Newman 203-236 Equilibrium adsorption and ion exchange. These surface areas are available for exchange of ions and molecules between the solids and surrounding solutions. This exchange process can be treated as an equilibrium process.
Adsorption takes place because of the attraction of ions to a surface.
At the same time, negative ions are attracted to the dark-blue octahedral hydroxyl surface. Example of vermiculite or smectite . Surface charge properties One way in which surface charge can develop is by adsorption of an ion where the solid acts as an electrode. The simultaneous adsorption of protons and hydroxyls as well as other potential determining cations and anions, leads to the concept of zero point of charge or ZPC , where the total charge from the cations and anions at the surface is equal to zero. The charge must be zero and this does not necessarily mean the number of cations versus anions in the solution are equal.