Origins of surface charge
Origin of charge - Ionic solids
|Fluorapatite, Ca5(PO4)3(F, OH)||pH = 6|
|Hydroxyapatite, Ca5(PO4)3(OH)||pH = 7|
|Calcite, CaCO3||pH = 9.5|
|Fluorite, CaF2||pCa = 3|
|Barite (syntheitc), BaSO4||pBa = 6.7|
|Silver iodide, AgI||pAg = 5.6|
|Silver chloride, AgCl||pAg = 4|
|Silver sulphide, Ag2S||pAg = 10.2|
Origin of charge for oxides
Adsorption of ionic surfactants
Origins of surface charge for colloids
Most particles in an aqueous colloidal dispersion carry an electric charge. There are many origins of this surface charge depending upon the nature of the particle and it's surrounding medium.
Below are the three mechanisms:
- Ionization of surface groups
- Dissociation of any acidic groups on a particle surface will give a negatively charged surface.
- Dissociation of any basic groups on a particle surface will give a positively charged surface.
- The magnitude of the surface charge depends on the acidic or basic strengths of the surface groups and on the pH of the solution.
- Differential loss of ions from the crystal lattice
- If a crystal of Agl is placed in water, it starts to dissolve.
- If equal amounts of Ag+ and l- ions were to dissolve, the surface would be uncharged.
- In fact Ag+ ions dissolve preferentially leaving a negatively charged surface.
- Adsorption of charged species
- Surfactant ions may be specifically adsorbed onto the surface of a particle.
- Cationic surfactants would lead to a positively charged surface.
- Anionic surfactants would lead to a negatively charged surface.