JEE Main 2020 · 3 September · evening · Single correct · +4 / −1
An ionic micelle is formed on the addition of :
- (a) excess water to liquid
- (b) excess water to liquid
- (c) liquid diethyl ether to aqueous NaCl solution
- (d) sodium stearate to pure toluene
Answer and solution
Answer (a) excess water to liquid
Solution
Chemistry ma’am · 3 steps
Hunt for an ion with a long greasy tail and a charged head, and make sure it is put into water.
- What a micelle needs. An ionic micelle forms when an ionic surfactant goes into water above its critical micelle concentration. The surfactant ion must have a polar ionic head and a long hydrocarbon tail. In water the tails cluster inside, away from water, and the charged heads face outwards.
Option Long hydrocarbon tail? Ionic head? Solvent is water? (a) 1,3-dimethylimidazolium octyl sulfate Yes, C₈H₁₇ chain on the anion Yes, -OSO₃⁻ Yes, excess water (b) 1,3-dimethylimidazolium PF₆⁻ No, only two methyl groups Yes Yes (c) diethyl ether + aqueous NaCl No surfactant at all No Yes (d) sodium stearate + toluene Yes, C₁₇H₃₅ chain Yes, -COO⁻ No, toluene is non-polar - Why the others fail. In (b) both ions are small, so they dissolve as free ions and do not aggregate. In (c) nothing is amphiphilic. In (d) the surfactant is right but the solvent is wrong: in toluene the charged heads cannot face outwards, so no normal ionic micelle forms.
- So. Only (a) has an amphiphilic ion, octyl sulfate, placed in water. Its tails pack together and its sulfate heads face the water, giving an ionic micelle.
The idea to keep. A micelle needs two things together: a surfactant ion with a long non-polar tail and an ionic head, and water as the solvent. Missing either one means no ionic micelle.
Answer from the NTA final answer key. Solution by PrepWiser.