Soaps and detergents are used to remove all kinds of soil, including everything from grease from an auto mechanic to the gravy stain from last night's dinner. Grease and water don't mix, so the cleansing action of detergents and soaps requires special chemistry.

Basic Concept

The principle of "like-dissolves-like" suggests that soap or detergent molecules must include one portion that is grease-soluble and another that is water-soluble. Although the chemistry of soaps and detergents is somewhat different, the basic mechanism remains the same.

The Mechanism

The fat-soluble portion of soap or detergent combines with greasy dirt particles. The water-soluble part surrounds or encases each particle in a sphere. This sphere dissolves in the wash water and is disposed of.

Soap

Soap is made from long-chain carboxylic acids derived from animal fat. These are not water-soluble. Neutralizing these acid chains with sodium or potassium hydroxide creates a small water-soluble ionic portion at one end of each chain.

Detergent

There are three types of detergents: anionic, cationic and non-ionic. The fat-soluble portion is once again derived from fatty acids, only this time it may be modified.

Detergent Chemistries

The water-soluble portion of anionic detergents is of alkyl sulfate or benzenesulfonate composition. Cationic detergents replace the acidic portion with ammonia. These are especially useful in shampoo. Non-ionic detergents use water-soluble portions that are not ionic. They make excellent dishwashing liquids.

Source:

Elmhurst College: Detergents and Surfactants"

More Information:

California State University Stanislaus: Soaps, Detergents and Cleaning