Overview

Is milk a liquid? Sort of. Is fog a gas? Sort of. Is Jell-O a solid? Sort of. These are all colloids. They are the “in-between” state of matter. They consist of tiny particles of one thing floating in another thing, without dissolving and without settling to the bottom.

Core Idea

The core idea is Dispersion Stability. Gravity wants to pull the particles down (sedimentation). Surface tension wants to clump them together (coagulation). A colloid stays mixed because the particles are too small to sink and often repel each other electrically.

Formal Definition

A mixture where one substance (dispersed phase) is suspended in another (continuous phase). Particle size: 1 nm to 1000 nm.

  • Sol: Solid in Liquid (Paint).
  • Emulsion: Liquid in Liquid (Mayonnaise).
  • Foam: Gas in Liquid (Whipped Cream).
  • Aerosol: Solid/Liquid in Gas (Smoke/Fog).

Intuition

  • Solution: Salt water. The salt disappears completely. Clear.
  • Suspension: Muddy water. The mud floats for a bit, then sinks to the bottom.
  • Colloid: Milk. The fat droplets float forever. They never sink, but they block light (Tyndall Effect).

Examples

  • Mayonnaise: Oil and vinegar don’t mix. But if you add egg yolk (an emulsifier), it coats the oil droplets and lets them float in the vinegar. Magic creamy sauce.
  • Aerogel: The lightest solid on earth. It’s a gel where the liquid has been replaced by air. It looks like frozen smoke.
  • Blood: A colloid of red blood cells floating in plasma.

Common Misconceptions

  • They are rare: Most of the food you eat (ice cream, butter, cheese, bread) and products you use (toothpaste, lotion) are colloids.
  • Brownian Motion: The particles in a colloid jiggle around randomly. This was the first proof that atoms exist (Einstein proved it).
  • Tyndall Effect: The scattering of light by particles. This is why you can see the beam of a flashlight in fog, but not in clear air.
  • Non-Newtonian Fluids: Cornstarch and water (Oobleck). A colloid that acts like a solid when you punch it and a liquid when you pour it.

Applications

  • Water Treatment: We add chemicals (flocculants) to dirty water to destroy the colloid state, making the dirt clump up and sink so we can remove it.

Criticism / Limitations

  • Instability: Colloids eventually separate (milk goes sour and curdles). Keeping them stable (shelf life) is a huge challenge for food scientists.

Further Reading

  • Shaw, Duncan. Introduction to Colloid and Surface Chemistry.
  • Poon, Wilson. Colloids.