Soaps vs Detergents

What is a Soap? What is a Detergent?

There is a certain amount of debate as to whether soaps are synthetic or natural products. Many people claim that soaps are a natural product as they are made from naturally occurring fats and oils such as palm oil and olive oil. Soaps such as sodium oleate are only a couple of steps away from the original natural products. As chemists however, we consider that the step of producing the soap molecule from these oils is still a synthesis step; soaps are thus synthetic products as well.

All of the soaps (sodium oleate etc) are fatty acid salts (later on, we will recognise them to be a type of anionic surfactant). They are characterised by:

• a long hydrocarbon chain, which may be monounsaturated (i.e. has one double bond, like sodium oleate), polyunsaturated (i.e. more than one double bond) or saturated (i.e. no double bonds)
• a carboxylate group at the end

Any surfactant that is not a soap is a detergent.

Properties of Soaps and Detergents

As we have already seen, the principal difference between a soap and a detergent is the behaviour in hard water. Soaps tend to complex with the metal ions in hard water forming a scum, while detergents do not.

Another difference between soaps and detergents is the sensitivity of soaps to acidic solutions. If you put a soap into an acidic solution (pH < 4.5), the carboxylate group will be protonated:

The protonation of oleate ion to form oleic acid can occur at pH below 4.5. This forms an uncharged, insoluble molecule, which is not surface-active.

The protonated soap molecule does not have a charged head, so it is no longer soluble in water. The soap molecules precipitate out forming a cloudy mixture, which (like hard water) leads to the formation of a scum. Soaps are not suitable for use in acidic conditions.

Other common soaps: sodium palmitate, sodium myristate and sodium stearate.