Question

What is a micelle? How do the micelles formed by soap molecules suspend greasy dirt in a solution?

Short answer

A micelle is an aggregate of surfactant molecules, like soap, dispersed in a liquid colloid such as water. It has a hydrophilic (water-loving) exterior and a hydrophobic (water-repelling) interior. Soap molecules form micelles in water, with the hydrophobic tails pointing inward, capturing greasy dirt particles. The polar heads of the soap molecules interact with the water, keeping the micelle-dirt complex suspended in the solution, and preventing the micelles from clumping together due to their negative charge. This allows the micelles with enclosed dirt particles to be easily washed away.

Step-by-step solution

Define a micelle

A micelle is an aggregate of surfactant molecules dispersed in a liquid colloid, such as water. Surfactants are molecules that have both hydrophilic (water-loving) and hydrophobic (water-repelling) parts. In a micelle, the hydrophilic parts face the surrounding water, while the hydrophobic parts are clustered together in the center, creating a protective sphere around them. This arrangement minimizes the contact between the hydrophobic parts of the surfactant molecules and the water.

Explain soap structure

Soap is a type of surfactant that consists of a long hydrophobic tail (usually made of hydrocarbon chains) and a polar, hydrophilic head. When soap is added to water, the hydrophobic tails of the soap molecules repel the water and gather toward each other, creating a spherical structure called a micelle. The hydrophilic heads remain on the outer surface of the sphere, interacting with the surrounding water.

Explain how micelles remove grease

Greasy dirt is not soluble in water, meaning it does not dissolve or mix easily with water. When soap is added to water containing grease, the soap molecules form micelles with the hydrophobic tails pointing inward. The hydrophobic tails in the center of the micelle can effectively 'capture' the greasy dirt particles, because they prefer interactions with the nonpolar grease molecules over the polar water molecules.

Describe the suspension of greasy dirt

Once the greasy dirt particles are captured by the micelles, the polar heads of the soap molecules that are in contact with water keep the micelle-dirt complex suspended in the solution. This is because the polar heads of the soap form a barrier around the micelle, and interact with the water molecules, forming a stable colloid. The hydrophilic head also has a negative charge, which prevents the micelles from clumping together due to electrostatic repulsion between the negatively charged heads. As a result, the micelles with the enclosed dirt particles stay dispersed in the water, allowing them to be easily washed away.