Introduction to Colloids and Dispersions

TL;DR

Colloids are mixtures where tiny particles are dispersed evenly in another substance, too small to settle but big enough to scatter light. Dispersions are a broader category, including solutions, colloids, and suspensions, distinguished by particle size. Understanding these helps explain how many everyday biological systems like blood and milk behave.

1. The Mental Model

Imagine particles mixed in a liquid. If they're molecularly blended, it's a solution. If they're big enough to eventually sink, it's a suspension; otherwise, if they're still too small to see but too big to be a solution, they're likely in a colloid.

2. The Core Material

When we mix one substance into another, we create a dispersion. This term is pretty general and just means one component is scattered (dispersed) throughout another. What makes dispersions interesting is the size of the dispersed particles, which dictates their properties and how we classify them.

2.1 Three Main Types of Dispersions

We generally categorize dispersions into three main groups based on the size of the dispersed phase particles:

Solutions

These are homogeneous mixtures where the "solute" particles are extremely small, typically less than 1 nanometer (nm) in diameter. They're individual molecules or ions, completely dissolved and evenly distributed. Think of sugar dissolving in water – you can't see the sugar, and it won't settle out. Solutions are transparent.

Colloids

Here, the dispersed particles are larger than those in solutions but still too small to be seen with the naked eye or settle out quickly due to gravity. Their size typically ranges from 1 nm to 1000 nm (or 1 micrometer, µm). Because of this intermediate size, colloids have some unique properties:
* Tyndall Effect: They scatter light, making the light beam visible (like sunlight through dust in the air). Solutions don't do this.
* Brownian Motion: The dispersed particles show random, jiggling movement due to collisions with molecules of the dispersion medium.
* Stability: They're generally stable and won't settle out over time, unlike suspensions.
* Heterogeneous at microscopic level: While they might look homogeneous, they're not truly mixed at the molecular level.

Common examples include milk (fat droplets in water), fog (water droplets in air), gels (protein in water), and blood plasma (proteins and other macromolecules