Problem 34 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following are surjactants? (I) \(\mathrm{CH}_{3}-\left(\mathrm{CH}_{2}\right)_{15}-\left(\mathrm{N}\left(\mathrm{CH}_{3}\right)_{3} \overline{\mathrm{Br}}\right)\) (II) \(\mathrm{CH}_{3}-\left(\mathrm{CH}_{2}\right)_{14}-\mathrm{CH}_{2} \mathrm{NH}_{2}\) (III) \(\mathrm{CH}_{3}-\left(\mathrm{CH}_{2}-\right)_{16}-\mathrm{CH}_{2} \mathrm{O} \mathrm{SO}_{3} \mathrm{Na}\) (IV) \(\mathrm{OHC}-\left(\mathrm{CH}_{2}\right)_{14}-\mathrm{CH}_{2}\left(\mathrm{COO}^{-} \mathrm{Na}\right)\) (a) I, II, III (b) II, III, IV (c) I, III, IV (d) I, II, III, IV

Step-by-Step Solution

Answer

Option (c) I, III, IV are surfactants.

Step 1: Understanding Surfactants

Surfactants are compounds that lower the surface tension between two liquids or a liquid and a solid. They usually consist of a hydrophobic tail (like a long hydrocarbon chain) and a hydrophilic head (like an ionic group).

Step 2: Evaluate Compound I

Compound I (\(\mathrm{CH}_{3}-\left(\mathrm{CH}_{2}\right)_{15}-\left(\mathrm{N}\left(\mathrm{CH}_{3}\right)_{3} \overline{\mathrm{Br}}\right)\)) has a long hydrophobic tail (\(15 + 1 = 16\) carbon atoms) and a quaternary ammonium head group, which is ionic. Thus, it is a surfactant.

Step 3: Evaluate Compound II

Compound II (\(\mathrm{CH}_{3}-\left(\mathrm{CH}_{2}\right)_{14}-\mathrm{CH}_{2} \mathrm{NH}_{2}\)) has a long hydrophobic tail (\(14 + 2 = 16\) carbon atoms) and an amino head group, which can interact with water, but it is not strongly ionic. While it may have some surfactant properties, it is less typical than others with ionic groups.

Step 4: Evaluate Compound III

Compound III (\(\mathrm{CH}_{3}-\left(\mathrm{CH}_{2}-\right)_{16}-\mathrm{CH}_{2} \mathrm{O} \mathrm{SO}_{3} \mathrm{Na}\)) has a hydrophobic tail and a sulfate group (ionic), which makes it a strong surfactant.

Step 5: Evaluate Compound IV

Compound IV (\(\mathrm{OHC}-\left(\mathrm{CH}_{2}\right)_{14}-\mathrm{CH}_{2}\left(\mathrm{COO}^{-} \mathrm{Na}\right)\)) features a carboxylate head group (ionic) with a long hydrophobic tail, classifying it as a surfactant.

Step 6: Determine the Correct Answer

Although Compound II has some surfactant properties due to its chemical structure, it is less effective than the others due to the non-ionic head group. Therefore, typically Compounds I, III, and IV are considered strong surfactants. Hence, option (c) I, III, IV is a suitable choice.

Key Concepts

Hydrophobic Tail

The hydrophobic tail is a crucial part of a surfactant molecule. It's typically a long chain of hydrocarbons — basically a string of carbon and hydrogen atoms. This tail doesn't like water. It prefers to stick with other non-polar substances, much like oil. That's why we call it "hydrophobic," which literally means "water-fearing." The length of the tail affects how the surfactant behaves.

A longer tail tends to make the molecule more effective in reducing surface tension. It can also influence whether a surfactant works better in water or oil. In our example compounds, each has a hydrophobic tail that's about 15-16 carbon atoms long. This common feature allows these molecules to stick to oil or grease while avoiding water, enabling them to break up and remove dirt more effectively.

• The tail length contributes directly to the oil-attracting properties of the surfactant.
• Hydrophobic tails are essential for forming micelles, which are structures that trap dirt and oil.

By understanding the role of the hydrophobic tail, it becomes clear how surfactants manage to clean and disperse substances that otherwise wouldn't mix, like oil and water.

Hydrophilic Head

In contrast to the hydrophobic tail, the hydrophilic head of a surfactant is "water-loving." This part of the molecule is typically polar or ionic, meaning it interacts well with water. The hydrophilic head is crucial because it allows surfactants to dissolve in either water or oils and helps them to achieve their role in reducing surface tension.

In our listed compounds, the heads vary:

• Compound I has a quaternary ammonium group, a positively charged ionic head that interacts with water.
• Compound III features a sulfate ( a negatively charged ionic group), ideal for attracting water.
• Compound IV contains a carboxylate head group, also ionic and compatible with water.

The effectiveness of the hydrophilic head comes from its ability to engage with a liquid environment. This interaction pulls the surfactant towards the surface of water, allowing the hydrophobic tail to extend into an oil phase or air, effectively reducing surface tension. Understanding this dual nature explains why surfactants are used in so many applications, from detergents to emulsifiers in food.

Ionic Groups

Ionic groups in a surfactant are key players in determining how well the molecule dissolves in water. These groups are generally charged and contribute to the head's property of being hydrophilic.

The types of ionic groups can vary, but their presence signals strong interactions with water, which is vital for the surfactant’s ability to stabilize mixtures of water with oils or other non-polar substances.

• In the surfactants mentioned, Compound I has a quaternary ammonium head that provides a positive charge.
• Compound III carries a sulfate group responsible for its strong negative charge.
• Compound IV possesses a carboxylate group, also negatively charged.

These charged components balance the hydrophobic tail by ensuring the surfactant can position itself between water and oil, creating new interfaces. They also help to form structures like micelles, which encapsulate and trap oils, allowing them to be rinsed away easily. Without ionic groups, surfactants would lack the versatility that makes them so effective in cleaning and emulsifying.