Coming Clean: How Soap Works

COMING CLEAN

…about soap and other personal products

Soap is one of those things we take for granted. But what is soap, anyway, and how does it work? Who invented it? Does it matter what kind we use?

The answer to the last question is yes, it sure does matter, and that makes it worth knowing more about soap, as well as about the ingredients in our other personal products.

HOW SOAP WORKS

It all begins with water. Why won’t water just wash away dirt? Water molecules are made up of one oxygen atom and two hydrogen atoms. The hydrogen atoms are on one side of the molecule and the oxygen atom on the other. The hydrogen atoms each have a single electron, which usually “hangs out” near the oxygen atom, which has eight electrons of its own. This creates a positive charge on the hydrogen side of the water molecule and a negative charge on the oxygen side. The positively charged hydrogen end of each water molecule is attracted electrically to the negatively charged end of other nearby water molecules. This attraction causes countless temporary bonds between water molecules, which is what makes water stick together.

Within a container, or even a drop, of water, these bonds create a pull on all the molecules in every direction – except at the surface. The molecules at the surface have no molecules above them to pull on them, so they are only pulled by the molecules underneath them. This creates surface tension, making it possible to fill a glass noticeably above the rim. It also holds dew drops together in little spheres. Water strider insects actually skim around on the skin-like surface of ponds and streams, depressing the water surface under their tiny pontooned feet.

Water will dissolve something like sugar or salt, as their particles also have electrical charges, but dirt is usually associated with oil, and water is simply not attracted to oily things because they have no electrical charge. In fact, water and oil repel each other. If you put a greasy dish into water, the grease will actually flatten itself against the plate to get away from the water molecules.

Soap changes all this, starting with the surface tension. A soap molecule is a long hydrocarbon chain that looks a bit like a caterpillar. The head end loves water and hates oil, the tail end loves oil and hates water. When you add soap to water, the soap molecules near the surface squeeze between the surface molecules of the water and all stand on their water-loving heads with their water-hating tails in the air, reducing the surface tension of the water to about a third of its usual strength. (Soap bubbles last longer than plain water bubbles because there is too much surface tension in water for it to remain in a bubble. In a soap bubble, the tails of the soap molecules are on the outside of the bubble, which also protects it from evaporation. If you want to see how long you can keep a soap bubble, try putting it in a jar. One bubble lover kept a bubble for 341 days this way.)

Soap that is dispersed in water forms little clusters called micelles, as a group of soap molecule “caterpillars” get together with their tails in the center of the cluster away from the water. This gives the clusters a negative charge, so they repel each other and disperse throughout the water. When they encounter a bit of grease or oil they grab it and form a new micelle with the grease held inside the micelle. These particles are washed away when we rinse the soapy dishes.

Washing a person is similar. The reason water alone does not do the job is that we are oily. What dirt attaches itself to us is embedded in this oil, and it repels water. Until we reach for that bar of soap.

But wait, what is that bar of soap actually made of? We’ll look at what’s in our soap shortly, but first…  [History of Soap]