Problem 87 - Chemistry: The Central Science

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) \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{11} \mathrm{COOH}\), (d) \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{11} \mathrm{COONa}\).

Step-by-Step Solution

Answer

The best emulsifying agent is (d) CH3(CH2)11COONa.

Step 1: Identify Hydrophilic and Hydrophobic Parts

For each molecule, let's identify the hydrophilic and hydrophobic parts: a) CH3COOH: The CH3 part is nonpolar (hydrophobic) while the COOH (carboxylic acid) part is polar (hydrophilic). b) CH3CH2CH2COOH: The CH3CH2CH2 part is nonpolar (hydrophobic) while the COOH (carboxylic acid) part is polar (hydrophilic). c) CH3(CH2)11COOH: The CH3(CH2)11 part is nonpolar (hydrophobic) while the COOH (carboxylic acid) part is polar (hydrophilic). d) CH3(CH2)11COONa: The CH3(CH2)11 part is nonpolar (hydrophobic) while the COONa (carboxylate salt) part is polar (hydrophilic).

Step 2: Evaluate Emulsifying Potential of the Molecules

We are now ready to evaluate each molecule's abilities as an emulsifying agent: a) CH3COOH: The molecule has one hydrophilic part (COOH) but only one hydrophobic carbon, which is not strong enough to interact with hydrophobic solvents. b) CH3CH2CH2COOH: The molecule has one hydrophilic part (COOH) and three hydrophobic carbons, which is slightly better than option (a) but still not ideal as an emulsifying agent. c) CH3(CH2)11COOH: The molecule has one hydrophilic part (COOH) and 12 hydrophobic carbons, which is relatively good as an emulsifying agent. d) CH3(CH2)11COONa: The molecule has one hydrophilic part (COONa) and 12 hydrophobic carbons. The COONa group (carboxylate salt) is more polar and more soluble in water than the COOH group (carboxylic acid), making this the most effective emulsifying agent among the options. Hence, the best emulsifying agent is:

Step 3: Answer

(d) CH3(CH2)11COONa.

Key Concepts

Hydrophobic and Hydrophilic

The terms hydrophobic and hydrophilic are critical to understanding how substances interact with water. Hydrophobic, meaning 'water-fearing', refers to nonpolar substances that do not mix well with water. On the other hand, hydrophilic, meaning 'water-loving', defines polar substances that easily dissolve or interact with water. An emulsifying agent takes center stage in this interaction, possessing both hydrophobic and hydrophilic regions.

For example, consider an oil droplet (hydrophobic) in water (hydrophilic). Without an emulsifier, the oil and water would separate. However, an emulsifying agent, with its hydrophobic tail, buries itself in the oil droplet, while the hydrophilic head remains in the water, thereby stabilizing the mixture into a homogeneous emulsion.

Carboxylic Acid

Carboxylic acids are organic compounds typically notated as R-COOH, where R represents a hydrocarbon chain, and COOH is the functional group responsible for the compound's acidic properties. The hydrogen (H) in the carboxyl group can release a proton, resulting in a negatively charged carboxylate ion (R-COO⁻) when it interacts with basic substances.

The COOH group is polar (hydrophilic) due to its ability to form hydrogen bonds with water. However, the effectiveness of a carboxylic acid as an emulsifying agent depends not only on its hydrophilic carboxyl group but also on the length and structure of its hydrophobic R group.

Carboxylate Salt

Carboxylate salts are the salt form of carboxylic acids, usually denoted as R-COONa or R-COOK, where Na or K is a sodium or potassium ion, respectively. These salts form when carboxylic acids are neutralized by a strong base. The resulting carboxylate ion is highly hydrophilic due to the ionic charge, which enhances its water solubility.

In the context of emulsifying agents, the ionic nature of carboxylate salts makes them more effective at stabilizing emulsions than their corresponding carboxylic acids. This is because the electric charge allows for a better interaction with water molecules, which increases the colloid's stability.

Colloid Stability

The stability of colloids, which are mixtures where one substance is dispersed evenly within another, is essential in the formation of stable emulsions. Emulsifiers help to increase the stability of colloids by reducing the surface tension between the substances, thereby preventing them from separating.

Stability is often enhanced by increasing the repulsion between particles, which can be achieved by adding an emulsifying agent with a charged group, like a carboxylate salt. The presence of this charge helps to keep the dispersed particles suspended and evenly distributed within the solvent, thus maintaining the stability of the colloid over time.