electrical double layer

A charged particle in a liquid is never naked. Like a celebrity surrounded by a crowd, it pulls oppositely charged ions toward itself, building up a shell of ions that screens its charge. That ionic shell, together with the particle's own surface charge, is the electrical double layer.

The double layer has two regions. Right against the surface is a tightly held compact layer of counter-ions (the Stern layer). Beyond it is a diffuse layer where counter-ions are still slightly concentrated but free to wander, gradually fading into the neutral bulk liquid. The potential is highest at the particle surface and falls off with distance through these layers.

The double layer matters because when two particles approach, their diffuse layers overlap and create a repulsive force that resists aggregation — the electrostatic half of the DLVO theory of colloid stability. Adding salt compresses the diffuse layer (the extra ions screen the charge over a shorter distance), shrinking the repulsion and promoting flocculation; this is exactly why electrolytes can destabilise a suspension and why ionic strength is a key formulation variable.