What is the physical (or chemical) mechanism by which soap (liquid and/or bar) cleans you?


I always assumed it has something to do with the foam, but I’m not quite sure what.

I also assume that shampoo and toothpaste work in a similar way, but with different chemicals that provide specific benefits to hair and teeth respectively.

In: Physics

A soap molecule has two parts. One loves water (hydrophilic – hydro-water, philic-loving) and the other hates water (hydrophobic – phobic-hating/staying away)

The hydrophobic part sticks to oil and dust and whatever is dirty. It kind of forms a sphere around the particle, with water-loving end outside. This collection of all suck spheres forms the foam. Remove the foam and thing is clean.

Soap has molecules that bind to oil and grime molecules and helps make them water soluble so that they’ll wash away.

Soap is a molecule where one end likes sticking to water and the other end likes sticking to oil, this molecule is a stretched out triangle shape which means when it chains together it makes circles and spheres around the dirt and oils with the end that likes water on the outside of this bubble, the dirt and oils can then be washed away along with the rest of the soapy water.

These bubbles filled with dirt and oil however are very small, the foamy bubbles you see are mostly so you know where you’ve put the soap and to lift the tiny dirty bubbles away from your skin so more soap can get in and give you a clean.

Shampoo contains certain oils (some soaps have moisturiser in them too) which linger in your hair, we are naturally oily and don’t do very well when all the oil is stripped off our skin and hair so need it put back once we have taken the dirt filled oil off our bodies.

Toothpaste is more of an abrasive, it scratches the surface of our teeth (yes, you can brush too much and too hard, though it is more of an issue for your gums than your teeth) to get the germs and bacteria off and traps them in the toothpaste for you to spit out, it can also contain things like fluorides to whiten your teeth and remove any stains, making it look like it’s removed more plaque than was actually there.

The process is called ‘emulsion’ I believe? Or soap is an ‘emulsifier’. I know it contains molecules attracted to both oil and water. Dirt = oil, so the soap attaches to the oil and washes away with water.

Soap is an emulsifier which means it makes oil soluble in water and this is what makes you clean
It doesn’t need to foam