What is Zeta Potential?

Zeta potential indicates the stability of a colloidal dispersion. A colloid is a particle, 10-8 to 10-6m in size which when mixed in a dispersing medium is termed a colloidal dispersion. Aerosols (solid or liquid colloids) in a gas (dispersing medium), emulsions (liquid colloids) in a liquid (dispersing medium) and gels (liquid colloids) in a solid (dispersing medium) are all examples of colloidal dispersions.

Each colloidal particle holds an electrical charge (positive or negative). The Zeta potential quantifies the potential difference across the phase boundaries between the colloidal particles and the dispersing medium.  

A high zeta potential indicates stability and the dispersion will resist aggregation; low zeta potential indicates less stability and a tendency to flocculate.

Importance of zeta potential

Knowing the zeta potential of a colloidal dispersion is critical to understanding the behaviour or stability of the overall fluid. Knowledge of the zeta potential is important in:

  • The pharmaceutical industry for optimal drug delivery as aggregation of colloidal particles must be minimised – a high zeta potential is required.
  • Wine and beer making as well as in wastewater treatment processes – as a low zeta potential makes it easier to remove aggregated particles in filtration systems.
  • The delivery of therapeutic agents in nanomedicine – a low zeta potential results in longer circulation of drugs due to low serum protein binding.
  • The shelf life of products in the food industry which is greatly dependent on the zeta potential. A high value minimises aggregation and maintains a one phase mixture (prevents mixtures separating).
  • Sludge reduction in paper manufacturing, the stability of cements, clays and drilling fluids, the strength of ceramics, raw mineral processing as well as optimum paint colour, quality, gloss and texture.

Measuring zeta potential

Zeta potential is measured in millivolts (mV) and can either be negative or positive. A stable system has a zeta potential of ± 50 mV (with a minimum value of ± 25mV) and an unstable system has a low zeta potential of ± 10mV.

The technique used to measure zero potential is electrophoretic light scattering (ELS). This technique measures the movement of a colloidal particle relative to the dispersing medium, under an electric field. This electrophoretic mobility is then converted into zeta potential using Henry’s equation.

Henry’s equation however, requires the radius of the colloidal particle, which can be determined by dynamic light scattering (DLS). This technology uses laser light which is scattered at different intensities by the movement of the colloidal particles (Brownian motion). These intensity fluctuations indicate the velocity of the Brownian motion, which in turn determines the size of the colloidal particles in a solution, emulsion or suspension (using the Stokes-Einstein relationship).

The Nicomp 380ZLS Zeta Potential analyser can measure both DLS and ELS in less than a minute. However  a more stable  measurement is taken after approx 5-10 minutes. Because it is necessary to  have the electric field, electrodes and cell setup correctly, the ability of the Nicomp to show the power spectrum as a preliminary measure of correct setup, is a valuable diagnostic tool.