General Description

• A glass of homogenized milk is a colloidal dispersion. Liquid fat globules are dispersed in a base of water containing soluble salts. It is a liquid-in-liquid colloid. Since water is the medium, milk is a hydrocolloid. However, there are other solid, liquid, and even gaseous colloids. Colloids may be prepared by condensing particles dissolved in a true solution. They may also be prepared by the reduction of larger-sized particles.
  
  

Types of Colloid Based on Physical State

• Examples of colloids wherein a gas is the medium are fog (liquid in gas) and smoke (solid in gas), and they are called aerosols.
  
  Frequently, a liquid is the colloidal medium. Gas dispersed in a liquid is a "foam." Ordinary whipping cream is one example of a foam colloid. Liquid dispersed in a liquid is an emulsion. One example is oil in vinegar. A solid dispersed in a liquid is called a sol. One example is ordinary household paint.
  
  Occasionally, even a solid may be the medium. One example is the plastic Styrofoam. A gel is a colloid formed from a liquid being dispersed in a solid. The solid is considered to be the medium. This results, counterintuitively, because of extensive cross-linking after initially combining ingredients. The most obvious example is gelatin.
  
  

Classification Based on Component Interaction

• The most common colloids --- liquid colloids --- may be classified by how the dispersed material interacts with the dispersion medium or solvent. It is called lyophilic (solvent loving) if the dispersed phase shares great attraction to the medium. If such a colloid loses its solvent, it can be readily reconstituted; it is called a reversible sol. If the colloid is lyophobic (solvent hating), the sol cannot readily be reconstituted; it is called an irreversible sol. In addition, a chemical substance may need to be added to an irreversible sol for stabilization.
  
  

Other Classifications

• Some molecules are large enough that when they are suspended in a liquid medium, they closely resemble a true solution, yet they are technically colloids. Examples include starches and proteins. These colloids are called macromolecular colloids. Some colloids involve the interaction of two or more molecules to form a particle that is suspended. Thus, some detergent molecules agglomerate to form micelles, which are classified colloidal.