How soap and shampoo help keep you and your hair clean

You use them every day to help keep your skin and hair clean, but do you know much about how soaps and shampoos work?

In the following post we'll explain the little of the history of soap and shampoo, and a lot of the science behind how they help keep your skin and hair clean.

How it started

The earliest known soaps are thought to have been made around 2800 BC. But it wasn't until 1823 when a French chemist called Michel-Eugène Chevreal  published his explorations of the chemistry of saponification that people started to really understand the chemistry of soap making and how it helps keep us clean.

While the rest of the world had been cleaning their hair with various plants, barks and herbal extracts for generations, it was almost a century after Monsieur Chevrel's explorations that Europeans started to use the process of saponification to make shampoos for their hair.

In fact: regular hair washing with liquid shampoos didn't really catch on in the West until the second half of the 20th Century. This was partially due to the increased production and marketing of shampoo during that period and partially due to improvements in household plumbing that made it easier to do (you don't need hot water to make shampoo work, but washing your hair is a much more pleasant experience when using hot water...)

That's the TL;DR version of the history of soap and shampoo making, now for the science bit.

The science of how soap and shampoo gets you clean

In order to clean your skin, or hair, you want to remove dirt, grime and oil that's collected on the surface.

You can do that to a degree with good old H-2-O (water) and a good scrub. To a degree...

No amount of brute force or rinsing with just water will remove really stubborn oils though. Because oil isn't water soluble. You can see this yourself by putting some water and some cooking oil in a container and shaking it up. Initially the oil and water seem to mix, but leave the container still for a while and you'll see the oil and water sitting in two separate layers again (that's why you need to give your oil based salad dressing a really good shake before dousing your favourite greens with added flavour).

Because oil won't mix with water, when you rinse your skin or hair with just water and no soap or shampoo the water can't wash away grease or oily grime that may have built up. So while a good soak will leave you feeling fresher, it won't leave you truly clean.

If we add soap or shampoo to the equation things change. Washing with soapy water allows oil to be mixed with the water and be washed away, leaving your hair and skin squeaky clean.

But how?

To understand we first need to know something specific about the chemical makeup of both water and oils (we did say we'd get scientific...):

Water is made up of molecules that are "dipoles". This means that each water molecule (made up of two Hydrogen atoms and one Oxygen atom, H20) has a charge and attracts its neighbours. They literately hold on to one another. But not in a particularly strong way; you can break the neighbourly molecular bonds simply by stirring the water.

To see this in action fill a glass with water, right up to the very top. Then add just a few more drops and you'll see the surface of the water rise above the top of the glass, just a bit more than you'd expect. The water molecules are holding on to each other just tightly enough to stop the overflow spilling over your glass. Slightly shake the glass and the water will spill though as you've done enough to break the weak bonds.

Most oils, however, are made of molecules that are not dipole. Which amongst other things means they don't bond with each other. Repeat the above overflow experiment with oil and the extra drops will spill down your glass without needing any shaking.

This is why you can't mix water and oil.

Stir them together and the water molecules want to attract other water molecules and in doing so "push" out any oil molecules that have been mixed between them. Given enough time any mixed water and oil will separate as the water molecules regroup and push away the oil molecules.

So if you wash your dirty skin or hair with just water, some of the fatty oils will never mix with the water and will never be rinsed away.

Soap (and shampoo) comes to the rescue.

This is where soap and shampoo come to the rescue. To understand how, we need to know what makes a soap (and shampoo) molecule.

A soap molecule is made up of two parts; a charged end with oxygen atoms called a carboxylate and a fat chain made up of carbon and hydrogen atoms.

The picture below shows an artist's impression...

Diagram of a soap molecule

A soap molecule is a fatty acid, albeit a very weak acid.

What we normally think of as "soap" (as in a bar of soap or a bottle of liquid soap) is technically soap fatty acid molecules mixed with other ingredients to form a bar or a liquid.

Soap's super power is that thanks to its molecular makeup, it has some properties of both oil and water.

The soap's carboxylate (head) has a dipole, so is charged and attracts water. The soap's fat chain, like oil has no charge and is not attracted to water.

This means that when mixed with water soap molecules arrange themselves in spherical structures, with the fat chains of the soap molecules gathered inside the spheres and the charged carboxylates facing outwards to the water like a skin. These structures are called micelles. The diagram below shows a cross section of how this looks.

Diagram of the cross section of a soap micelle in water

If you put soap, water and oil together and stir, all their molecules are mixed together. When the stirring stops and the molecules start to reorganise themselves some of the oil molecules get trapped inside these micelle spheres.

Rinse away the water containing these micelle spheres, and you remove the oil trapped inside them too.


So how exactly does shampoo help wash your hair?

The keen eyed reader will have spotted that what we've described here refers a lot to how soap helps you wash. So you may be wondering how shampoo helps when washing your hair?

Well, basically in exactly the same way. A shampoo's chemical makeup is very similar to soap's (just adjusted to be more suitable for use on hair) and so they perform the same way when mixed with water: trapping dirt and oil molecules from your hair and scalp in micelle spheres that get washed away when you rinse after lathering.

Here endeth the science lesson.

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