Sodium Chloride & Soap Viscosity

When Joe is performing quality control tests on liquid soap, it is important for him to consider the viscosity of it because it is important for soap to have a certain thickness to it in order for the product to feel nice and comfortable on the consumer's hands. Joe increases the viscosity by adding sodium chloride --a table salt that I have come across many times in my career as a high school chemistry student. So how does it actually work?

First off, soap is prepared through saponification, since natural soaps are sodium or potassium salts of fatty acids. Saponification is the reaction that occurs between an ester and a strong base, such as sodium hydroxide or potassium hydroxide. When the reaction occurs, an alcohol and a salt of the acid is formed. The latter is the soap. For instance, when methyl methanoate reacts with sodium hydroxide, it forms methanol (the alcohol) and a sodium salt of ethanoic acid (the soap).

The saponification of methyl methanoate in the presence of sodium hydroxide (first line of the image)

Sodium chloride can be added to the soap in order to precipitate it, thus making it more viscous. This is related to the common-ion effect, which is responsible for the reduction in solubility of a compound when a soluble compound consisting of one of the ions in the other compound is added to the solution in equilibrium. Therefore, adding sodium chloride to soaps causes it to precipitate, increasing the thickness. Not only is this due to the common-ion effect, but it is also due to increased ionic strength. Since sodium chloride is considered an electrolyte, it can be decomposed into sodium and chloride ions. Therefore there is an increase in ionic strength due to the formation of ions that causes precipitation. The concept of electrolytes, ions, and precipitation which I have learned throughout high school chemistry have been adapted to the adjustment of viscosity in soap in the field of quality control.

How NaCl is an electrolyte since it decomposes into Na+ and Cl- ions.