Question

In the context of coagulation-flocculation used in water treatment, the compression of the electrical double layer around a colloidal particle, brought about by the addition of an electrolyte, primarily reduces the thickness of which ONE of the following layers?

• A. Bulk solution layer
• B. Shear plane only
• C. Stern layer and diffuse layer
• D. Van der Waals layer

Hint:

Recall that the electrical double layer is made of the Stern layer plus the diffuse layer, and adding electrolyte (increasing ionic strength) compresses this double layer, reducing the repulsive barrier between particles.

Answer 1

The Correct Option is C

Step 1: Recall the structure of the electrical double layer (EDL) around a charged colloidal particle. A colloidal particle surface carries a fixed charge, which attracts a layer of tightly bound counter-ions right against the particle surface, called the Stern layer, followed by a more loosely associated, diffuse cloud of counter-ions extending further into the solution, called the diffuse (Gouy-Chapman) layer. Together these two layers make up the electrical double layer.

Step 2: Recall how coagulant electrolytes act on the EDL. Adding an electrolyte (coagulant) increases the ionic strength of the solution. This compresses (reduces the thickness of) the diffuse layer specifically, since more counter-ions are now available close to the particle to neutralize its surface charge, and the associated Stern layer, being the innermost compact layer, is also affected as the overall double-layer thickness (Debye length) shrinks with increasing ionic strength.

Step 3: Explain why this promotes coagulation. As the double layer compresses, the repulsive electrostatic energy barrier between approaching particles is reduced, so van der Waals attractive forces dominate at a shorter separation distance, allowing particles to approach closely enough to aggregate (coagulate).

Step 4: Eliminate the wrong options. Option (A), the bulk solution layer, is unrelated to the localized double layer structure around a particle. Option (B) is incomplete, since compression affects the diffuse layer thickness, not merely a shear plane concept, and the Stern layer is also affected. Option (D) is a distractor - van der Waals forces are attractive forces, not a physical layer structure at all.

Step 5: Conclude. The addition of an electrolyte compresses the Stern and diffuse layers together (the electrical double layer), which is option (C).