11.5 Colloids

Colloids

Colloids are mixtures that fall between true solutions and suspensions. The particles in a colloid are too small to settle out but large enough to scatter light. You encounter colloids constantly: milk, fog, and mayonnaise are all examples.

Colloids vs Solutions and Suspensions

The main difference between these three types of mixtures comes down to particle size.

• Colloids have particle sizes ranging from 1–1000 nm. These particles stay suspended indefinitely because of Brownian motion, the constant random movement caused by collisions with molecules in the surrounding medium. Examples: milk, fog, mayonnaise.
• Solutions are homogeneous mixtures with solute particles smaller than 1 nm. The particles are too small to see or to scatter light. Examples: salt water, sugar water, air.
• Suspensions are heterogeneous mixtures with particles larger than 1000 nm. These particles are heavy enough that gravity pulls them down, so they settle over time. Examples: sand in water, dust in air.

Tyndall Effect in Colloid Identification

The Tyndall effect is the scattering of light by colloidal particles. When you shine a beam of light through a colloid, you can see the beam's path as a visible cone or streak. Think of how headlights become visible beams in fog.

This effect is useful for telling mixtures apart:

• Solutions do not show the Tyndall effect because their particles are too small to scatter light.
• Colloids clearly show the Tyndall effect.
• Suspensions may scatter light too, but you can distinguish them from colloids because suspension particles will eventually settle to the bottom.

Preparation and Stability of Colloids

There are two broad approaches to making colloids:

1. Dispersion methods start with large particles and break them down.

   • Mechanical dispersion: grinding or homogenizing material into colloidal-sized particles
   • Electrical dispersion: using an electric arc to break material into fine particles

2. Condensation methods start with individual atoms or molecules and build them up.

   • Physical condensation: cooling a vapor so particles form (this is how fog forms)
   • Chemical condensation: running a chemical reaction like precipitation or hydrolysis to produce colloidal particles

Once a colloid is made, several factors determine whether it stays stable or falls apart:

• Electrostatic stabilization: Colloidal particles often carry the same surface charge. Like charges repel, so the particles push each other away and resist clumping together.
• Steric stabilization: Polymers or surfactants coat the particle surfaces, creating a physical barrier that keeps particles from touching and sticking.
• Zeta potential: This measures the electrical potential at the boundary between a particle and the surrounding fluid. A higher zeta potential (whether positive or negative) means stronger repulsion between particles and greater stability.

When stability breaks down, two things can happen:

• Flocculation: Particles loosely clump together, but this process is reversible.
• Coagulation: Particles irreversibly aggregate and settle out of the mixture.

Types and Applications of Colloids

Colloids come in different forms depending on the phases of the dispersed substance and the medium:

• Sol: Solid particles dispersed in a liquid. Paints and blood are common examples.
• Gel: A semi-solid system where the dispersed phase forms a network throughout the liquid, giving it structure. Gelatin and hair gel are gels.
• Surfactants are molecules that lower the surface tension between two phases (like oil and water). In solution, surfactant molecules arrange into clusters called micelles, with their water-attracting ends facing outward and their water-repelling ends tucked inside. This is how detergents work to trap grease.
• Dialysis is a technique for purifying colloids. A semi-permeable membrane lets small dissolved molecules and ions pass through while keeping the larger colloidal particles behind. This is the same principle used in kidney dialysis to filter waste from blood.