Zeta Potential measurement

What is zeta potential?

Zeta potential (ZP) is the difference between the potential on the shear surface of the particle and the potential of the solution (source). ZP measures the degree of repulsion between the charged particles in the dispersion; the high ZP indicates the higher level the particles are charged.

The following picture shows the position of the ZP in a suspension system; Note the terms:

  • Stern layer. Stern layer separates liquid-particle into two parts:
    • A inner region where ions are strongly bound to particles
    • A diffusive region where ions are less firmly associated with the particles
  • Diffusion layer.
  • Slip plane. It is a notional boundary within diffusion layer, inside which the ions and particles form a stable entity. The particles and ions within slip plane moved together when forces added into the system. Meanwhile, ions beyond the slip plane stay with the bulk dispersant. The potential at this boundary is the zeta potential (reference).
Source

According DVLO theory, to make a stable suspension, the repulsion force between particles should be higher than the repulsion force. Therefore, higher ZP leads to higher electric repulsion force and prevents aggregation of the particles. The ZP value of −30 mV is considered optimum for good stabilization of a nano-dispersion. (reference)

Colloid stability

Two ways to improve stability: steric effect and electrical repulsion as shown above (source: Malvern).

Measurement

Source: Malvern company

Electrophoretic light scattering (ELS)

Electrophoresis is the migration of macromolecules under the influence of an electric field. The particle electrophoretic velocity can be measured by Doppler effect, illuminated by laser team when particles migrate under electrical field.

Source

The particle hydrodynamic radius Rh can be measured simultaneously with mobility by DLS (ref), an independent module in the tool (reference).

if the Rh (or a in the following figure) is much larger or smaller than the thickness of the ionic double layer (reciprocal of Debye length, k-1), Zeta potential can be calculated by the following equation under each conditions.

Source: Malvern

References

  1. Zeta Potential Tutorial | Part 1: Intro to Zeta Potential

2. Zeta Potential Tutorial | Part 2: Performing a Measurement