Animated Solution for Chemistry - Surface Chemistry: Sodium stearate ${\text{CH}}_3({\text{CH}}_2{)}_{16}{\text{COO}}^{-}{\text{Na}}^{+}$ is an anionic surfactant which forms micelles in oil. Choose the correct statement for it from the following. [2021, 22 July Shift-I]

A Tale of Two Solvents

Have you ever wondered how soap, which is essentially a salt of a fatty acid, manages to wash away stubborn oil and grease? The secret lies in a fascinating microscopic structure called a micelle. Let's dive into the world of surface chemistry and explore how molecules like sodium stearate behave, and more importantly, how their behavior completely flips depending on their environment.

Sodium stearate, ${\text{CH}}_3({\text{CH}}_2{)}_{16}{\text{COO}}^{-}{\text{Na}}^{+}$, is a classic example of a surfactant. It has a split personality. One part of the molecule is a long hydrocarbon chain, ${\text{CH}}_3({\text{CH}}_2{)}_{16}$, which is hydrophobic (water-hating) but lipophilic (oil-loving). The other part is the carboxylate ion, ${\text{COO}}^{-}$, which is hydrophilic (water-loving) but lipophobic (oil-hating).

Read the Solvent Carefully!

In a standard textbook scenario, we dissolve soap in water. In water, the hydrophobic tails hide in the center to avoid the water, while the hydrophilic heads point outwards. This forms a standard micelle.

However, this question contains a brilliant trap! It explicitly states that the micelles are formed in oil. Oil is a non-polar solvent. This completely changes the game.

When sodium stearate is placed in oil, the molecule's preferences are flipped relative to the solvent. The long non-polar hydrocarbon tails, ${\text{CH}}_3({\text{CH}}_2{)}_{16}$, are now in their favorite environment. They will happily point outwards to interact with the surrounding oil.

Conversely, the polar carboxylate heads, ${\text{COO}}^{-}$, despise the non-polar oil. To minimize their energy and avoid the oil, they huddle together, pointing inwards to form a polar core.

This inside-out spherical structure is known as a reverse micelle.

Therefore, in oil, sodium stearate forms spherical micelles with the ${\text{CH}}_3({\text{CH}}_2{)}_{16}$ group pointing outwards on the surface of the sphere. This makes option (c) the correct statement.