How Soap Cleans

Step 1
Decreases the water's surface tension, making it a better wetting agent

Soap first lowers suface tension so that the water carrying the micelles can get to the dirt. When the soap micelles reach the embedded dirt, the soap molecules that form these micelles once again find themselves at a surface.



Step 2
Converts greasy and oily dirt into micelles that become dispersed in the soapy water

Now, as the hydrophilic heads of the soap molecules remain surrounded by water molecules, the soap micelles break up and the hydrophobic hydrocarbon tails, which had remained in the interior of the spherical micelles, become embedded in the grease. With this greasy dirt providing as compatible a chemical environment for the hydrophobic tails as the water provides for the hydrophilic heads, the tails are just as much at home in the grease as the heads are in the water.

Step 3
Keeps the grease micelles in suspension, thereby preventing them from coalescing back to large globules of grease that could be redeposited on a clean surface.

Agitation now breaks the grease into micelles whose surfaces are covered by the negatively charged carboxylate groups, the hydrophilic -CO2- groups of the embedded soap molecules. The grease droplets repel each other and remain suspended in the wash water instead of coalescing and redepositing on the material being cleaned. In the end, the suspended droplets go down the drain with the wash water. (While all this is going on the sodium ions move about freely and independently in the wash water.)