Origins of surface charge

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Origin of charge - Ionic solids

Substance PZC
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

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Origin of charge for oxides

Oxide PZC Oxide PZC Oxide PZC
Ag2O 11.2 HgO 7.3 SnO2 5.6
Al2O3 9.1 La2O3 10.1 Ta2O5 2.8
BeO 10.2 MgO 12.4 ThO2 9.2
CdO 11.6 MnO2 5.3 TiO2 (rutile) 5.7
CeO2 8.1 MoO3 2 TiO2 (anatase) 6.2
CoO 10.2 Nb2O5 2.8 V2O3 8.4
Co3O4 7.4 NiO 10.2 WO3 0.4
Cr2O3 7.1 PuO2 9 Y2O3 8.9
CuO 9.3 RuO2 5.3 ZnO 9.2
Fe2O3 8.2 Sb2O5 1.9 ZrO2 7.6
Fe3O4 6.6 SiO2 2

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Adsorption of ionic surfactants


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Origins of surface charge of 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.

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