7.2 Classification of Colloidal System

The colloidal solution can be divided into three categories based on the type of particles of the dispersed phase.

1. Multimolecular colloids

When on dispersion of a substance in the dispersion medium, a large number of atoms or smaller molecules of the substance (with diameter less than $1$ $nm$) aggregate together to form species having size in the colloidal range, the species thus formed are called multimolecular colloids. These are held together by van der Waals forces.

2. Macromolecular colloids

When certain substances having big size molecules, called macromolecules, having large molecular masses are dissolved in a suitable liquid, they form a solution in which the molecules of the substance, i.e., the dispersed particles have size in the colloidal range. Such substances are called macromolecular colloids.

These macromolecular substances are usually polymers with very high molecular masses. As these molecules have large sizes and have dimensions comparable to those of colloidal particles, their dispersions are called macromolecular colloids. Their colloidal solutions are quite stable and resemble true solutions in many respects.

3. Associated colloids (Micelles)

The substances which when dissolved in a medium at low concentrations behave as a normal, strong electrolytes at higher concentrations exhibit colloidal state properties due to formation of aggregated particles are called associated colloids. The aggregated particles thus formed are called micelles.

The formation of micelles takes place only above a particular temperature called Kraft temperature $({T_k})$ and above a particular concentration called Critical Micelle Concentration (CMC). The most common example of associated colloids is that of surface active agents such as soaps and synthetic detergents.

Cleansing Action of Soaps

Suppose some grease or oil is sticking on the surface of cloth.When it comes in contact with soap solution, the stearate ion arrange themselves around it in such a way that hydrophobic parts of the stearate ions are in the oil (or grease) and the hydrophilic parts project outside the grease droplet. As hydrophilic is part is polar, these polar groups can interact with the water molecules present around the oil droplet.

As a result, the oil droplet is pulled away from the surface of the cloth into water to form ionic micelle which is then washed away with the excess of water. In fact, the stearate ions of soap molecules help in making a stable emulsion of oil with water which is washed away with the excess of water . It may be noted that a sheath of negative charge is formed around the oil globule which prevents them to come together and form aggregate.