the electric double layer
A charged particle sitting in water is never bare — it wears a coat of ions. If the surface is negative, positive ions from the water crowd toward it to balance the charge, forming a cloud that is dense right at the surface and thins out with distance. This structure — the charged surface plus its answering cloud of counter-ions — is the electric double layer: two layers of charge, the fixed one on the particle and the mobile one in the liquid around it.
It comes in two parts. Right against the surface a thin, tightly bound sheet of counter-ions sticks fast — the Stern layer, only an ion or two thick. Beyond it stretches a looser, fuzzy cloud called the diffuse layer, where counter-ions are still attracted but free to wander and get shuffled by thermal motion. The electric potential is highest at the surface and decays through these layers toward zero far away; how fast it decays is measured by the Debye length (1/kappa), the cloud's effective thickness. That thickness depends sharply on how much salt is dissolved: roughly 1/kappa (in nm) is about 0.3 / sqrt(I) with the ionic strength I in mol/L, so about 10 nm in very pure water at 0.001 M but under 1 nm at 0.1 M salt. When particles move — settling, or dragged by flowing liquid — they slip through the water at a surface inside the diffuse layer called the shear (or slipping) plane, and the potential right there is the measurable zeta potential.
The double layer is the whole basis of electrostatic stabilization. Two particles repel only when their diffuse clouds overlap and resist being squeezed together; a thick double layer (low salt) means a long-range, strong repulsion, while a thin one (high salt) lets particles approach until van der Waals attraction wins and they flocculate. This is why adding salt, or moving toward the isoelectric point where the charge vanishes, coagulates a slurry — you are collapsing the double layer. Kill the double layer and you have lost your only electrostatic defence against clumping.
Take a nicely dispersed silica sol and stir in ordinary table salt. As the salt concentration climbs, the double layer around each particle squeezes from tens of nanometres down to a fraction of a nanometre; past a critical amount the repulsion can no longer hold particles apart and the clear sol turns cloudy and gels — the double layer has been crushed.
Salt shrinks the ion cloud; shrink it far enough and electrostatic repulsion collapses, so the sol flocculates.
The double layer is not a rigid shell but a statistical cloud that is thicker in fresh water and thinner in salty water; the surface potential itself is not directly measurable, which is why we work with the zeta potential at the shear plane instead.