Reasons for Stability of Lyophobic Colloidal Particles

To determine the factors responsible for the stability of lyophobic colloidal systems, let us analyze the physical chemistry principles governing colloidal stability:

1. Origin of Charge and Electrostatic Repulsion (Option A): Lyophobic colloidal particles are solvent-repelling and do not have strong solvation layers to keep them stable. Instead, their stability is primarily due to the presence of an electrical charge on their surface. This charge arises mainly from the preferential adsorption of common ions from the dispersion medium onto the surface of the colloidal particles.

   Because all particles in a given colloidal solution selectively adsorb the same type of ion, they acquire identical electrical charges (either all positive or all negative). The resulting electrostatic repulsion between like-charged particles prevents them from approaching close enough for van der Waals attractive forces to cause aggregation or coagulation. Thus, Option A is correct.

2. Solvation Layer (Option B): Preferential adsorption of solvent molecules onto the particle surface creates a protective hydration or solvation sheath. This mechanism provides stability to lyophilic (solvent-attracting) colloids, not lyophobic colloids. Thus, Option B is incorrect.

3. Interparticle Attraction (Option C): Attraction between oppositely charged particles leads to mutual neutralization of charges and subsequent coagulation (precipitation) of the colloidal system rather than its stabilization. Therefore, Option C is incorrect.

4. Helmholtz Electrical Double Layer and Zeta Potential (Option D): When colloidal particles preferentially adsorb a specific layer of ions (the fixed layer), counter-ions from the medium form a secondary, mobile layer (the diffuse layer or diffused layer) surrounding the fixed layer.

   The potential difference developed across the interface between the fixed layer and the diffuse layer is known as the electrokinetic potential or zeta potential ($\zeta$): $\zeta = \frac{4\pi \eta u}{\varepsilon}$ where $\eta$ is the viscosity of the medium, $u$ is the electrophoretic mobility, and $\varepsilon$ is the dielectric constant of the medium.

   A high zeta potential indicates a strong electrostatic repulsive force between approaching colloidal particles, preventing coagulation and thereby imparting stability to the lyophobic sol. Thus, Option D is correct.

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Conclusion: The correct choices are (A) and (D).