Classification of Colloids: Methods & Examples

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

Colloids have several different methods of classification. In this lesson we will learn how they are classified based on physical state, interaction type, and particle size.

Classification Methods

Now that we've learned what a colloid is, you have probably realized that there are a lot of colloids all around you - from the food you eat to the rubber in your tires. With so many different colloids there are also different methods to classify these colloids. The different methods are used based on the needed applications or in order to think about colloids in different ways.

Colloids can be classified based on the physical states of the dispersion medium and dispersed phase, or the type of interaction between the dispersion medium and dispersed phase, or based on the particle types in the dispersed phase.

Physical State

The dispersion medium can be in the gas, liquid or solid state. The same is true of the dispersed phase. Based on the phase of the dispersion medium and the dispersed phase we can classify the colloids into 8 categories:

  • Aerosol
  • Solid Aerosol
  • Foam
  • Emulsion
  • Sol
  • Solid Foam
  • Gel
  • Solid Sol

The only combination that does not occur is with both the dispersion medium and the dispersed phase being in the gaseous form.

Physical state

With aerosols we have a liquid in a gas, using the example of fog we have water (liquid) dispersed in the air (gas). While smoke is an example of a solid aerosol, because we have soot particles (solid) in the air (gas).

When liquids act as the dispersion medium we get foams, emulsions, and sols. Foams put a gas into a liquid, this includes whipped cream where air (a gas) has been whipped into the cream (a liquid). Emulsions put a liquid with a liquid, many of your salad dressings will use this method by mixing oil (liquid) with water (liquid). Paints and ink are a sol, which put solids (the color particles) into a liquid (can be an oil or a liquid polymer).

Solids as the dispersion medium form solid foams, gels, and solid sols. With cakes the gas forms as the cake is cooking, solidifying the gas into the solid cake, forming a solid foam. Jelly is a an example of a gel colloid (the name makes sense) based on the fact that water (liquid) is suspended within the starch particles (solid). And colored glass puts color pigments (a solid) into glass (another solid) forming a solid sol

Interaction Type

While colloids can be classified based on the physical type, the affinity of each type variates greatly even within each categories. This affinity refers to the affinity between the dispersion medium and the dispersed phase. This only breaks into two categories - strong and weak affinity. A strong affinity between the dispersed phase and the dispersion medium is called lyophilic. A weak affinity between the dispersed phase and the dispersion medium is called lyophobic. You can remember which is which by remembering that -philic means 'loving' while -phobic mean 'fearing'. So, when there is a strong affinity, the two love each other. When there is a weak affinity they hate each other.

Things such as jelly and rubber are lyophilic. It is pretty easy get the water and starch to mix and stay together in a gel. In science when we say something such as this is 'easy' to do, it usually means that it doesn't take much energy.

Salad dressings is an example of a lyophobic affinity. Oil and water do not like to mix, so it takes a lot of energy to get them to mix together. Think about how you sometimes need to mix the salad dressing bottle a lot before pouring it onto your salad. This is adding the needed energy to get these two substances to mix.

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