Question

Sodium stearate $CH_3(CH_2)_{16}COO^- Na^+$ is an anionic surfactant which forms micelles in water. Choose the correct statement for it from the following :

• A. It forms spherical micelles with $CH_3(CH_2)_{16}-$ group pointing outwards on the surface of sphere.
• B. It forms non-spherical micelles with $-COO^-$ group pointing outwards on the surface.
• C. It forms spherical micelles with $CH_3(CH_2)_{16}-$ group pointing towards the centre of sphere.
• D. It forms non-spherical micelles with $CH_3(CH_2)_{16}-$ group pointing towards the centre.

Hint:

Remember: Hydrophobic = Water-fearing (goes inside); Hydrophilic = Water-loving (stays outside). Soap works by trapping grease in the hydrophobic center of these micelles.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Surfactants are molecules with a hydrophilic (polar) head and a hydrophobic (non-polar) tail.
In water, these molecules aggregate into micelles to minimize the energy of the system.
Step 2: Detailed Explanation:
In sodium stearate:
1. \(CH_3(CH_2)_{16}-\) is a long hydrocarbon chain (non-polar, hydrophobic tail).
2. \(-COO^-\) is the polar hydrophilic head.
When placed in water, the hydrophobic tails cluster together in the interior of the micelle to avoid water molecules, while the hydrophilic heads face outwards to interact with water.
This aggregation typically forms a spherical structure.
Therefore, the hydrocarbon groups point towards the center of the sphere.
Step 3: Final Answer:
Statement (C) is correct.