Effect of Salt on Zeta Potential
Theories describing how the charge density around a particle varies with distance always use the concept of the diffuse double layer. In the simplest theory, the electrostatic potential decays exponentially with distance away from the shear plane. The inverse of the decay constant is a distance called the Debye double layer thickness. It is a function of free salt ion concentration (as embodied in the value of the ionic strength): the higher the concentration, the faster the decay, the smaller the double layer thickness. At high enough salt, the double layer collapses to the extent that the ever-present attractive van der Waals forces overcome the charge repulsion. This is one example of the so-called "salting out" effect. Electrostatically stabilized colloidal suspensions will become unstable with the addition of enough salt.