Ever wondered how soap works? Soap is magic. The more I learn about soap, the more it amazes me. Soap is an old substance. Yet with all of our scientific advancements, we can’t make it better. It stands as the first line of protection against every bacteria, every virus, every fungus that seeks to do us harm.
With all that we understand about germ theory and antibiotics and sanitizing, we can’t do better than what the Babylonians (likely) came up with 5000 years ago.
What is this miracle worker?
Let me take you down to the molecular level. Most substances either dissolve in water or they dissolve in oil. Soap does both. This is because on a soap molecule, one end is water-soluble and the other is oil-soluble.
Yes! Be astounded!
Let me say it a couple different ways, which all get us to the same place:
One end is water-soluble, the other is oil-soluble.
One end is Hydrophilic and the other is Hydrophobic.
- Hydrophilic – Greek for “water loving,” meaning it is attracted to water
- Hydrophobic – Greek for “water fearing,” meaning it repels and is repelled by water.
One end is Hydrophilic and the other is Lipidphilic.
- Lipidphilic – Greek for “oil loving.” I like this statement; there’s no hate, only love.
One end is Polar and the other end is Nonpolar.
- Polar – has a charge. Water is polar, and since like dissolves like, polar dissolves polar.
- Nonpolar – has no charge.Oils are nonpolar, and nonpolar dissolves nonpolar.
All this adds up to one thing: soap dissolves into both water and oil. Honestly, this stuff gets me terribly excited.
One more bit of Greek and then I’ll stick with English. The soap molecule as a whole is classified as an Amphiphilic molecule. In Greek this means “loves both.” If only we all were amphiphilic.
Do you get what this means? With one end dissolved in water and one end dissolved in oil, soap binds oil to water. It connects them, mixes them and not even if you turn your back on them will they unmix.
This is the equivalent of achieving world peace! There is so much we can learn from the example of soap.
Where does soap come from?
Soap is made by a beautifully efficient one-step chemical reaction. Two inputs. Three outputs. All of the results are useful. No waste. No unwanted leftovers. Beautiful.
Input 1: an animal or plant fat
Input 2: a strong alkali
An animal or plant fat can be tallow (rendered beef fat), lanolin (rendered sheep fat), olive oil, coconut oil, palm oil, palm kernel oil, avocado oil… really any animal or plant fat. My brother David even once used butter for a science fair project. (I don’t recommend making soap out of butter.)
The strong alkali is usually sodium hydroxide – aka lye, aka NaOH – or potassium hydroxide – aka KOH. The former produces solid bar soap. The latter produces liquid soap.
Taking it again down to the molecular level, picture a fat or oil molecule as a big capital “E.” The vertical post is a glycerin molecule, and the three horizontal bars are fatty acids. This earns them the name “triglycerides” – “tri” for the three fatty acids and “glyceride” for the glycerin backbone.
When you combine the oil with the super strong alkali, the ensuing reaction blasts the fatty acids off the glycerin and divides the sodium or potassium ion from its hydroxide OH–.
All these parts reform:
- The fatty acids bond with the sodium or potassium, forming soap.
- The OH– (hydroxide) ions get together and form H2O – water!
- The glycerin decides it likes the solo life and hangs out by itself.
Two inputs. Three outputs. Oil and alkali in. Soap, water, and glycerin out. It’s just so tidy.
Soap in action: surfactant & emulsifier
Before soap gets to its peacemaking action mentioned above, it has to prepare the water. Water is an exclusive substance. It’s rather clique-ish. All it wants is to hang out with itself. The scientific term for this snobbery is surface tension.
Surface tension is the cohesive force a liquid has between molecules of its kind. With water, surface tension is pretty high, so much so that it somewhat resembles an elastic membrane. Surface tension is why water beads up on a glass surface, how some bugs can walk on water, and why belly flops are so very painful.
Water’s surface tension also makes it skim over surfaces instead of penetrating down into them. Pure water doesn’t make things totally wet. You may be thinking, “I’m pretty sure water makes me wet. It doesn’t skim over me.” True you get wet, but not as wet as you could.
Soap breaks this surface tension of water, forcing the water molecules to let go of each other, to look around and see that there are other things worth exploring. Things like fibers and pores and other microscopic crevices in surfaces that need to be cleaned. Soap makes water wetter.
When soap breaks the surface tension of water, it allows the water to penetrate more fully down to the surfaces that need to be cleaned, whether it’s a fabric, a counter, hair, or your skin. This ability makes soap a “surfactant” – which is a snazzy little mashed up word drawn from the phrase, “Surface Active Agent.”
Then, once the water with the assistance of soap has gotten itself down to the surface in need of cleaning, soap then gets to work with its great peacemaking mission I described above.
Soap’s ability to bond oil and water has a scientific name: emulsifying. Soap is an emulsifier. But it doesn’t do this emulsifying on a one-to-one, one soap molecule for every oil molecule basis. Instead, it acts in large groups.
A whole bunch of soap molecules – around 60 – will surround each bit of oil, forming a sphere around it, tucking the oil away safely in the middle. This little oil/soap nugget is called a micelle. It takes some time and jostling for soap to get itself situated in this way, which is why there’s the recommendation for 20 seconds of handwashing.
The surface of the fully formed micelle consists solely of those water-loving soap ends, which is all the surrounding water senses. The water has no idea that its archenemy oil is hiding inside. As the water passes by, it picks up those micelles by all those hydrophilic heads and carries them away. This is how soap cleans.
But what about germs!
All along here, I’ve been talking about soap’s action against oils. But it’s not only oils on my hands that concern me. A bigger concern is germs – bacteria, viruses, and such. Happily for us, germs are coated in a lipid layer. Remember lipid means fat. So as far as the soap can sense, bacteria and viruses are oil. Soap attaches to them the same way it would to a bit of olive oil. Its oil-soluble end dissolves right into that lipid layer, tucking it into a micelle, and swishing it away. Good-bye, germs. Hello, health.
What about Antibacterial Agents?
In recent decades soap had a challenger to its cleaning supremacy: Antibacterial Cleansers. Novel antibacterial agents seemed to elevate the effectiveness of hand and surface cleaning. These seemed like they might keep us even safer than soap ever could.
However, after a couple decades of dominance, where humble soap appeared to be bested, research caught up with the marketing. The Food & Drug Administration (FDA) demanded proof from manufacturers that antibacterial soaps were indeed more effective than regular soap, as marketing and labeling implied. Despite the huge market share that was at stake, no convincing research was found. Further, antibacterial agents give people a false sense of security and may possibly contribute to the rise of antibiotic resistant bacteria. The FDA banned the sale of antibacterial hand and body cleansers containing any of 19 active antibacterial agents, including the ubiquitous Triclosan. They reaffirmed their stance that soap is all you need.
- Soap is a surfactant and an emulsifier.
- Soap works primarily by removing, not by killing, though some germs are killed in this process.
- Soap needs some time and action to get itself situated in all those tidy micelle clusters. Thus, the recommended 20 seconds for handwashing.
- Soap needs to be rinsed with water. It does not work waterlessly.
This is my ode to soap. I think soap is pretty great. I hope knowing how it works gives you more confidence in it and makes you want to spend a little more time with it.
If you’re tired of this new-fangled digital business and yearn for a tangible book, check out Theodore Gray’s excellent tome Molecules, especially Chapter 4 “Oil and Water.” Soap is the superhero. Don’t just look at the pictures, riveting though they are. Read the words.