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Which of these substances forms cationic micelles above a certain concentration?

1. Sodium dodecyl sulphate
2. Sodium acetate
3. Urea
4. Cetyltrimethylammonium bromide.

Correct answer: Cetyltrimethylammonium bromide.

Solution

Cetyltrimethylammonium bromide is a cationic surfactant; its quaternary ammonium head is positively charged, so above the CMC it forms cationic micelles. Sodium dodecyl sulphate is anionic, while sodium acetate and urea are not micelle-forming surfactants.

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