What makes things dirty? Why does soap clean?
Let’s say that something is dirty if it is coated with foreign matter. “Something” for our purposes is going to be some sort of body part, so, skin. “Foreign matter” could be tiny bits of dead skin, mud from the garden, grease from the garage, or other such grime, also known as dirt.
Let’s also talk about liquids. Water is a liquid; oil is also a liquid. Liquids present a phenomenon called surface tension that holds droplets together and keeps them more or less round, and it is this surface tension that grabs and holds on to particles of dirt. There’s a couple of great videos on surface tension from experiments on the space station: Cmdr Chris Hadfield on what happens when you wring out a wet washcloth in microgravity and Don Pettit using surface tension to hold water inside a wire loop as a thin film.
Skin produces watery liquids in the form of sweat, and oily liquids in the form of sebum. Thanks to surface tension, these liquids trap little particles of everything your skin comes into contact with — even dust from the air, if you’re not touching anything. Thanks, surface tension.
So now that we know what we want to clean (skin), what we want to get rid of (dirt), and why they end up stuck together (sweat & oil) we can consider soap and what characteristics make it effective.
First, soap contains at least two kinds of surfactants. Surfactants reduce the surface tension of water, making it less “sticky” to particles of dirt. Additionally, most surfactants are snakelike molecules that have a water-loving “head” and an oil-loving “tail”. This lets you suspend small bubbles of oil in water (or small bubbles of water in oil, if you’re in ancient Rome) so they can be washed away, trapped dirt and all. The primary class of surfactants in soap are the alkali fatty acid salts created when the lye busts apart the triglycerides in the source oil, replacing the glycerol with the lye’s cation (so sodium or potassium ion). The ionic bit then forms the head of the surfactant molecule, and dissolves easily in water, while the fatty acid forms the tail. The other class of surfactants in soap comes from the excess oil in a soap recipe. Most handmade soaps include 5-15% more oil than can be converted by the lye, and these oils may naturally include fatty alcohols like monolaurin (from coconut oil) or oleyl alcohol (from beef tallow). Fatty alcohols use the alcohol part as the head and the fat part as the tail, and act as a milder surfactant than the alkali fatty acid salts.
Secondly, soap is alkali, meaning it has a pH greater than 7. When the ions in an alkali solution come into contact with the triglycerides in oil, they recombine to make soap molecules instead. The more alkali the solution, the more oil can be converted to soap. And soap, as we already know, is much more soluble in water than oil is, so it can be washed away.
So essentially, soap works in two ways: by making liquids less “sticky” so the dirt can be removed, and by removing the liquids (oil or water) that trapped the dirt there in the first place.
Now we have a problem: we want to get rid of the dirt, but skin is supposed to have oil in it. The lipids that make up your cell walls are a kind of oil that has the same kind of water-loving head and oil-loving tail that surfactants do, except in your cells, the oil-loving side faces out. Your skin can shift and flex and stretch because of oil that sits between your skin cells, lubricating everything as well as serving as a water barrier so that the water inside you stays in and the water outside you stays out. If all that oil gets washed away, whether with surfactants or by turning it to soap with alkali, the skin left behind will be less flexible and might crack, and it will be easier for water to evaporate out of the body. This is… less good.
It all comes down to this: A soap with greater cleaning power will strip away more of the skin’s oils & be more irritating. A soap that leaves more skin oils in place will have less cleaning power. This is the fundamental challenge of writing a good soap recipe, and forms the rationalization behind a lot of my labeling decisions. Want to know more about my labels?
Incidentally, this is why the cosmetics and other industries have dedicated so much energy toward researching surfactants — since ideally, we could un-stick the dirt from your skin and leave everything else behind. Unfortunately, while we’ve discovered a lot of really effective surfactants, they often come with side effects that are worse than the oil-stripping soaps they’re meant to replace. One of the most infamous examples is C8, which was used by DuPont for decades before its ill effects on health became too serious to ignore. More on C8 and DuPont here.