Soaps and detergents

04 Carbon and its compound

A soap is the salt of a strong base (sodium hydroxide) and a weak acid (carboxylic acid), so a solution of soap in water is basic in nature.
Being basic, a soap solution turns red litmus paper to blue.

Heating of animal fat or vegetable oil with concentrated sodium hydroxide solution (caustic soda solution) forms soap.
Reaction of fats or oils with sodium hydroxide forms soap and glycerol:

A soap molecule makes up of two parts:
- A long hydrocarbon part
- A short ionic part containing – COO^–Na⁺ group.
The soap molecule is said to have a tadpole structure.

The long hydrocarbon chain is hydrophobic (water-repelling), so the hydrocarbon part of soap molecule is insoluble in water but soluble in oil and grease.
The ionic portion of the soap molecule is hydrophilic (water-attracting) due to the polar nature of water molecules. So, the ionic portion of soap molecule is soluble in water but insoluble in oil grease.
If on the surface of water, soap is present then the hydrophobic tail which is not soluble in water will align along the water surface.

Dirt is mostly oily in nature and hence it does not dissolve in water.
Sodium or potassium salts of long chain carboxylic acids make up the molecule of soap so that the carbon chain dissolves in oil and the ionic end dissolves in water.
Thus the soap molecules form structures called micelles .One end of micelles is towards the oil droplet and the other end which are ionic faces outside.

So, it forms emulsion in water and helps in dissolving the dirt.
In a soap micelle, the soap molecules arrange radially with hydrocarbon ends directed towards the centre and ionic ends directed outwards.

On putting a dirty cloth is put in water containing dissolved soap, attaches the hydrocarbon ends of the soap molecules in the micelle to the oil or grease particles present on the surface of the dirty cloth.
In this way the soap micelle entraps the oily or greasy particle by using its hydrocarbon end.
The ionic ends of the soap molecules in the micelles, however, remain attached to water as colloidal solutions.
The oily and greasy particles present on its surface, entrapped by soap micelles get cleaned thoroughly by rinsing in clean water a number of times.
The soap solution appears cloudy as it forms a colloidal solution which scatters light.

Detergents are also called ‘soap-less soaps’.
This is because though they act like soap in having the cleansing properties, they do not contain the usual ‘soaps’ like sodium stearate, etc.
A detergent is the sodium salt of a long chain of benzene sulphonic acid (or the sodium salt of a long chain alkyl hydrogen sulphate) which has cleansing properties in water.

DIFFERENCES BETWEEN SOAPS AND DETERGENTS

Soaps	Detergents
Soaps are the sodium salts (or potassium salts) of the long chain of carboxylic acids (fatty acids). The ionic group in soaps is – COO-Na+	Detergents are the sodium salts of long chain benzene sulphonic acids or long chain alkyl hydrogen sulphates. The ionic group in a detergent is –SO3-Na+
Soaps are not suitable for washing purposes when the water is hard.	Detergents can be used for washing even when the water is hard.
Soaps are biodegradable.	Some of the detergents are not biodegradable.
Soaps have relatively weak cleansing action.	Detergents have a strong cleansing action.

Detergents have a number of advantages over soaps due to which they are replacing soaps as washing agents.
Detergents are better than soaps because of the following reasons:
- Detergents can be used even with hard water whereas soaps are not suitable for use with hard water.
- Detergents have a stronger cleansing action than soaps.
- Detergents are more soluble in water than soaps.
A notable disadvantage of detergents over soaps is that some of the detergents are not biodegradable.