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Which of the following forms cationic micelles above certain concentration :-
A
sodium acetate
B
Urea
C
Cetyl trimethylammonium chloride
D
Sodium dodecyl sulphonate
Detailed Solution
Micelles are formed by surfactant molecules having a long hydrophobic hydrocarbon tail and a polar (ionic) head. Above the critical micelle concentration (CMC) and Kraft temperature, these molecules aggregate into micelles.
If the surface-active part of the surfactant is a cation, the micelle carries a positive charge, i.e. it is a cationic micelle.
Cetyl trimethylammonium chloride: $CH_3(CH_2)_{15}N^+(CH_3)_3\,Cl^-$. The long chain cation $C_{16}H_{33}N^+(CH_3)_3$ is the surface-active part, so it forms cationic micelles.
Sodium dodecyl sulphonate, $C_{12}H_{25}SO_3^-Na^+$, has a surface-active anion, so it forms anionic micelles.
Sodium acetate (very short chain) and urea (non-ionic, no long chain) do not form micelles.
So the compound that forms cationic micelles is cetyl trimethylammonium chloride.
If the surface-active part of the surfactant is a cation, the micelle carries a positive charge, i.e. it is a cationic micelle.
Cetyl trimethylammonium chloride: $CH_3(CH_2)_{15}N^+(CH_3)_3\,Cl^-$. The long chain cation $C_{16}H_{33}N^+(CH_3)_3$ is the surface-active part, so it forms cationic micelles.
Sodium dodecyl sulphonate, $C_{12}H_{25}SO_3^-Na^+$, has a surface-active anion, so it forms anionic micelles.
Sodium acetate (very short chain) and urea (non-ionic, no long chain) do not form micelles.
So the compound that forms cationic micelles is cetyl trimethylammonium chloride.