Problem 102 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following is not a surfactant? (a) \(\mathrm{CH}_{3}-\left(\mathrm{CH}_{2}\right)_{15}-\mathrm{N}^{+}-\mathrm{CH}_{3} \mathrm{Br}^{-}\) (b) \(\mathrm{CH}_{3}-\left(\mathrm{CH}_{2}\right)_{14}-\mathrm{CH}_{2}-\mathrm{NH}_{2}\) (c) \(\mathrm{CH}_{3}-\left(\mathrm{CH}_{2}\right)_{16}-\mathrm{CH}_{2} \mathrm{OSO}_{2}^{-} \mathrm{Na}^{+}\) (d) \(\mathrm{OHC}-\left(\mathrm{CH}_{2}\right)_{14}-\mathrm{CH}_{2}-\mathrm{COO}^{-} \mathrm{Na}^{+}\)

Step-by-Step Solution

Answer

Option (b) is not a surfactant.

Step 1: Understanding Surfactants

First, recognize that a surfactant is a compound that reduces surface tension between two substances, such as a liquid and a solid or two liquids. It generally contains both hydrophobic (non-polar) and hydrophilic (polar) parts, allowing it to interact with both types of compounds.

Step 2: Analyze Each Compound

Examine each given compound to check if they possess both hydrophobic and hydrophilic regions: - (a) \(\mathrm{CH}_{3}-\left(\mathrm{CH}_{2}\right)_{15}-\mathrm{N}^{+}-\mathrm{CH}_{3}\mathrm{Br}^{-}\): This compound has a long hydrophobic alkyl chain and a positive quaternary ammonium, making it amphipathic (a typical surfactant structure).- (b) \(\mathrm{CH}_{3}-\left(\mathrm{CH}_{2}\right)_{14}-\mathrm{CH}_{2}-\mathrm{NH}_{2}\): This compound has a long hydrophobic tail and a polar amino group but lacks ionizable group, making it less effective as a surfactant.- (c) \(\mathrm{CH}_{3}-\left(\mathrm{CH}_{2}\right)_{16}-\mathrm{CH}_{2}\mathrm{OSO}_{2}^{-} \mathrm{Na}^{+}\): This compound has a hydrophobic tail and a negatively charged sulfonate group with a sodium ion (typical surfactant structure).- (d) \(\mathrm{OHC}-\left(\mathrm{CH}_{2}\right)_{14}-\mathrm{CH}_{2}-\mathrm{COO}^{-} \mathrm{Na}^{+}\): This compound has a hydrophobic chain and a carboxylate group with a sodium ion (typical surfactant structure).

Step 3: Identify the Odd One Out

Based on the analysis, option (b) does not have an ionizable group like the other options, meaning it might not effectively reduce the surface tension as a surfactant would. Hence, (b) lacks the complete structure characteristic of common surfactants.

Key Concepts

Understanding Hydrophobic Properties

Hydrophobic literally means "fear of water." These types of molecules do not interact well with water. Instead, they are attracted to other non-polar substances. This happens because hydrophobic molecules, such as long chains of carbon and hydrogen (alkyl chains), avoid forming bonds or interactions with polar molecules, like water.
Imagine a drop of water on a leaf; the water droplets form on top without spreading. This is a classic hydrophobic effect. In the context of surfactants, hydrophobic regions allow the molecule to bond with oils and greases, helping to remove them. Without a hydrophobic part, a substance won’t act effectively as a surfactant.
Surfactants can thus collect oil or grease inside their hydrophobic tails while their opposite hydrophilic parts can dissolve in water. It's a clever trick that amplifies cleaning capabilities.

Discovering Hydrophilic Properties

On the other side of the surfactant equation sits the hydrophilic part. Hydrophilic means "water-loving." These parts of molecules have charges or polar sections that attract interactions with water molecules.
An easy way to think of this is to consider how sugar dissolves in water; this is thanks to its hydrophilic parts.
For surfactants, the hydrophilic region is typically charged or polar, allowing it to dissolve in water. This capability is what makes surfactants able to reduce surface tension and mix water with oils. The surfactant acts as a bridge between the two, with one end in water while the other is stuck in oil.
The combination of water and oil-friendly ends in a molecule explains why surfactants are used in soaps and detergents to clean surfaces and fabrics, drawing both water and oil into the solution.

Understanding Quaternary Ammonium

Quaternary ammonium compounds are a specific group of chemicals used as surfactants. These compounds have a nitrogen atom surrounded by four alkyl groups, and one of those groups is typically a long, hydrophobic chain.
This formation gives quaternary ammonium salts a reliable amphipathic nature. They carry a permanent positive charge, making the nitrogen part hydrophilic. Meanwhile, the long alkyl chain remains hydrophobic.

• The permanent positive charge is a key indicator of quaternary ammonium compounds.
• They are effective at breaking down surface tension due to their dual action.
• Quaternary ammonium compounds often serve in disinfectants, thanks to their structure allowing them to disrupt cell membranes in bacteria and viruses.

Their excellent performance in both cleaning and disinfecting makes them a popular choice in various cleaning products. This dual-action results from the combination of strong interactions with liquid environments and disruption of biological membranes.