Zeta potential (ζ-potential) is the electric potential at the slipping (shear) plane of a particle suspended in a liquid.
It represents the potential difference between the dispersion medium and the stationary fluid layer attached to the particle.
High zeta potential (positive or negative) means strong repulsion between particles and, usually, colloidal stability.
Units: millivolts (mV)
Measurement Techniques
- Electrophoretic Light Scattering / Laser Doppler Electrophoresis: Measures particle speed in an electric field via Doppler shift, then converts to zeta potential.
- Streaming Potential: Used for surfaces or membranes; relates movement of fluid to electric current.
- Electrokinetic/Electroacoustic Methods: Used for certain complex or concentrated systems.
- Nanoparticle Tracking Analysis: Observes Brownian motion and electrophoretic mobility for nanoparticles.
Careful control of temperature, pH, and ionic strength is needed for accurate measurements.
Factors Affecting Zeta Potential
- pH: Changes the surface charge, with isoelectric point (ζ = 0) meaning lowest stability.
- Ionic Strength: High salt compresses the electrical double layer, reducing zeta potential.
- Medium Properties: Solvent type, dielectric constant, and viscosity matter.
- Surface Modifiers: Surfactants, polymers, or adsorbed ions can strongly alter ζ-potential.
- Temperature: Affects particle mobility and calculation outcomes.
Stability Guidelines of Zeta Potential
- |ζ| > 30 mV: Generally stable, particles repel each other.
- |ζ| < 30 mV: Generally unstable, particles aggregate easily.
These are approximate; other forces may impact actual stability.
Applications of Zeta Potential
- Characterization of nanomaterials and colloids
- Formulation science: pharmaceuticals, paints, food, cosmetics
- Water and waste treatment (aggregation of suspended solids)
- Environmental fate of colloids
- Biotechnology: drug carriers, protein stability
Limitations of Zeta Potential
- ζ-potential is not a direct measure of actual surface charge (it's at the slipping plane).
- Strongly dependent on medium properties (pH, ionic strength, electrolyte type).
- Assumes uniform particle surfaces (real particles may have heterogeneous or rough surfaces).
- Accuracy sensitive to calibration, especially in high-conductivity or non-aqueous media.
Summary Table
| Parameter | Description |
|---|---|
| Definition | Potential at slipping plane (mV) |
| Measurement Techniques | Electrophoresis, Streaming Potential, NTA |
| Factors | pH, ionic strength, modifiers, solvent, temperature |
| Stability Guidelines | |ζ| > 30 mV: Stable; < 30 mV: Unstable |
| Applications | Nanotech, pharma, water treatment, environment |
| Limitations | Not surface charge, medium-dependent, accuracy issues |