Surface charge of oxides in water

Several mechanisms can cause surface charges of oxide particles [JOL 94] [HUN 87]. The main ones are i) the reaction of hydroxyl groups present at the srrrface of oxides, ii) the adsorption of specific iorrs or charged polyelectrolytes called 

The surface of oxide particles consists of hydroxyl groups whose 

These two equations show that, depending on the pH of the medium, the charge of the particles can be positive or negative. A characteristic point of an oxide is the point of zero charge (PZC). It reflects the acidic-basic character of the surface. The PZC can be defined in terms of pKa of reactions [5.1] and [5.2]  

The value of the PZC is linked to the nature of the oxide and will depend directly on the polarization of the OH group by the cation M of the oxide. A cation of high charge (Si +) will result in an acid surface and a weak PZC. The PZC of sihca (Si02 is about 2. The PZC of alumina (AI2O3), with a bivalent cation (AP+) is about 9. 

The evolution of the potential, when we move away from the particle surface, is calculated by expressing the electrochemical potential difference between the particle surface and the solution. In the structured part of the double layer (Stem layer), the ions are supposed to be located on distinct planes. The decreases in the potential between the surface and the IHP, and between the IHP and the OHP are therefore as linear as inside a condenser. The potential decreases exponentially with the distance in the diffuse layer starting from the OHP. The electrically disturbed zone extends to about 100 angstroms. Its thickness depends highly on the concentration and the charge of the electrolyte ions. The greater the charge and the concentration, the more the diffuse layer is compressed. 

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