Which one of the following substances is not used for preparing lyophilic sols? (a) Metal sulphide (b) Gum (c) Gelatin (d) Starch

In the realm of chemistry and material science, a colloidal system is a heterogeneous mixture where one substance (dispersed phase) is distributed uniformly throughout another substance (dispersion medium). The particles in a colloidal system range in size from 1 to 1000 nanometers. Because of this size, colloids display unique properties, particularly with respect to light scattering (Tyndall effect), movement (Brownian motion), and charges on particle surfaces.

Applications of colloidal systems span across industries — from milk, which is an emulsion of fat globules in water, to ink, which is pigment particles dispersed in a liquid. In medicine, colloids serve as carriers for drug delivery because they can interact with biological systems without being readily absorbed or filtered out. Understanding the characteristics of colloidal systems is vital for designing materials with specific behaviors and applications.

Focusing on the concept of dispersed phase, this refers to the particles that make up one of the two main components of a colloidal system. The dispersed phase is the substance in tiny particles, droplets, or bubbles that is interspersed throughout the dispersion medium. It’s notable that the stability and properties of a colloid greatly depend on the nature of the dispersed phase.

Substances that form the dispersed phase can be solid, liquid, or gas. The particle size is crucial because if they are too large the mixture forms a suspension which will settle out, or if they are too small, it will form a true solution, where particles are completely dissolved. Dispersed particles also carry an electrical charge which prevents them from aggregating and allows the colloid to remain stable.

When we discuss lyophobic colloids, we are referring to 'solvent hating' colloidal systems. Lyophobic means 'solvent fearing', and typically these colloids are not thermodynamically stable. This implies that they require specialized methods like vigorous agitation, ultrasonication, or addition of stabilizing agents to be formed. Unlike lyophilic colloids, which exhibit significant interaction and affinity towards the dispersion medium, lyophobic colloids have little to no affinity.

Examples of lyophobic colloids include those with dispersed phases such as metals, their sulfides, and oxides. To prepare these colloids, one needs to overcome the natural tendency of the dispersed particles to aggregate. Substances like metal sulphides don't spontaneously emulsify in water, hence they are lyophobic colloids. They are distinct from lyophilic colloids due to their lack of spontaneous solubility in the dispersing medium.

The term chemical affinity conveys an important aspect of colloidal stability and formation. It describes the tendency or ability of a substance to react or bind with another. In the context of colloid chemistry, it's particularly important when distinguishing between lyophilic and lyophobic colloids. Lyophilic colloids have a high chemical affinity for the dispersion medium, meaning the particles are held in suspension by the strong interactions with the solvent molecules.

This affinity can result from various intermolecular forces such as hydrogen bonds, electrostatic, and van der Waals forces. In contrast, lyophobic colloids have low affinity for the dispersing medium, often requiring energy input or stabilizers to maintain the colloidal state. The chemical nature of the colloid's dispersed phase significantly impacts its behavior and methods of preparation.