A colloidal dispersion has been defined as a suspension of small particles in a continuous medium because of their ability to scatter light and very low osmotic pressure; these particles are much larger than simple small molecules like water and alcohol and simple salts like NaCl.
It was assumed that they were aggregates of many small molecules held together in an amorphous state quite different from the normal crystalline state of the substance.
Aggregates are, in fact, single molecules that have a very high molar mass. The size limits are difficult to specify, but dispersed particles are between 1μm and 1 nm, and the system is a colloidal dispersion.
There are two subdivisions of colloidal systems:
- Lyophilic or solvent–loving colloids (gel)
- Lyophobic or solvent–fearing colloids (sols)
Lyophilic colloids
These are invariably polymeric molecules, and the colloidal solution consists of a dispersion of single molecules. The stability of the lyophilic colloidal is a consequence of the strong, favourable solvent-solute interaction. Lyophilic systems would be proteins or starch in water, rubber in benzene, cellulose nitrate, or cellulose acetate in acetone.
The process of dissolution may be slow. The first addition of solvent is slowly absorbed by the solid, which swells. This stage is called imbibitions. Further addition of solvent and mechanical kneading slowly distribute the solvent and solute uniformly.
In the case of ordinary gelatin, the dissolution process is aided considerably by raising the temperature. As the solution cools, the long and twisted protein molecules get entangled in a network with much open space between the molecules.
The presence of the protein induces some structure in the water, which is physically trapped in the interstices of the network. The result is a gel. Adding gross amounts of salts to a hydrophilic gel will ultimately precipitate the protein.
This is a consequence of competition between the protein and the salt for the solvent, water. Lithium salts are particularly effective because of the large amount of water the lithium-ion can bind. The charge of the ion is not a primary determinant of its effectiveness as a precipitant.
Lyophobic colloids
These colloids are invariably substances that are highly insoluble in the dispersion medium. The lyophobic colloids are usually aggregates of small molecules.
