Zeta Potential and Colloidal Behavior

• Bulk solids have lower free energy than colloids and are stable systems whereas colloids exist in a meta-stable state - they are constantly under the influence of attractive forces
• Repulsive forces keep them from aggregating
• In most polar liquids these forces come from particle charge
• Changes in the free energy may lead to state instability
• Stability can be adjusted and measured
  • Zeta potential is a measure of dispersion stability
  • Higher zeta potential implies more stable dispersions
  • Low values can indicate colloid instability which could lead to aggregation.

• Counterions build up near the surface
• Due to diffusion, counterions and co-ions eventually equilibriate
• At the shear plane, the potential difference is defined to be the zeta potential

Isoelectric Point

• Zeta potential changes with
  • Salt concentration
  • Potential determining ion concentration
    • pH for example
  • Surfactant concentration
• The concentration of the potential determining ion at which the zeta potential is zero is defined as the isoelectric point (IEP)
• The isoelectric point is a very important measure and relates strongly to stability

IEP values for some common oxides

Effect on Zeta Potential of Surface Modifications for TiO₂

• R960 (TiO2) has an IEP at pH ~7
• R931 doped with silica, has an IEP at pH ~ 5
• R900 doped with alumina, has an IEP at pH ~9
• Surface doping shifts IEP as expected
• Zeta potential depends on surface chemistry, not bulk chemistry