Identify the correct molecular picture showing that happens at the critical micellar concentration (CMC) of an aqueous solution of a surfactant (polar head: shaded circle; non-polar tail: wavy line; water: black dot).
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At the CMC, surfactant molecules form micelles in the bulk (heads outward, tails inward) while the surface is covered by a monolayer with the tails sticking out of the water into the air.
At the critical micellar concentration the surface (air-water interface) is saturated with surfactant: the polar heads stay in water and the non-polar tails point out of the water into the air. In the bulk, further surfactant aggregates into micelles with the non-polar tails pointing inward (hydrophobic core) and the polar heads facing the water. Picture (A) has only a surface layer and no micelles (below CMC). Pictures (B) and (C) show surface molecules with the tails dipping into the water, which is wrong. Picture (D) shows tails above the surface and micelles with tails inside and heads outside, so (D) is correct. Option (3) corresponds to (D). Answer: (3) (D)
