides.

OK! But what is SOAP??

When oils and fats are mixed with a strong base (sodium or potassium hydroxide), soap is made! A base is a molecule that can release an OH group. Lye is a solution of sodium or potassium hydroxide (NaOH or KOH) dissolved in water. When lye made with sodium hydroxide reacts with triglycerides, it breaks the bonds between the glyceride part of the molecule and the fatty acid parts. The glyceride gets its Hydrogens back to become glycerine again, and the fatty acids acquire sodium (Na+), becoming “salts.”

For example, when stearic acid reacts with lye, it loses a Hydrogen (from the OH on the end of the molecule) and attracts a sodium ion (Na+).

what is soap, soap chemistry, triglyceride, glycerine, fatty acid

Stearic Acid Sodium Salt Structural Formula, Wikimedia Commons

So SOAP is a mixture of salts of fatty acids and glycerine, and it has a high (basic) pH because lye is a strong base that releases OH molecules.

There you have it, a molecular explanation of what soap is!

Next week’s post will talk about how soap WORKS!

If you’d like to try some natural soap, consider One Earth Fair-Trade, palm oil free, sustainable soaps!

4 Comments

  • Trudy O’Donovan Posted June 7, 2019 6:10 pm

    Thank you so very much for the science ed. I am planning to start an artisan natural soap and skincare business here in rural Ireland. However I want to use a tallow base for all my products and hopefully keep the ingredient to a minimum of 75% local I realise that this goes against the vegan trend. However without having looked much into the environmental and social costs of importing vegetable oils like coconut and cocoa butters I realised that the carbon footprint of these oils is far greater than of the lovely grass feed beef we produce here . I am able to make top grade tallow for my labour and minimal energy cost alone here. This to me means that I can keep my carbon footprint and production cost much lover than others who use vegetable fats only. Having read your post questioning vegan environmental friendly? Soaps I am happy that my gut-feelings are correct and my products will have only minimal negative impact of the social and environmental fabric of other less developed countries. Mind you I always will love good quality dark chocolate. However I do not need to use cocoa butter for my skincare products. So thank you very much for confirming my taught and enlighten me further.

    • annealtor Posted June 13, 2019 9:58 am

      Hi Trudy,
      These issues are definitely complex. When you have a quality source of local materials, it makes sense to use them – especially if they would otherwise be wasted. You are clearly thinking through the impacts of the materials you’ll be using. I wish all producers did so. Best wishes with your company!

  • Chuck Posted March 6, 2019 9:14 pm

    A great overview and very technical explanation of what is involved in soap manufacture. It is obvious that you have done much research and are an expert in the science of this product. I do feel that the ‘vegan’ label is fine can and should be expanded to infer environmental and social criteria as you have suggested. Thank you for your very thoughtful and scientific explanation of the multiple issues involved in the term ‘vegan’. It is very important that this is exposed and clarified.

    • annealtor Posted March 6, 2019 9:38 pm

      Thank you very much for your comment and thoughts on the soap chemistry and vegan soap posts. I think it’s important to help people understand what’s behind the labels that we sometimes take for granted. Thanks for reading!

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