Problem 118 - A Complete Resource Book in Chemistry for JEE Main

Question

Potassium stearate is obtained by the saponification of an oil or fat. It has the formula \(\mathrm{CH}_{3}-\left(\mathrm{CH}_{2}\right)_{16}-\mathrm{COO}^{-} \mathrm{K}^{+} .\)The molecule has a lyopho- bic end \(\left(\mathrm{CH}_{3}\right)\) and a lyophilic and \(\mathrm{COO}^{-} \mathrm{K}^{+}\). Potassium stearate is an example of (1) lyophobic colloid (2) associated colloid or micelle (3) multimolecular colloid (4) macromolecular colloid (a) 2 only (b) 1 and 2 (c) 3 and 4 (d) 4 only

Step-by-Step Solution

Answer

Potassium stearate is an associated colloid (micelle); answer is (a) 2 only.

Step 1: Understanding Potassium Stearate Structure

Potassium stearate consists of a long hydrocarbon chain, \( \mathrm{CH}_{3}-\left(\mathrm{CH}_{2}\right)_{16}- \), which is the hydrophobic (water-repelling) part, and a carboxylate group \( \mathrm{COO}^{-}\mathrm{K}^{+} \) which is the hydrophilic (water-attracting) part.

Step 2: Identifying Lyophobic and Lyophilic Ends

The structure naturally gives potassium stearate amphiphilic properties, meaning it has both lyophobic and lyophilic parts which leads to the formation of micelles in aqueous solutions. This amphiphilic nature makes it effective in forming colloids such as micelles.

Step 3: Defining Associated Colloid (Micelle)

Associated colloids, also known as micelles, are composed of amphiphilic molecules like potassium stearate. When a certain concentration (critical micelle concentration - CMC) is reached in an aqueous solution, these molecules align themselves in a spherical shape - the hydrophilic heads facing the water and the hydrophobic tails inwards.

Step 4: Analyzing Options Based on Potassium Stearate

Based on the structure and behavior of potassium stearate, it can form micelles in a solution, classifying it as an associated colloid. Thus, option (2) 'associated colloid/micelle' reflects the behavior of potassium stearate correctly.

Step 5: Reviewing Answer Options

Given the colloidal nature of potassium stearate as an associated colloid, the correct match is (a) 2 only. None of the other options are relevant because potassium stearate does not form lyophobic, multimolecular, or macromolecular colloids.

Key Concepts

Amphiphilic Molecules

Amphiphilic molecules have a unique structure that features both water-attracting (hydrophilic) and water-repelling (hydrophobic) parts. This dual nature allows them to interact with different environments. The hydrophilic part is often composed of polar or charged groups, while the hydrophobic part usually consists of non-polar hydrocarbon chains.

This combination permits amphiphilic molecules to position themselves at air-water or oil-water interfaces, reducing surface tension. It is this property that enables them to stabilize various forms of colloids, like micelles.

Such molecules are crucial in processes like the formation of soap bubbles, where the hydrophilic head interacts with water and the hydrophobic tail interacts with oils and dirt. Together, these characteristics make amphiphilic molecules integral in numerous applications, from detergents to biological membranes.

Micelles

Micelles are spherical structures formed by amphiphilic molecules in an aqueous environment. When these molecules are introduced into water, the hydrophobic tails avoid contact with water, while the hydrophilic heads remain in contact with the aqueous phase.

The molecules orient themselves such that the hydrophobic tails are tucked inside the sphere, away from water, while the hydrophilic heads face outward. This arrangement minimizes the energy and makes micelles thermodynamically stable.

Micelles are important in various applications, including cleaning products and drug delivery systems, where they can encapsulate hydrophobic substances and transport them in a water-based environment. This ability to isolate non-polar substances makes micelles powerful tools in both industrial and biological processes.

Critical Micelle Concentration

The critical micelle concentration (CMC) is the threshold concentration at which amphiphilic molecules begin to form micelles in a solution. Below this concentration, the molecules exist primarily as individual entities, while above it, they aggregate to form micelles.

The CMC is a crucial parameter for understanding the efficiency of surfactants (surface-active agents), as it indicates the minimum amount needed for maximum effectiveness. Factors such as temperature, electrolyte presence, and the molecular structure can influence the value of the CMC.

Knowing the CMC helps in optimizing formulations in various applications, such as shampoos or emulsions, ensuring that there is enough amphiphilic substance to achieve desired properties without unnecessary excess. Understanding and controlling CMC is a key aspect in the development of products that rely on micelle structures.