Question

Action of soap is due to emulsification and micelle formation. Comment.

Answer 1

Soap is a sodium or potassium salt of a higher fatty acid and may be represented as RCOO^–Na⁺ (e.g., sodium stearate CH₃(CH₂ )₁₆COO Na⁺ which is a major component of many bar soaps). When dissolved in water, it dissociates into RCOO^– and Na⁺ ions. The RCOO^– ions, however, consist of two parts – a long hydrocarbon chain R (also called the non-polar ‘tail’) which is hydrophobic (water repelling), and a polar group COO^– (also called the polar-ionic ‘head’), which is hydrophilic (water loving).

The RCOO^– ions are, therefore, present on the surface with their COO^– groups in water and the hydrocarbon chains R stay away from it and remain at the surface. But at critical micelle concentration, the anions are pulled into the bulk of the solution and aggregate to form a spherical shape with their hydrocarbon chains pointing towards the center of the sphere with COO^– part remaining outward on the surface of the sphere. An aggregate thus formed is known as an ‘ionic micelle’.

The cleansing action of soap is due to the fact that soap molecules form micelles around the oil droplet in such a way that the hydrophobic part of the stearate ions is in the oil droplet and the hydrophilic part projects out of the grease droplet like the bristles. Since the polar groups can interact with water, the oil drop that is surrounded by stearate ions is now pulled into the water and removed from the dirty surface. Thus, soap helps in emulsification and washing away of oils and fats. The negatively charged sheath around the globules prevents them from coming together and forming aggregates.

(a) Grease on cloth

(b) Stearate ions (from soap) arranging around the grease droplets

(c) Micelle formed