soaps and detergents

Soaps are formed by the alkaline hydrolysis (breaking up) of fats and oils by sodium or potassium hydroxide by boiling under reflux conditions:

¢Hydrolysis of esters such as fats/oil produces glycerol and fatty acids. Fats and oils are triglycerides meaning they are esters which contain 3 molecules of fatty acid condensed to 1 molecule of the trihydric alcohol, glycerol. So during hydrolysis, three molecules of soap are made per molecule of glycerol. (3:1 ratio of fatty acid:glycerol)

¢The hydrolysis is carried out using alkalis (NaOH or KOH) as catalyst and the fatty acids formed are changed into sodium or potassium salts (soaps)

The soaps are ionic and water-soluble

A soap is a sodium or potassium salt of some long chain carboxylic acids (fatty acid). Sodium salts of fatty acids are known as hard soaps and potassium salts of fatty acid are known as soft soaps.

Some example so soaps are sodium stearate, C₁₇H₃₅COO^¾ Na+, sodium palmitate, C₁₅H₃₁COO ^-¾ Na+ and sodium oleate, C₁₇H₃₃COO^¾ Na+

A molecule of soap is made up of two parts:

(i) An ionic part which is hydrophilic (water loving) and

(ii) A hydrocarbon chain which is hydrophobic (water hating).

When soap is at the surface of water, the hydrophobic tail of soap will not be soluble in water and the soap will align along the surface of water with the ionic end in water and the hydrocarbon ‘tail’ protruding out of water.

Inside water, the soap molecules form clusters to keep the hydrocarbon part out of water. In a cluster, the hydrophobic tails are in the interior of the cluster and the ionic ends are on the surface of the cluster. This formation is called a micelle. 

Soap is prepared by heating oil or fat of vegetable or animal origin with concentrated sodium hydroxide solution (caustic soda solution). Hydrolysis of fat takes place and a mixture of sodium salt of fatty acids and glycerol is formed. Since the salt of fatty acids thus formed are used as soap so alkaline hydrolysis of oils and fats is commonly known assaponification

Nature of soap: soap is basic in nature on account of the presence of some amount of free sodium hydroxide. it changes red litmus into blue colour.

The mechanism of the cleansing action of soaps : When soap is at the surface of water, the hydrophobic tail protrudes out of water while the ionic end remains inside water.

Inside water, the molecules form clusters with the hydrophobic tails in the interior of the cluster and the ionic ends on the surface of the cluster. This formation is called a micelle.

Soap, in the form of micelle collects the oily dirt in the centre of the micelle. The micelles stay in solution as a colloid and do not precipitate due to ion-ion repulsion. Thus, the dirt suspended in water is washed away during rinsing.

Explain the formation of scum when hard water is treated with soap. (V. Imp.)

Ans. A sample of water which contains Ca2+ or Mg2+ ions dissolved in it, is called hard water. Soap is a sodium salt of long chain carboxylic acid and is represented by RNa+ where ‘R’ stands for the long chain of fatty acids.

When Ca2+ or Mg2+ ions come in contact with soap molecules they form compounds R2Mg2+ or R2Ca2+ which are insoluble in water and appear in the form of scum.

What are detergents chemically? Why are they more effective than soaps in cleansing action? How can detergent molecules be altered to make them biodegradable? 

Ans. Detergents are sodium salt of long chain benzene sulphonic acid or sodium salt of long chain alkyl hydrogen sulphate.

Synthetic detergents are more effective than soaps because their calcium and magnesium salts are soluble in water and do not form scum.

If a straight chain hydrocarbon is used in the detergent instead of a branched hydrocarbon, then the detergent becomes biodegradable