which of the following forms cationic micelles above certain concentration?

Options

A. Sodium ethyl sulphate
B. Sodium acetate
C. Urea
D. Cetyl trimethyl ammonium bromide

Explanation

To determine which of the given compounds forms cationic micelles above a certain concentration, we need to analyze the properties of micelles and the structures of the provided options.

### Step-by-Step Solution:

1. **Understanding Micelles**:
- Micelles are aggregates of surfactant molecules that form in solution above a certain concentration known as the critical micelle concentration (CMC).
- They typically consist of a long hydrophobic (non-polar) tail and a hydrophilic (polar) head.

2. **Identifying the Properties of Cationic Micelles**:
- Cationic micelles are formed by surfactants that have a positively charged head group.
- They require a sufficiently long hydrophobic tail to stabilize the micelle structure.

3. **Analyzing the Options**:
- **Option A: Sodium Ethyl Sulfate**:
- Structure: Ethyl group (2 carbons) with a sulfate group.
- Analysis: Lacks a long carbon chain; thus, it cannot form micelles.

- **Option B: Sodium Acetate**:
- Structure: Methyl group (1 carbon) with a carboxylate group.
- Analysis: Also lacks a long carbon chain; cannot form micelles.

- **Option C: Urea**:
- Structure: Contains amine and carbonyl groups.
- Analysis: Does not have a hydrophobic tail or polar head suitable for micelle formation; cannot form micelles.

- **Option D: Cethyl Trimethyl Ammonium Bromide**:
- Structure: Long carbon chain (16 carbons) with a trimethyl ammonium group.
- Analysis: Has a long hydrophobic tail and a positively charged polar head, which satisfies the criteria for forming cationic micelles.

4. **Conclusion**:
- Based on the analysis, **Option D (Cethyl Trimethyl Ammonium Bromide)** is the only compound that can form cationic micelles above a certain concentration due to its long carbon chain and polar head.

### Final Answer:
The correct option is **D: Cethyl Trimethyl Ammonium Bromide**.