The formation of micelle takes place only:

(A) above a particular temperature called kraft temperature

(B) above a particular concentration called critical micelle concentration

(C) when catalyst is added

Choose the correct answer from the options given below :

(A), (B) Only

The correct answer is option 4. (A), (B) Only.

Micelles are aggregates of surfactant molecules formed in a solution when specific conditions are met. Surfactants are molecules that contain both hydrophilic (water-loving) and hydrophobic (water-repelling) regions. In an aqueous solution, these molecules tend to arrange themselves in such a way that the hydrophobic tails are tucked inside, away from the water, while the hydrophilic heads face outward toward the water. This leads to the formation of spherical structures known as micelles. Micelle formation is crucial in various applications, such as in cleaning (detergents), biological systems (cell membranes), and pharmaceuticals (drug delivery). For micelles to form, two key conditions must be met:

Kraft Temperature \((T_k)\)

The Kraft temperature is the minimum temperature at which surfactant molecules can aggregate to form micelles in a solution. Below this temperature, the surfactant exists as a solid or in an insoluble form, and micelles cannot form. Above the Kraft temperature, surfactant molecules dissolve properly in water and form micelles. Below the Kraft temperature, the solubility of the surfactant is too low for micelle formation. Only when the solution is heated above this temperature do the molecules gain enough energy to overcome intermolecular forces and arrange themselves into micelles.

Example: Sodium stearate (a soap) has a Kraft temperature around 30°C. Below this temperature, it is not very soluble, and micelles do not form. When the temperature is raised above 30°C, the solubility increases, and micelle formation occurs.

Thus, micelle formation takes place only above the Kraft temperature.

Critical Micelle Concentration (CMC)

The Critical Micelle Concentration (CMC) is the minimum concentration of surfactant molecules required for micelles to form in solution. Below the CMC, surfactant molecules exist as individual units dispersed in the solution. Once the concentration reaches or exceeds the CMC, the surfactant molecules start to aggregate and form micelles. At concentrations below the CMC, the surfactant molecules are too few in number to form micelles. As the concentration of surfactant increases and reaches the CMC, the solution becomes saturated with surfactant molecules, and any additional molecules aggregate into micelles to reduce the system's free energy.

Example: Sodium dodecyl sulfate (SDS), a common surfactant, has a CMC of approximately 8 mM in water. At concentrations below 8 mM, SDS molecules remain as individual molecules in the solution. When the concentration is above 8 mM, micelles begin to form.

Thus, micelle formation takes place only when the concentration of the surfactant is above the CMC.

Role of Catalysts

Catalysts are substances that speed up chemical reactions by lowering the activation energy but remain unchanged at the end of the reaction. However, catalysts have no role in micelle formation. Micelle formation is not a chemical reaction but a physical process of molecular aggregation. The factors that influence micelle formation are temperature (Kraft temperature) and concentration (CMC), not the presence of a catalyst. Therefore, the addition of a catalyst does not influence micelle formation**.

Summary of Options:

(A) Above a particular temperature called Kraft temperature:

True. Micelles form only above the Kraft temperature because the solubility of the surfactant increases at higher temperatures.

(B) Above a particular concentration called critical micelle concentration:

True. Micelles form only when the surfactant concentration exceeds the CMC, as this is the threshold for aggregation.

(C) When a catalyst is added:

False. Catalysts do not play a role in micelle formation since it is a physical aggregation process, not a chemical reaction.

Conclusion: The correct answer is (4) (A) and (B) only. Micelle formation is dependent on both temperature (Kraft temperature) and concentration (CMC), but not on the presence of a catalyst.