Soap Science!

Soap Science!

SoG+ community member J Bennet shares the cool chemistry behind his soap making hobby with us. Have you ever tried making soap? 

Originally shared by J Bennett (The Speculating Ape)

I plan to make a batch of soap this weekend, and I thought it might be interesting to bring up here. Soapmaking is a simple home craft, but there’s some interesting science behind it.

For one thing, what exactly is soap? It’s sort of waxy, but it’s not wax. It’s sort of oily, but it’s not oil. It turns out that soap is a kind of salt, the thing you get when you react acid and metal. The soap we use for bathing is made with a lye solution (sodium hydroxide and water) and any number of vegetable or animal oils.

In the soap-making reaction, called saponification, the sodium in the lye is the metal. That’s for bar soap. For liquid soap you use potassium hydroxide. The acid comes from the oils, which are made of triglycerides. A triglyceride is a glycerol molecule with three fatty acid molecules attached.

The characteristics of your soap depend on the fatty acids in the oils you choose, since each type of acid yields a different salt. You might even see the names on some soap packages. Sodium cocoate is the salt (really a mixture of salts) you get when making soap with cocoa oil. Sodium palmate comes from the palmitic acid in palm oil, and it’s also where the “palm” in napalm comes from.

Like any chemical operation, you have to be careful about amounts. Too much lye for the oils you use, and your homemade soap will have drain cleaner as the secret ingredient. It’s a harsh way to exfoliate. That’s why soap recipes use extra oil, typically around 4% to 10%. Soapers call that superfatting. It makes sure that all your lye will react, and the extra oil helps moisturize your skin.

It’s a fun hobby, relatively safe if you’re careful with the lye, and a good example of how even non-scientists can use science to make life smell better.

A final interesting (I hope) point. You make soap with lye, water, and oil. A similar process using lye, alcohol, and oil yields biodiesel.