Origins of surface charge

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Origin of charge - Ionic solids

Reference

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

Reference

Reference

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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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