A colloidal dispersion has been de?ned as a suspension of small particles in a continuous medium. The colloidal dispersion clearly has two phases, one is the bulk medium called disperse medium and another phase of the suspended particles (aggregates of molecules or single molecules of size 1 µm to 1 nm) called the dispersion phase. Colloids have the ability to scatter light and they apparently lack osmotic pressure. Bellow we give some examples of colloidal systems with the corresponding dispersion phases and disperse medium.
A sol is a colloidal suspension of solid particles in a continuous liquid medium. Examples include blood, pigmented ink, and paint. They are also known as lyophobic colloids. These are invariably substances that are highly insoluble in the dispersing medium. The lyophobic colloids are usually aggregates of small molecules. They can be prepared in a colloid mill, or by chemical methods like Hydrolysis. They are very sensitive to electrolytes.
Gels are a dispersion of molecules or particles within a liquid in which the solid is the continuous phase and the liquid is the discontinuous phase. These are solid, jelly-like material that can have properties ranging from soft and weak to hard and tough. These are also known as lyophilic colloids. Typical lyophilic systems would be proteins (especially gelatin) or starch in water, rubber in benzene etc.
Emulsions are colloids where both the dispersed and the continu- ous phase are liquid. In an emulsion, one liquid (the dispersed phase) is dispersed in the other (the continuous phase). Examples of emulsions in- clude vinaigrettes, the photo-sensitive side of photographic film, milk and cutting fluid for metal working.