Problem 3 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following forms cationic micelles above certain concentration? (a) sodium dodecyl sulphate (b) sodium acetate (c) urea (d) cetyltrimethyl ammonium bromide

Step-by-Step Solution

Answer

Cetyltrimethyl Ammonium Bromide (d) forms cationic micelles.

Step 1: Understanding Micelles

Micelles are aggregates of surfactant molecules dispersed in a liquid colloid. They form when the concentration of surfactant molecules in the solution reaches or exceeds a certain critical micelle concentration (CMC).

Step 2: Identify Surfactant Type

Review the options to identify which one is a surfactant that forms cationic micelles. Surfactants can be cationic, anionic, or nonionic based on their headgroup charge. Cationic micelles are formed by surfactants with a positively charged headgroup.

Step 3: Analyze Each Option

- Sodium Dodecyl Sulphate (SDS) is an anionic surfactant. It forms anionic micelles and not cationic ones. - Sodium Acetate is not a surfactant; it is an ionic compound that does not form micelles. - Urea is a non-ionic compound and does not form micelles. - Cetyltrimethyl Ammonium Bromide (CTAB) is a cationic surfactant, which means it forms cationic micelles.

Step 4: Conclusion

From the analysis, Cetyltrimethyl Ammonium Bromide (CTAB) is the compound that forms cationic micelles as it is a cationic surfactant. Therefore, the correct choice is CTAB, option (d).

Key Concepts

Surfactants

Surfactants are unique compounds that play a crucial role in various industries and scientific applications. These substances are surface-active agents that consist of molecules with two distinct parts: a hydrophilic (water-attracting) "head" and a hydrophobic (water-repelling) "tail." This dual structure allows surfactants to reduce the surface tension between different substances, such as oil and water, effectively enabling them to mix.
For instance, during cleaning, surfactants help break up and lift away dirt and grease by surrounding them with their hydrophobic tails, making them easier to wash away. Surfactants are also responsible for the formation of micelles, which are essential structures in the field of chemistry and biology.
There are three main types of surfactants based on the charge of their headgroup:

• Cationic surfactants have a positively charged head.
• Anionic surfactants have a negatively charged head.
• Nonionic surfactants have no charge on their head.

Each type of surfactant behaves differently in solution and is chosen based on the desired application or outcome.

Critical micelle concentration

The concept of critical micelle concentration (CMC) is vital in understanding how surfactants work. CMC is the specific concentration at which surfactant molecules in a solution start to aggregate and form micelles. Prior to reaching the CMC, surfactant molecules exist mainly as monomers, spreading out evenly in the solution.
Once the concentration exceeds the CMC, the formation of micelles becomes energetically favorable. This transition leads to dramatic changes in the physical properties of the solution, such as reduced surface tension and increased solubilization of hydrophobic substances.
Understanding CMC is crucial for various applications:

• In detergency, achieving above the CMC ensures optimal cleaning since micelles can encapsulate grease and oils.
• In pharmaceuticals, CMC helps design drug delivery systems where micelles can carry and release drugs in a controlled manner.

Researchers calculate CMC through various methods, including surface tension measurements and conductivity tests, to optimize the use of surfactants in different applications.

Cationic surfactants

Cationic surfactants are a specific class of surfactants distinguished by their positively charged head groups. In solutions, the positive charge of the head group allows these surfactants to strongly interact with negatively charged surfaces or particles. This property makes them highly effective as:

• Fabric softeners: They neutralize static charges, allowing fabrics to remain soft.
• Antimicrobial agents: Their positive charge disrupts negatively charged microbial cell membranes, effectively killing bacteria.
• Hair conditioners: Cationic surfactants bind to the hair cuticle, providing a smooth, silky feel.

One common example of a cationic surfactant is cetyltrimethyl ammonium bromide (CTAB), which forms cationic micelles when its concentration exceeds the CMC. Cationic surfactants are highly versatile and extensively used in industrial and personal care products due to their unique interaction properties.