Soap -- a Detergent

A detergent is anything that cleans, especially if it removes oily or greasy dirt. One particular kind of detergent is soap.

Soaps are detergents in the sense that they help clean oily and greasy dirt from fabrics, metals, our skin and hair. We restrict the term soap to the sodium salts of long-chain carboxylic acids. A carboxylic acid is marked by the presence of a carboxyl group, -CO2H. With the anion of the carboxyl group balanced by a sodium cation and tied by a covalent bond to a long chain of -CH2- groups that terminate in a CH3- group, we have a soap molecule. We can generalize the molecular structure of a soap molecule as

CH3 - (CH2)n - CO2 - Na2+

CH3 - (CH2)n - resembles quite closely the long chains of the hydrocarbon molecules. Like the molecules of gasoline and mineral oil, this part of the soap molecule tends to dissolve readily in materials that are or that resemble hydrocarbons, but not in water. All these long chains of -CH2- groups of soaps and of hydrocarbons and hydrocarbon-like materials intermingle easily, but they don't mix readily with the H2O molecules of water. The other end of the molecule, though is ionic:



O
||
- C - O - Na+

Like sodium chloride and other ionic compounds, that ionic end tends to dissolve in water, but not in hydrocarbon solvents. As a result, one molecule has two opposite and contradictory tendencies. A hydrophilic structure is attracted towards water molecules but shun hydrocarbons and other oily and greasy substances. A hydrophobic structure shuns water but mixes easily with those very oily, greasy substances that repel the hydrophilic, or charged, part.