Soap, Detergents and Polymers: Cleansing Agents, Hydrophilic and Lipophilic Parts (For CBSE, ICSE, IAS, NET, NRA 2022)

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Cleansing Agents

Soaps and detergents are widely used as cleaning agents. Chemically soaps and detergents are quite different from each other. The common feature of soaps and detergents is that when dissolved in water the molecules of soap and detergent tend to concentrate at the surface of the solution or at interface. Therefore, the surface tension of the solution is reduced, it causes foaming of the solution.

Hydrophilic and Lipophilic Parts

  • Both soap and detergent molecules have two parts. One part of the molecule is polar (ionic) in nature. Polar nature is due to the presence of groups like carboxylate or sulphate . The polar group is a hydrophilic group. The hydrophilic group makes soaps and detergents soluble in water.
  • The other part of the soap or detergent molecule is nonpolar (non-ionic) that is lipophilic. The lipophilic part (a long chain alkyl or a long chain substituted aryl group) makes the molecule oil soluble.

Soaps

Soaps are the sodium or potassium salts of long chain fatty acids. These fatty acids are present in oils and fats in the form of glycerides. The glycerides present in oils and fats are the esters of glycerol and long chain carboxylic acids for example palmitic acid and stearic acid.

Saponification

Saponification is the process of making soap. Saponification is done by hydrolysis of oils or fats (of vegetable or animal origin) with the help of alkali like sodium hydroxide or potassium hydroxide .

Saponification is the Process of Making Soap

Synthetic Detergents

Synthetic detergents are used as cleaning agents much like soaps. Detergent molecules are similar to that of soap molecules, that is they have an oil-soluble (lipophilic) long chain of carbon atoms and a polar (hydrophilic) water- soluble part.

Synthetic Detergents

For example, sodium lauryl sulphate has a 12-carbon atom hydrocarbon like alkyl chain. The long carbon chain is oil-soluble (lipophilic) part and the sulphate is polar (hydrophilic) part that makes the molecule water-soluble.

Synthetic Detergents

Cleansing Action of Soap and Detergents

The long chain of hydrocarbon, which is hydrophobic is nature (insoluble in water) directed towards the center while the head (hydrophilic water-soluble part) is on the surface in control of water.

Cleansing Action of Soap and Detergents
  • The initial concentration at which micellization begins is called critical micellization concentration . The formation of micelles starts above a definite temp, that is called kraft temperature .
  • The cleansing action of soap is due to micelles. The micelles are absorbed by grease/dirt in cloth. Because both are non-polar. The polar head is directed towards water the rubbing by hands or mechanical stirring break the grease particles into smaller droplets and form emulsion with water. As a result, the cloth gets free from dirt and grease. The droplets and washed away with water.

Advantages and Disadvantages of Synthetic Detergents

  • The synthetic detergents are better than soaps in certain respects. Synthetic detergents can be used for washing of clothes even if the water is hard. Calcium and magnesium ions present in hard water make corresponding salts with detergent molecules.
  • However, detergent containing branched alkyl benzene sulphate is not completely biodegradable. Soap is completely biodegradable. Therefore, excessive use of synthetic detergents is a cause of worry.

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