Soaps:
- Soaps are sodium or potassium salts of higher fatty acids (are organic acids that have carbon atoms within 9-17 in their molecules) like stearic, palmitic and oleic acids.
- Soap is a metallic salt of saturated (CnH2n+1COOH) or unsaturated (CnH2n-1COOH) higher fatty acid.
- There may be Pb, Mg, Ca or other metallic salts.
- The sodium soaps are called hard soaps and The potassium soaps are known as soft soaps.
- Soaps are obtained from oils and fats.
- For example, tri-stearin is got from beef and mutton tallow, tri-palmitin from palm oil and tri-olein from lard (pig fat), olive oil and cotton seed oil.
Properties of soaps:
1. Soap dissolves very slowly in cold water but rapidly in hot water.
2. Soap dissolves in hot alcohol but is only carefully soluble in other organic solvents such as acetone, ether or petroleum.
3. Sometimes there are free fatty acid molecules in soap to form what is known as acid soaps.
4. Soap reacts with hard water to form insoluble soap.
5. The number of carbon atoms in the aliphatic chain of soap is very important.
The number of carbon atoms (C) < 9 : No detergency power
The number of carbon atoms within (C) 9-17 : Extremely good detergent
The number of carbon atoms (C) 17 : Poor solubility
Cleansing action of soap:
Soap has two dissimilar ends. At one end it has the hydrocarbon chain
that is non-polar and hydrophobic (soluble in oil/dirt). At the other
end there is the carboxylate ion that is polar and hydrophilic (water
soluble). When soap is added to water, its molecules make a unimolecular
film on the surface of water with their carboxyl groups dissolved in
water and the hydrocarbon chains standing on end to form a hydrocarbon
layer. When a dirty cloth is soaked into a soap solution, soap dissolves
fat or oil with dust by micelle formation.
The
fat or oil with dust is dissolved in water by hydrophobic hydrocarbon
chains of soap. The water soluble carboxylate ions make a hydrophilic
surface in water and render the micelles of oil or fat from the cloth to
the water. Thus, the micelle is dissolved in water and is washed away.
Soap tends to concentrate on the solution surface and therefore lowers
its surface tension, causing foaming. This helps it to penetrate the
fabric. It emulsifies fat and dirt to form micelles and make all the
micelles water-soluble. Thus, the water washes the dirt away.
The structure of a micelle
The
hydrophobic tails ‘burrow’ into the droplet of oil or grease. The
hydrophilic heads are left to face the surrounding water. This results
in the formation of a ball-like structure (a micelle).
Saponification:
In simple terms, saponification is the name for a chemical reaction between an acid and a base to form a salt.
The vegetable oil is a glyceride of fatty acid (Oleic, stearic, palmitic acid), which is immiscible with water.
When such oil is heated with a solution of sodium hydroxide in water, the oil is split into fatty acid and glycerine.
This fatty acid reacts with sodium hydroxide solution and form sodium salt like soap which is soluble in water.
This
reaction is called saponification. As a result of saponification, the
insoluble and water immiscible oil is converted to water soluble
products.
SOAP
Soaps are sodium or potassium salts of long chain fatty acids.
When triglycerides in fat/oil react with aqueous NaOH or KOH, they are converted into soap and glycerol.
This is called alkaline hydrolysis of esters. Since this reaction leads to the formation of soap, it is called the Saponification process.
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