Soaps and Detergents
Chemistry (Year 12) - Polymers, Proteins and Soaps
What are Soaps?
Soaps are molecules made through a process called saponification. This process is a type of ester hydrolysis. In an ester hydrolysis reaction, the ester bond is broken. This bond can be broken by water or a base (hydroxide).
Tip: Think of the water or the base as molecular scissors. When they interact with an ester they will always cut the O-R(2) bond.
To understand the saponification reaction, we need to become familiar with the molecule triglyceride. For chemists triglycerides are trifunctional molecules that contain 3 esters groups, as shown below:
Triglycerides are formed through the esterification reaction between glycerol (a naturally occurring alcohol that contains 3 hydroxyl groups) and different fatty acids.
The Soaponification Reaction
Triglyceride reacts with a base in the saponification reaction, typically it is the strong base sodium hydroxide (NaOH). NaOH will act as the molecular scissors in this reaction:
This results in the formation of glycerol and a soap (formally referred to as sodium stearate):
The reaction summary is as follows:
The Structure of Soap
Soap contains a polar head and a non-polar tail. The tail is made up of long hydrocarbon chains, while the head is made of a carboxylate group.
The structure can be simplified to the following schematic:
Detergents consist of a very similar structure. They also have a long non-polar hydrocarbon tail and an ionic head; however, they have a benzene group attached to the non-polar tail and a sulphonate group as the polar head:
When soaps and detergents are added to water a spherical structure, termed a micelle is formed, shown below:
Water is polar. As such, the polar heads of the soap molecule will interact with water, while the non-polar tails try to get as far away from the water as possible.
The non-polar tails that are hydrophobic (water hating) point towards each other inwards, forming the centre of a micelle, while the hydrophilic (water loving) heads point outwards towards the water.
The Cleaning Action of Soaps and Detergents
Grease, oils and fats are all non-polar and therefore will interact with the non-polar tails of the soap/detergent molecules.
The micelle structures position themselves around the grease particles. Again, the polar heads position themselves to face the water and the non-polar tails dissolve themselves in the grease.
It is good to agitate stains to break them down into smaller particles, this increases the particle’s surface area and greatly improves cleaning actoon. Through vigorous agitation the grease held within the micelle can be washed away.
Hard Water and the Cleaning Process
Hard water is water that contains a high concentration of metal cations. When exposed to hard water, soaps will form precipitate with these cations, that is called ‘scum’. This precipitayion reduces the effectiveness of soaps.
An advantage of detergents is that they do not form this scum precipitate. Yay!
This Table provides a summary of detergents and soaps: