The science of how things work has always fascinated me. So, How Does Soap Work?
Soap, water, and oil are all made up of molecules. Some molecules are hydrophilic, (hydro=water and philic=loving) these molecules are attracted to water. Some molecules are hydrophobic, (hydro=water and phobic=fearing), they are repelled by water.
Molecules that readily mix with water are hydrophilic.
Molecules that readily mix with oil are hydrophobic.
Since we all know that water and oil do not mix, then we also know that hydrophilic and hydrophobic compounds do not mix.
Two other words we need to know are polar and nonpolar. Nonpolar compounds, like oil and grease, cannot dissolve in water. Polar compounds can dissolve in water.
Most of what we call dirt is grease or oil which will not come off with just water. This is because oil and grease are non-polar, which means they will not dissolve in the water.
Soap can mix with both water and with oil. Why? The soap molecule has two different ends, one that is hydrophilic (polar head) that binds with water and the other that is hydrophobic (non-polar hydrocarbon tail) that binds with grease and oil.
When greasy dirt or oil is mixed with soapy water, the soap molecules arrange themselves into tiny clusters called micelles.
The water-loving (hydrophilic) part of the soap molecules sticks to the water and points outwards, forming the outer surface of the micelle.
The oil-loving (hydrophobic) parts stick to the oil and trap oil in the center where it can't come into contact with the water.
With the oil tucked safely in the center, the micelle is soluble in water. As the soapy water is rinsed away the greasy dirt goes along with it. Micelle Picture from Wikipedia
So basically, the soap is attracted to the fat/oil/grease because of its fat-loving side but then tears up the grease by pulling it into the water using its water-loving side. Sounds kind of like a football play--surround the oil particles and move them away from one another.
Ever wonder why it is easier to clean dirty, greasy hands (and other things) in hot or warm water rather than cold water? It is because the fats and oils soften or melt in hot water, which allows them to attach more readily to the hydrophobic end of the soap molecule. In turn, that makes it easier to rinse away.
Soap is a natural surfactant. A surfactant is any substance that tends to reduce the surface tension of a liquid in which it is dissolved.
Almost all cleansing products are based on surfactants. Surfactants not only reduce the surface tension of the water but the way they are constructed (with one hydrophilic end and one hydrophobic end) makes them compatible with both water and oils. This property is what makes them good for cleansing. When surfactants lower the surface tension of water, they basically make the water molecules more slippery, so they are less likely to stick to themselves and more likely to interact with oil and grease.
Natural soap needs no synthetic additives to create lather or to clean because natural soap is a natural surfactant. So it not only makes great bubbles and lather, but it also helps clean oily dirt from your skin--naturally!
You can think of soap as the middle-man that helps bring oil and water together so that the dirt and grease on your skin can be easily rinsed away.