How Does Soap Identify Dirt

At first glance it would appear that soap has properties that allow it to discern dirt from everything else. In fact, the reality is that soap has no such ability. Simplistic as it may sound, nearly everything we label dirt is either oily or is stuck to us with oil. What makes soap special is this ability to remove oil.

How Dirt Sticks

Anything we don't want attached to us (ie. dirt) can be stuck to us in one of two simple ways.

1. trapped in microscopic crevices. For instance the dirt you get on a dry dusty road.
2. or moisture makes it adhere. For instance the dirt you get from a muddy road.

In both cases, a good hosing down with plain water with perhaps a little rubbing will do a resonable job of removing the dirt. Soap isn't really necessary yet.

But what if the dirt particles have a slightly oily coating? They will stick to your skin like wet mud. In addition, the dirt doesn't have to even bring its own oily coating. Your skin many times has enough oil on it to make dirt particles stick. But unlike the mud, this dirt is going to stay stuck because oil doesn't evaporate and dry up as water does. Nor will a spray of plain water dislodge it because it will simply roll off the dirt as water rolls off the oil. Assuming that the force of water isn't sufficient enough to dislodge the oil of course.

Removing Oil Substances

To unstick the oil adhering dirt is to seek out and destroy the sticky oil itself. A liquid can then be applied and the dirt will fall off and be swept away.

You could fill a sink with alcohol, kerosene, or gasoline as they are all good solvents for oil. In fact dry cleaners tumble our clothes in a barrel full of a solvent such as perchlorethylene an organic solvent that is a phenomenal dissolver of oil. The problem with these solvents is they are all toxic to greater degrees.

The answer is soap. But soap dosn't really disolve oil like solvents do. It works by enticing the oil into the water so the oil and its dirt can be flushed away. It also disturbs the surface tension of water. In other words, it makes water lay flatter and not ball up like a rain drop. This allows the water to get into micro crevices even better and wash away the dirt.

But How Does Soap Work Then

Soap molecules are long and stringy just like oil molecules are. They have two very distinct ends. The head of this soap molecule string loves water and will attach to other water molecules. Its tail on the other hand loves oil and will search out and attach to other oil molecules. This gives us a very interesting situation where the head of the soap molecule will attach to other water molecules and the tail will find and attach to other oil molecules. The effect is that the soap molecule will latch onto the oil with its tail and be flushed away with other water molecules attached with its head. To make matters even better, soap molecules disrupt the surface tension of water by crowding around the water surface with their water loving heads which disrupts the waters ability to stay tightly coupled together in a sphere like a rain drop. So in a way it flattens waters ability to remain in this sphere configuration and makes the water molecules flatter thus wetting everything even the smallest crevices.