What is the difference between the molecules of soaps and detergents, chemically? Explain the cleansing action of soaps.

Chemical Difference

Soaps are sodium or potassium salts of long-chain carboxylic acids (fatty acids). For example, Sodium Stearate.

Detergents are generally ammonium or sulphonate salts of long-chain carboxylic acids. Unlike soaps, detergents do not form insoluble precipitates (scum) with the calcium and magnesium ions present in hard water.

Cleansing Action of Soaps

Soap molecules have two ends with different properties:

1. Hydrophilic end: A polar/ionic head (e.g., $-COO^-Na^+$) that dissolves in water.
2. Hydrophobic end: A long hydrocarbon tail that dissolves in oil or dirt.

When soap is added to water containing oily dirt, the molecules arrange themselves into clusters called micelles. In a micelle, the hydrophobic tails attach themselves to the dirt/oil droplet at the center, while the hydrophilic heads face outwards into the water. This traps the dirt in the center of the micelle. When the water is agitated, the micelles (carrying the dirt) are pulled away from the surface and washed away.