Question

How are the colloids classified on the basis of the nature of interaction between dispersed phase and dispersion medium? Describe an important characteristic of each class. Which of these sols need stabilising agents for preservation? 

Answer 1

Colloids are classified based on the nature of interaction between dispersed phase and dispersion medium into two main types: lyophilic sols and lyophobic sols.

1. Lyophilic sols (liquid-loving) are formed when the dispersed phase has a strong affinity for the dispersion medium. Examples include sols of gum, gelatin, starch, and proteins in water (hydrophilic sols). These sols are stable, self-stabilized, and do not precipitate easily. They are reversible, if dried, the dispersed phase can be redispersed simply by mixing with the medium again. This reversibility and inherent stability are their important characteristics.
2. Lyophobic sols (liquid-hating) involve a dispersed phase that has little affinity for the dispersion medium, such as metal sols like gold or sulfur sols. These sols are prepared by special methods and are less stable. They coagulate or precipitate on addition of small amounts of electrolytes, heating, or agitation. They are irreversible, meaning once coagulated, the dispersed phase cannot be redispersed easily. Due to their instability, lyophobic sols require stabilizing agents (protective colloids) to prevent coagulation and ensure preservation.

In summary, lyophilic sols are stable, reversible, and self-stabilizing, whereas lyophobic sols are unstable, irreversible, and need stabilizing agents for preservation.