How Does Oil & Dirt Get Washed Away With Soap?

by Dr. George Grant

We use soap each day in our lives in the form of detergents, shampoo, shower crème, hand soap or bar soap. We are so used to using soap that we rarely stop and wonder how this wonderful compound manages to help us clean ourselves day after day. Have you ever thought about what would happen if there were no soap? How else can we rid the dirt off our bodies or our clothes?

Most of the time, dirt comes in the form of grease or oil which sticks itself onto surfaces and will not come off if only water is just used. This is because oil and grease are non-polar, which means that the oil molecules are not charged and therefore are not attracted to polar substances such as water. Because of this, oil tends to stick with its own molecules or other non-polar substances.

On the other hand, water is a polar substance which is made up of one positive and one negative charge, and therefore is a fragmented substance. With this, water dissolves salt easily because salt is made up of charged ions in which the positive charge will be attracted to the negative ions in water.

Due to the fact of the nature of oil and water, you will see that oil will not dissolve in water but remain clustered on the surface. Also, oil and grease will stick onto plates and cutlery during cleaning, and no amount of water can completely remove it. That’s when soap comes in. All it takes is just one layer of soap with water and the oil will be removed. How does this happen?

Well, 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 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 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.

In conclusion, our lives have been made cleaner and easier through the wonders of a simple substance called soap. Without it, we would be having a difficult time removing dirt, oil and grease in our everyday cleaning.