The Chemistry behind Soap!

Soap occurs in KS3 and GCSE Chemistry courses during the study of alkalis, water quality, carboxylic acids (including fats and oils and emulsifiers) and investigations of industrial chemistry. It would form a good topic for cross-curricular project work.

Soap is one of those “superstuffs” that we use every day and take for granted.  Soap is used for washing ourselves, cleaning clothes, pots and pans, floors – just about everything in fact. Yet most of the products we use today actually don’t contain soap at all. 

All these different cleaning materials are detergents that act as emulsifiers. They consist of long molecules with a charged group at one end and a long hydrocarbon chain at the other. The charged end is hydrophilic and dissolves in water. The hydrocarbon chain is hydrophobic. It does not dissolve in water but does mix with fats and greases. Mixing a detergent with fats and water breaks up the fat into globules surrounded by the detergent molecules with the hydrophilic end sticking into the water so preventing the globules joining up.
Soap is a good detergent.  It is the sodium or potassium salt of long chain fatty acids e.g.
CH3CH2.......CH2COO- Na+. The length and structure of the chain depends on the fat or oil that the soap is derived from. No-one knows when it was discovered that mixing a fat or oil with ashes from a fire produced a soap but the process has been in use for thousands of years. Goat fat and wood ash was a favourite early recipe. Salt was often added to the mixture to separate out the soap.

The value of a good soap was well recognised. For instance, King Charles I of England handed out licences to soap boilers in London and placed a tax on the manufacture of soap at £4 per ton. The tax made soap expensive for working class people and for the woollen industry where soap was used to wash fleeces.  The soap tax was not removed until 1853.

A Frenchman, Michel-Eugene Chevreul worked out what was happening in the soap-making process in 1823. Alkali (from the wood ash) splits up the fat molecules into the fatty acids and glycerol.  Once soap makers understood the process they could improve the quality of their soap and separate off the glycerol as a useful by-product. An important change was to use vegetable oils such as palm or olive oil instead of animal fats.  In the 1880s William Hesketh Lever established a factory at Port Sunlight on the Mersey estuary and made a fortune producing his superior quality soaps. His company, now known as Unilever, still makes a variety of well-known cleaning materials.

While soap is an excellent and versatile detergent it forms a dirty scum in hard water. Hard water contains calcium or magnesium ions that form an insoluble salt with the fatty acids of soap. The scum wastes soap and remains on the clothes or pots being washed. 

Today we use detergents that are designed not to form precipitates in hard water. A common detergent is sodium lauryl sulfate. It is made by reacting lauryl alcohol with sulfur trioxide and then forming the sodium salt of the acid. Lauryl alcohol is made by reducing the fatty acids obtained from coconut and palm oil. Sodium lauryl sulfate has a sulfate group instead of the carboxylic acid group at the end of the hydrocarbon chain. The sulfate group forms soluble salts with calcium and magnesium ions so that no scum is formed in hard water.  Sodium laureth sulfate is a similar substance with a slightly more complex hydrophilic group.