Problem 87 - Chemistry: The Central Science in SI Units, Global Edition

Question

An "emulsifying agent" is a compound that helps stabilize a hydrophobic colloid in a hydrophilic solvent (or a hydrophilic colloid in a hydrophobic solvent). Which of the following choices is the best emulsifying agent? (a) \(\mathrm{CH}_{3} \mathrm{COOH},\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{COOH},\) (c) \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{11}\) COOH, (d) \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{11} \mathrm{COONa}\).

Step-by-Step Solution

Answer

(d) \(\mathrm{CH}_3\left(\mathrm{CH}_{2}\right)_{11} \mathrm{COONa}\)

Step 1: (a) CH3COOH

(Acetic acid) This compound has a hydrophilic carboxyl group (COOH) and a hydrophobic methyl group (CH3). Its hydrophobic part is relatively small compared to other given compounds.

Step 2: (b) CH3CH2CH2COOH

(Butanoic acid) This compound has a hydrophilic carboxyl group (COOH) and a hydrophobic butyl group (CH3CH2CH2). The hydrophobic part is larger than that in Acetic acid, but still smaller than those in the next two compounds.

Step 3: (c) CH3(CH2)11COOH

(Lauric acid) This compound again has a hydrophilic carboxyl group (COOH), but now it has a large hydrophobic part consisting of 12 carbon atoms in a chain. It has a much stronger hydrophobic nature than the previous two compounds.

Step 4: (d) CH3(CH2)11COONa

(Sodium laurate) This molecule has the same hydrophobic part as in Lauric acid with 12 carbon atoms. However, the hydrophilic part in this case is a carboxylate anion (COO-) that is negatively charged and has an ionic nature along with the Na+ ion. It provides a much stronger hydrophilic nature compared to the other compounds with only a COOH group. Now, we need to find the compound that has the best balance between hydrophobic and hydrophilic nature, which would help it act as an effective emulsifying agent. As Sodium laurate (option d) has the significant hydrophobic part and the strongly hydrophilic part due to the ionic nature, this would make it the best emulsifying agent. It will be able to interact with both hydrophobic and hydrophilic parts of the mixture and help in stabilizing it. Thus, the correct answer is: (d) \(\mathrm{CH}_3\left(\mathrm{CH}_{2}\right)_{11} \mathrm{COONa}\).

Key Concepts

Hydrophobic Colloids

Hydrophobic colloids are particles that tend not to mix well with water or other polar solvents. They have surfaces that repel water molecules, making them insoluble in a hydrophilic environment. This is because they possess nonpolar groups that do not interact favorably with water.
To understand this better:

• "Hydrophobic" means "water-fearing".
• These colloids tend to cluster together in aqueous environments to minimize their exposure to water.
• They often require the help of emulsifying agents to be stabilized in polar solvents.

An example of a hydrophobic colloid would be lauric acid, which is prominent in some emulsifying agents due to its long hydrophobic carbon chain.

Hydrophilic Solvent

Hydrophilic solvents are mediums in which polar or charged particles dissolve easily. The term "hydrophilic" means "water-loving", and such solvents can dissolve substances by surrounding polar or ionic groups with water molecules.
Key characteristics of hydrophilic solvents include:

• High dielectric constant which helps in dissolving ionic compounds.
• Good interaction with solutes that contain polar or ionic groups.

Water is perhaps the most common example of a hydrophilic solvent. It's crucial for water to mix well with both the hydrophilic and hydrophobic parts of an emulsifying agent to stabilize emulsions.

Ionic Nature

The ionic nature of a compound refers to its ability to dissociate into ions in a solution. Ionic molecules contain charged particles, enabling stronger interactions with polar solvents like water. This ionic property is crucial for emulsifying agents, as it gives them strong hydrophilic characteristics.
Here are some important points:

• Ionic compounds have both positive and negative charges which allow them to attract water molecules.
• This makes them highly soluble in water and useful in stabilizing colloids in hydrophilic solvents.

Sodium laurate is an example of a compound with significant ionic nature, owing to its carboxylate anion (COO^-) and sodium cation (Na⁺), ensuring its effectiveness as an emulsifying agent.