لخّصلي

Double electric layer is made from potential determining ions that are tightly bound by the solid surface, and the equivalent amount of oppositely charged ions (counter ions). The layer of counter ions is divided into two parts: the first layer is called adsorption layer and it is made from oppositely charged ions interacting with ions from potential determining layer; the second layer is called diffusion layer since those ions are not directly adsorbed on a surface.
Electrothermodynamical potential (φ) exists between the layer of potential determining ions and the layer of counter ions. Electrokinetical potential (ζ) exists between the adsorption layer of counter ions and the diffusion layer of counter ions.. The border between the adsorption layer and the diffusion layer of counter ions is sometimes called a sliding boarder.
As an example we can take a sol formed after the reaction between water solutions of two salts: the excess of potassium iodide (KI) and relatively low amount of silver nitrate (AgNO3). As a result of this reaction insoluble silver iodide is formed (AgI).
KI + AgNO3 → KNO3 + AgI↓
Iodide anions are adsorbed on a surface of an aggregate, since they are able to build up its crystal lattice. That is how the layer of potential determining ions (PDI) is formed.
The charge of this layer is negative (in this case). Potential determining ions are responsible of electrothermodynamical potential formation. Aggregate and the layer of potential determining ions form a nucleus of colloid particle. The next layer is formed by counter ions (K+) adsorbed on negatively charged surface of a nucleus. Nucleus together with adsorption layer forms a granule of colloid particle.
Actually, granule is a real particle that exists in colloid solution . Its charge is equal to the sum of the overall charge of potential determining ions and the overall charge of adsorption layer of counter ions. . Сharge of a granule has the same sign as the charge of potential determining ions (in our case it is negative). We say that colloid particle has an overall charge equal to zero. To make this rule obey we should say that all the counter ions that are necessary to compensate the charge of a granule are situated in the diffusion layer. Schematically we should show the structure of colloid particle in the following way.
{mAgI∙nI–∙(n – x)K+}–x∙xK+
{aggregate∙PDI∙counter ions from adsorption layer}∙diffuse layer
{nucleus∙ counter ions from adsorption layer}∙diffuse layer
{granule}∙diffuse layer
In this scheme “m” shows the number of units of AgI in the aggregate; “n” shows the number of potential determining ions; “x” shows the number of counter ions in the diffusion layer. The charge of a granule in this scheme is equal to “–x”. The boarders of a granule are shown by braces. The numbers of “m”, “n” and “x” are variables. They are different even for different colloid particles of the same sol