Fatty Acids in Soap

There are so many different oils we can use to make soap, choosing which oils to use in a new recipe can get really confusing to the newbie soapmaker (and some of us who've been around the block a couple of times too!).   Soapcalc (my favorite lye calculator, and topic of a future post), lists the fatty acid make up of all the oils in its database.  However, unless you know what each fatty acid brings to the party, the information is only so helpful.  Soapcalc does list the hardness, cleansing, conditioning, and lather properties of each oil. However, I find that as I develop new recipes to try out, I'm looking more and more often at the amounts of specific fatty acids in my oils, because I want to increase or decrease specific properties in my soap.

This post will describe the different fatty acids found in soap, and how they affect the properties of our finished soap.  The next post will look at some of the more common oils we use in soap making, and the major fatty acid make up of each one. I'll start by explaining some basic chemistry of fatty acids.  If high school or college chemistry gave you a headache, feel free to skip this next paragraph and go right to the list of fatty acids found in our soap making oils.

What IS a fatty acid?  As the name implies, it is both a fat and an acid.  The head of the molecule is a carboxylic acid,  and the tail is a chain of carbon and hydrogen atoms of varying length. There may be some double bonds between the carbon atoms, making the fatty acid unsaturated, or there may be no double bonds, making it saturated (it has as many hydrogen atoms attached as is possible). The number and location of the double bonds contributes to the flexibility of the molecule, and the melting temperature of the oil.  A non-scientific way of determining the relative saturation of oils is to see how solid the fat is at room temperature.  The more solid the fat, the fewer double bonds it has. Saturated fatty acids (like stearic acid and palmitic acid), are quite hard at room temperature, and have high melting temperatures, while highly unsaturated oils (like olive oil), are liquid at room temperature, and stay liquid until they get fairly cool.  Because of saturation/non-saturation, one can see that the more saturated the fatty acids in the oils and butters you use, the harder the finished bar of soap will be.  Unsaturated oils will add more conditioning properties to soap.

Fatty Acids Found in Soap and Their Properties

• Lauric Acid:  A saturated fatty acid that contributes to a hard bar, fluffy and stable lather, and high cleaning. Too much of this fatty acid can make a soap drying.
• Linoleic Acid:  An unsaturated fatty acid adds conditioning to the soap.  Oils high in this fatty acid tend to go rancid quickly, and more easily develop DOS (dreaded orange spots).
• Linolenic Acid:  Not to be confused with linoleic acid.  Adds conditioning properties and is very mild. 
• Myristic Acid:  A saturated fatty acid, so it adds to the hardness of the bar, fluffy lather, and cleansing ability.  Soaps high in myristic can be very drying to the skin.
• Oleic Acid:  An unsaturated fatty acid, so it adds conditioning properties.  This fatty acid does not contribute much to the lather of a soap, but it does contribute to the "slippery" feeling of soap.
• Palmitic Acid:  Another saturated fatty acid, so it will contribute to a hard bar and can by drying if too much is used.  It contributes to a creamy, rather than fluffy lather. 
• Ricinoleic Acid:  Unsaturated, it adds conditioning properties, and is great for a fluffy, bubbly lather.  Found almost exclusively, and in great quantities in castor oil. 
• Stearic Acid:  Saturated, so it contributes to a hard bar. It is very similar to palmitic acid, and is generally interchangeable with it.  Contributes to a creamy lather. 

Feel free to copy and paste this information for your future use. However, please reference this page as your source. 


A list of my sources of information: 
Wikipedia entry on fatty acids
Wikipedia entry on carboxylic acids
Wikipedia entry on ricinoleic acid
Buzzle article on fatty acids
Fatty acid information