What Is a Fatty Acid Salt?

Soap is an example of a fatty acid salt. Soap is usually made by mixing sodium or potassium salts, such as lye, with fatty acids. Soap also sometimes has air bubbles mixed in to decrease its density, enabling it to float on water.

Potassium fatty acid salts have a softer, creamier lather. The reaction between a fatty acid and a metal hydroxide forms a fatty acid salt and is called hydrolysis. This reaction occurs in two steps, with the oxygen-based ester bond of the fatty acid being broken down in the first step and the metal ion bonding in place of the oxygen atom in the second step.

The efficiency of reacting the fatty acid portion to become a fatty acid salt depends on the basicity and concentration of the metal hydroxide solution used. The type of fatty acid and the length of the alkyl chain it contains determine the property of the produced soap. Animal fats and tallow produce sodium stearate, a fatty acid salt possessing 18 carbon atoms. This soap is very hard and insoluble, as the longer the fatty acid chain gets, the more insoluble the soap becomes. Zinc stearate is a fatty acid salt used as a water-repellent for this reason.