Soaps consist of compounds such as sodium stearate, CH3(CH2)16COO–Na+, that have both hydrophobic and hydrophilic parts. Consider the hydrocarbon part of sodium stearate to be the “tail” and the charged part to be the “head.” (a) Which part of sodium stearate, head or tail, is more likely to be solvated by water? (b) Grease is a complex mixture of (mostly) hydrophobic compounds. Which part of sodium stearate, head or tail, is most likely to bind to grease? (c) If you have large deposits of grease that you want to wash away with water, you can see that adding sodium stearate will help you produce an emulsion. What intermolecular interactions are responsible for this?

Verified step by step guidance

1. Step 1: Understand the structure of sodium stearate. Sodium stearate is a soap molecule with a long hydrocarbon chain (CH3(CH2)16) known as the 'tail' and a charged carboxylate group (COO–Na+) known as the 'head'. The tail is hydrophobic, meaning it repels water, while the head is hydrophilic, meaning it attracts water.
2. Step 2: Determine which part of sodium stearate is more likely to be solvated by water. Water is a polar solvent, and it tends to solvate or dissolve polar or charged substances. The hydrophilic 'head' of sodium stearate, which is charged, is more likely to be solvated by water due to its ability to interact with water molecules through ion-dipole interactions.
3. Step 3: Analyze which part of sodium stearate is most likely to bind to grease. Grease is composed of hydrophobic compounds, which do not mix well with water. The hydrophobic 'tail' of sodium stearate is more likely to bind to grease because it can interact with other hydrophobic molecules through van der Waals forces, allowing the soap to surround and encapsulate grease particles.
4. Step 4: Explore how sodium stearate helps wash away grease with water. When sodium stearate is added to water, it forms micelles. The hydrophobic tails of the soap molecules cluster together, while the hydrophilic heads face outward towards the water. This arrangement allows the micelles to trap grease within their hydrophobic core, making it easier to wash away with water.
5. Step 5: Identify the intermolecular interactions responsible for forming an emulsion. The formation of micelles and the subsequent emulsification of grease involve several intermolecular interactions: hydrophobic interactions between the tails of sodium stearate and grease, and ion-dipole interactions between the heads of sodium stearate and water. These interactions stabilize the emulsion, allowing grease to be dispersed in water.

Key Concepts

Hydrophilic and Hydrophobic Properties

Hydrophilic substances are attracted to water and can interact favorably with it, while hydrophobic substances repel water and do not mix well with it. In the context of sodium stearate, the hydrophilic 'head' interacts with water, making it soluble, whereas the hydrophobic 'tail' does not. This dual nature allows soaps to interact with both water and grease.

Intermolecular Interactions

Intermolecular interactions refer to the forces that occur between molecules, influencing their behavior in mixtures. In the case of sodium stearate, the ionic interactions between the charged 'head' and water molecules facilitate solvation, while van der Waals forces between the hydrophobic 'tails' and grease allow for binding. These interactions are crucial for the formation of emulsions.

Emulsification

Emulsification is the process of mixing two immiscible liquids, such as oil and water, into a stable mixture. Soaps like sodium stearate act as emulsifiers by reducing the surface tension between the two phases, allowing the hydrophobic tails to bind with grease and the hydrophilic heads to interact with water. This enables the grease to be dispersed in water, facilitating its removal.