Chemistry for Every Man: Forming Soap from Fat

Our ancestors have known of soap for more than 4000 years and has been used throughout the ages as a cleaning agent. Soap today is most frequently made from animal fat and lye (also called caustic soda or sodium hydroxide, chemical formula NaOH), or potassium hydroxide (chemical formula KOH). Potassium hydroxide tends to produce a softer soap, or even a liquid soap.

Castile soap is made exclusively from vegetable oil.

In its earliest days, the alkaline hydroxides were obtained from the ashes of fires. Ashes and animal fats made the first crude soaps.

Animal fat consists of glyceride esters of fatty acids. If the fatty acids are given the shorthand notation, HOOC-R1, HOOC-R2, and HOOC-R3, then the fat's composition may be written as:

CH2OOC-R1

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CH-OOC-R2

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CH2-OOC-R3

When the above fat molecule is reacted with three molecules of NaOH, the acid portions of the fat molecule are split off as they react with them. Thus, the reaction becomes,

CH2OOC-R1

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CH-OOC-R2 + 3NaOH =>

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CH2-OOC-R3

CH2OH

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CHOH + Na-OOC-R1 + Na-OOC-R2 + Na-OOC-R3

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CH2OH

The above resultant products are glycerine,¹ and three different types of soap molecules. Now the curious thing about the soap molecules is their possessing two portions with remarkably different solubilities.

The sodium end of the molecule is what is called ionic in nature. That means the sodium has a charge on it (plus charge) and the atoms right next to it carry a different charge (negative charge). This portion of the molecule is water-soluble in nature.

The part identified as R1, R2, or R3, is said to be covalent in nature. There is no charge to speak of in this part of the molecule, and this part tends to be water-insoluble. Rather, it tends to mix with oils and greases. Since those materials at the covalent end, with the other end dissolving in water, the composite can be used to clean greasy objects, so that the water will contain the dirt, and it can be disposed of, such things as pots and pans, or dirty hands, clean!

This, then, is the basic chemistry of soap.