Colloidal stability in lyophobic colloids arises from the electric double layer (EDL) around each particle.

Layer Structure:

1. Inner Stern Layer (Compact Layer): A firmly adsorbed layer of ions (from the preparation electrolyte or selectively adsorbed from solution) directly on the particle surface. These ions are immobile. For Fe(OH)_{3} sol, $Fe^{3+}$ ions from excess $FeCl_{3}$ form this layer, giving the particle a positive charge.

2. Outer Diffuse Layer (Gouy-Chapman Layer): A diffuse layer of counter-ions (ions of opposite charge to the particle) that are attracted electrostatically but subject to thermal motion. These extend into the bulk solution, becoming less concentrated with distance.

Zeta Potential: The potential at the slipping plane (the boundary between mobile and immobile fluid around the particle). High |zeta potential| = strongly charged = strong repulsion between particles = stable colloid. Coagulation typically occurs when |zeta potential| falls below ~15–20 mV.

Effect of Adding Electrolyte: When electrolyte is added, counter-ions from the electrolyte compress the diffuse double layer (decrease its thickness, called Debye length). This reduces the zeta potential. When the double layer is sufficiently compressed (coagulation value reached), particles can approach close enough for van der Waals attraction to dominate → coagulation.

Hardy-Schulze Explanation via EDL: Higher-valency counter-ions are more effective at compressing the double layer (because they carry more charge per ion, more effectively neutralizing the particle charge). This is why $Al^{3+}$ (z=3) is far more effective than $Na^{+}$ (z=1) — quantitatively, by ~z^6 ≈ 729× at the same concentration.