Stupendous Suds: The extraordinary cleaning powers of soap

Soumya Satheesan
22 August 2017

Above: Image © LCOSMO, istockphoto.com

When I was a child, washing my hands took three long, extremely boring minutes. To make it more fun, I would rub my soapy hands together. I’d make an “O” with my thumb touching my index and middle fingers, creating a thin film of soap. I’d blow into that film and a beautiful soap bubble would expand out. I’d try to make bigger and bigger bubbles every time.

I would play with soap several times a day. But I had no idea that I was playing with the product of a complex chemical reaction!

Soaps may seem magical. The truth is, they’re so good at cleaning because they are surfactants and emulsifiers. As surfactants, soaps can wet surfaces easily because they reduce the surface tension of water. More on that in a minute. As emulsifiers, soaps can suspend oil or dirt so that it can be removed.

Let’s learn more about these processes in detail.

But first: what is soap?

Many people don’t know that soap is actually a salt. It results from a complex chemical reaction called saponification. This is a type of hydrolysis, a chemical decomposition (breakdown) with water. Soap comes from the hydrolysis of fat and an alkali (a soluble base).

Did you know? You probably think of soap as something to clean with. But in the past, people used soap for all kinds of things - even as medicine!

Soap manufacturers use various additives to make the soap smell, feel, look, and function better. For example, they add perfumes and fragrances to the soap mixture to make the soap smell fresh. They also add abrasives, such as talc or pumice (volcanic ash) to give it a nice texture. Then they add dyes to make it more pleasing to your eye.

Some manufacturers may also add enzymes to help the soap remove biological stains such as grass stains or blood on your clothes.

Why does soap clean effectively?

In order to clean, water needs to spread and dampen surfaces quickly. It can’t always easily do this on its own. That’s because it has a high surface tension. This means the molecules in the water are strongly attracted to each other, which prevents it from easily spreading. Lowering its surface tension helps water spread.

Substances that can lower surface tension are called surfactants (or surface active agents). Soap is a type of surfactant called an emulsifier. Emulsifiers can cause two liquids to mix even if they normally wouldn’t. For example, think of oil and water. They don’t naturally mix. But since soap is an emulsifier, it can hold the oil in such a way that water can remove it.

One end of a soap molecule is hydrophilic - that is, it mixes with water. It holds onto compounds that can dissolve in water. The other end is hydrophobic - that is, it does not mix with water. It holds onto compounds that do not dissolve in water, such as oil.

Both ends work together to hold all components in the mixture together, forming an emulsion that can easily be removed from a surface. This mixing is also what decreases the surface tension of water. The exact explanation is a bit complex, but a simple way to think of it is this: as the hydrophilic ends of the soap molecule get in between the water molecules, it interferes with the attractive forces between the water molecules.

So the next time you wash your hands, you’ll know that underneath those bubbles, some complex chemical reactions are taking place!

Did you know? Sodium-based soap is often called “hard soap,” and is often used as laundry soap. Potassium-based soap, a more water-soluble soap, is often called “soft soap.” It’s commonly used in shaving products.

Learn more

Dear Science: How does soap make things clean? (2017)
S.Kaplan, Washington Post

Soap and Detergent
A.S. Davidsohn, Encyclopaedia Britannica

Soumya Satheesan

I’m currently working towards a BScE in Chemical Engineering at the University of New Brunswick.