Identify the correct molecular picture showing that happens at the critical micellar concentration (CMC) of an aqueous solution of a surfactant ( ◍ polar head; ∼ non-polar tail; ● water).

Solution

Chemistry ma’am · 3 steps

At CMC, look for two things: tails up in the air at the surface, and a micelle with tails hidden inside.

1. Surface first. The non-polar tail avoids water and the polar head likes it. So at the water surface, heads stay in the water and tails point up into the air. As more surfactant is added, this surface layer fills up.
2. Then micelles. Once the surface is full, the critical micellar concentration is reached. Extra molecules in the bulk cluster into a ball, called a micelle, with tails packed in the centre and heads facing the water.

   Picture	Surface layer	Micelle in bulk	Matches CMC?
   (A)	heads in water, tails in air	none	no, only the surface stage
   (B)	heads in air, tails in water (wrong)	none, only loose molecules	no
   (C)	heads in air, tails in water (wrong)	yes, tails inside	no
   (D)	heads in water, tails in air	yes, tails inside	yes

3. So. Only picture (D) shows a correctly oriented surface layer together with a micelle in the bulk. That is option (c).

The idea to keep. A surfactant always keeps its tail away from water. At the surface the tail sticks into the air, and in the bulk, from the CMC onwards, the tails hide together inside a micelle.

Answer from the NTA final answer key. Solution by PrepWiser.