Soaps

Soaps have been mentioned several times as examples of carboxylate salts. They are such an important subgroup that we will spend some time dealing specifically with them.

Soaps are the salts (usually the sodium or potassium salts) of fatty acids. As such, they have an ionic portion which is soluble in water and a nonpolar section which is soluble in such things as grease and grime.

Structure of a soap. [64str07.JPG]

Often the long carbon chains are represented by a zigzag line in which each corner represents a carbon atom with two hydrogen atoms attached and the end represents a carbon atom with three hydrogen atoms attached.

Alternate structure of a soap showing zigzag line for carbon chain. [64str09.JPG]

This picture tries to give you an idea of how a soap or a detergent operates in terms of cleaning away grease and grime. You have to remember that these soaps contain ions of very long-chained carboxylic acids. The ionic end is very soluble in water; the alkyl end is very soluble in nonpolar organic compounds such as grease.

Diagram showing soap molecules surrounding oil droplet. [64str10.JPG]

As you are aware, grease is not very soluble in water, so the fatty acid ion is essentially used to stick the grease and the water to one another. The way this happens is that the nonpolar end of the fatty acid ion dissolves in the grease, and the ionic end dissolves in the water. By doing this, the soap is able to pull the grease along with it when the water is washed away.

Detergents

Detergents, by the way, are similar to soaps, but they are the salts of inorganic acids other than carboxylic acids, such as the sulfonate shown here.

Structures of soap and detergent. [64str08.JPG]

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