Question

when soap is used to clean motor oil (a mixture of high molar mass alkanes) what specifically happens to the oil when the soap is applied?

Answer 1

Soap is a unique substance of potassium fatty acid salts, produced through a chemical reaction called saponification. Its molecules are made up of a hydrocarbon chain, which is non-polar, as well as a carboxylate molecule which is polar. Therefore, the non-polar part of the soap – the hydrocarbon chain, is not attracted to water but to motor oil (lipophilic). On the other hand, the carboxylate molecules which are negatively charged, are attracted to the positively charged water molecules (hydrophilic).

In this case, when soap is applied to motor oil and grease, the lipophilic parts of soap will attach itself to the non polar molecules of oil. However, the other component of soap, the hydrophilic component, will be left on the surface. When water is applied onto this surface with a sponge, the hydrophilic component will be attracted to the water molecules and is lifted from the surface, together with the oil. This way, both oil and soap is removed with the wipe of the sponge. At the same time, because soap molecules have been combined with oil, other soap molecules will also be attracted to it. This is why you can see clusters of oil that are surrounded by soap within the water that has been used for washing. Of course, once soap has been used up to attract the oil, more soap would need to be added to work on the access oil.