Soaps and detergents

What is soap?

Soap is a sodium or potassium salt of a long chain carboxylic acid. For example, sodium stearate, $\text{C}_{17}\text{H}_{35}\text{COONa}$.

It is made by saponification: a fat or an oil, which is an ester, is heated with sodium hydroxide. The products are soap and glycerol.

The shape of a soap molecule

A soap molecule has two ends.

• The head, the $-\text{COO}^-\ \text{Na}^+$ part, is ionic. It loves water.
• The tail, the long carbon chain, is a hydrocarbon. It hates water, and it loves oil and grease.

How soap cleans

A cloth is dirty with grease, which does not dissolve in water. In soap solution:

1. The tails of the soap molecules go into the grease.
2. The heads stay in the water, on the outside.
3. Many molecules form a ball, a micelle, with the grease trapped in the middle.
4. The micelles stay apart because each has the same charge on its outside, so they do not stick together. Scrubbing or rinsing carries them away with the water.

Hard water

Hard water has calcium and magnesium salts. Soap reacts with these ions and makes an insoluble scum.

Soap is wasted, and there is no lather until all the calcium and magnesium have been used.

You should find that doubling the calcium in the water doubles the soap lost as scum, before any lather forms.

Detergents

Detergents are the sodium salts of sulphonic acids, or ammonium salts with a long chain. They work like soap, with a long hydrocarbon tail and an ionic head. But they do not form scum with hard water, because their calcium salts are soluble. That is why they are used in washing machines and in shampoos.

Biodegradable. Detergents with branched chains are not broken down by bacteria, and they pollute rivers with foam. Detergents with straight chains are broken down, and are biodegradable.

Practise with new numbers

Try it in Python

This program finds the masses in the making of soap from the fat tristearin.

Remember these

• Soap is the sodium or potassium salt of a long chain carboxylic acid, made by saponification.
• The tail goes into the grease and the head stays in the water. This makes a micelle.
• In hard water, soap makes scum. A detergent does not.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Which end of a soap molecule faces the water in a micelle?

2. Why do micelles not stick together into one big lump?

3. Why are detergents with straight chains preferred to those with branched chains?

4. What is saponification?