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Surface Chemistry question

2008 · Shift 2 · Q9
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Surface Chemistry question

2008 · Shift 2 · Q9

JEE AdvancedChemistrySurface ChemistryMCQ+3 / −1
Among the following, the surfactant that will form micelles in aqueous solution at the lowest molar concentration at ambient condition is:
  1. A
    CH 3{}_33​(CH 2{}_22​) 15{}_{15}15​ N +{}^++(CH 3{}_33​) 3{}_33​ Br −{}^-−
  2. B
    CH 3{}_33​(CH 2{}_22​) 11{}_{11}11​ OSO 3−{}_3^-3−​ Na +{}^++
  3. C
    CH 3{}_33​(CH 2{}_22​) 6{}_66​ COO −{}^-− Na +{}^++
  4. D
    CH 3{}_33​(CH 2{}_22​) 11{}_{11}11​ N +{}^++(CH 3{}_33​) 3{}_33​ Br −{}^-−
View written solutionFree

Correct answer: A

Step-by-step Solution:

  1. Understanding Micelle Formation and Critical Micelle Concentration (CMC): Surfactants are amphiphilic molecules, meaning they have a hydrophobic (water-repelling) tail and a hydrophilic (water-attracting) head. In an aqueous solution, above a certain concentration called the Critical Micelle Concentration (CMC), these molecules aggregate to form micelles. In a micelle, the hydrophobic tails are shielded from the water in the core, and the hydrophilic heads are exposed to the water on the surface.

  2. Identifying the Goal: The question asks which surfactant will form micelles at the lowest molar concentration. This is equivalent to asking which surfactant has the lowest Critical Micelle Concentration (CMC).

  3. Factors Affecting CMC: The primary factor that determines the CMC for a homologous series of surfactants is the length of the hydrophobic part (the alkyl chain). The relationship is as follows:

    • Longer hydrophobic chain: A longer alkyl chain is more hydrophobic. This means it has a stronger tendency to avoid contact with water. The system can more effectively lower its free energy by forming micelles even at a lower concentration. Therefore, as the length of the hydrophobic chain increases, the CMC decreases.

  4. Analyzing the Given Surfactants: Let's examine the structure of each surfactant and determine the length of its hydrophobic alkyl chain.

    • A: CH₃(CH₂)₁₅N⁺(CH₃)₃Br⁻

      • Hydrophobic tail: CH₃(CH₂)₁₅- . The total number of carbon atoms in this chain is 1 + 15 = 16.

    • B: CH₃(CH₂)₁₁OSO₃⁻Na⁺

      • Hydrophobic tail: CH₃(CH₂)₁₁- . The total number of carbon atoms in this chain is 1 + 11 = 12.

    • C: CH₃(CH₂)₆COO⁻Na⁺

      • Hydrophobic tail: CH₃(CH₂)₆- . The total number of carbon atoms in this chain is 1 + 6 = 7.

    • D: CH₃(CH₂)₁₁N⁺(CH₃)₃Br⁻

      • Hydrophobic tail: CH₃(CH₂)₁₁- . The total number of carbon atoms in this chain is 1 + 11 = 12.

  5. Comparing the Chain Lengths: We compare the number of carbon atoms in the hydrophobic tails of the four options:

    • A: 16 carbons
    • B: 12 carbons
    • C: 7 carbons
    • D: 12 carbons

The surfactant in option A has the longest hydrophobic chain (16 carbons).

  1. Conclusion: Since a longer hydrophobic chain leads to a lower CMC, the surfactant with the longest chain will form micelles at the lowest concentration. The surfactant in option A, with its 16-carbon chain, is the most hydrophobic and will therefore have the lowest CMC among the given choices.

Therefore, CH₃(CH₂)₁₅N⁺(CH₃)₃Br⁻ will form micelles at the lowest molar concentration.

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