Question

Explain the cleansing action of soaps.

Explain the mechanism of cleaning action of soap.

Answer 1

Soap molecules form micelles around an oil droplet (dirt) in such a way that the hydrophobic parts of the stearate ions attach themselves to the oil droplet and the hydrophilic parts project outside the oil droplet. Due to the polar nature of the hydrophilic parts, the stearate ions (along with the dirt) are pulled into water, thereby removing the dirt from the cloth.

Answer 2

Soaps contain two parts: a large hydrocarbon, which is hydrophobic (water repelling), and a negatively charged head, which is hydrophillic (water attracting). In solution water molecules, being polar in nature, surround the ions and not the organic part of the molecule. When a soap is dissolved in water, the molecules gather together as clusters, called micelles. The tails stick inward and the head tward. The hydrocarbon tail attaches itself to oily dirt. When water is agitated, the oily dirt tends to lift off from the dirty surface and dissociate into fragments. The solution now contains small globules of oil-surrounded detergent molecules. The negatively charged heads present in water prevent the small globules from coming together and forming aggregates. Thus the oily dirt is removed from the object.

Answer 3

The mechanism of cleansing action of soap:

1. A molecule of soap has two dissimilar ends. The hydrophobic end of the hydrocarbon chain is water repellent, while the hydrophilic end is polar and water soluble due to presence of carboxylate anion. Thus a soap molecule may be represented as:
   
2. When a soap is dissolved in water, many molecules come together and form a group called a micelle because their hydrocarbon chains come together and the polar ends are projected outward.
   
       Micelle formation
3. When a cloth with a spot of oil and dirt is soaked into the soap solution, soap dissolves a tiny oil droplet by the hydrophobic end in the middle of the micelle. Due to the outwardly projected polar ends, these micelles dissolve in water and are washed away. Thus the clothes get cleaned.
   
    Oil droplet in the middle of the micelle