Zeta Potential Analysis
Zeta Potential is the characterization of the electrokinetic potential of liquid-liquid or solid-liquid colloidal dispersions. Under the influence of an electric field, charged particles will exhibit certain electrokinetic effects including electrophoresis, electroosmosis, streaming potential, or sedimentation potential.2 At PTL, zeta potential is measured using either a Zetasizer Nano ZS (Malvern Panalytical) system or Litesizer 500 (Anton Paar) system. Both calculate zeta potential using the principle of electrophoresis.
During a zeta potential analysis, charged colloidal dispersions are placed into a zeta cell and upon application and upon application of an external electric field, the particles travel toward the electrode that has a charge opposite to that of the particle. Their velocity, or electrophoretic mobility, under the influence of an electric field, is measured using LASER Doppler velocimetry.3 It is then used to calculate the zeta potential by applying the Henry equation:
Zeta potential is often helpful in terms of ruling out candidate product batches during stability testing thereby saving time and money. If the suspended particles have a large magnitude of charge, they will likely repel one another and have lower tendencies to flocculate, indicating stability and long-term shelf life. On the other hand, low zeta potential is sometimes desired, e.g. for water purification systems. Low inter-particle repulsion will enhance flocculation and aid in proper filtration.4
To take a closer look, the graphic depicts a negatively charged particle dispersed in water. A tight layer of counter ions (in this case cations) called the stern layer, forms around the surface of the particle, where its attractive forces are strongest. Cations in the diffuse layer are also attracted by the negatively charged particle but to a lesser extent, since the interactive force between two charged particles is proportional their charge magnitudes, and inversely proportional to the distance that separates them.1 Additionally, anions that are simultaneously repelled by the negatively charged particle and attracted by stern layer cations are also found in the diffuse layer. Within the diffuse layer is a boundary called the slip plane (shear plane), where the particle’s zeta potential is measured. When a particle moves, ions within the slip plane travel with it because of their strong attraction to the particle, but ions outside of the slip plane do not since the force of the applied electric field and subsequent particle velocity overcomes any bond they have to it, creating a plane of shear.5
1 Coulomb’s law. Dictionary.com Unabridged. Retrieved April 5, 2017. website http://www.dictionary. com/browse/coulomb-s-law.
2 The Zeta Potential. Colloidal Dynamics: leader in colloidal measurement. 1999. Retrieved April 5, 2017. website http://www.colloidal-dynamics.com/docs/CDElTut1.pdf.
3 Zetasizer Nano Series User Manual. Malvern Instruments Limited. MAN0317 Issue 2.1. July 2004. Printed in England.
4 Zeta Potential. Malvern Panalytical website https://www.malvernpanalytical.com/en/products/measurement-type/zeta-potential
5 Zeta Potential An Introduction in 30 Minutes. Malvern Instruments Limited. Zetasizer Nano Series Technical Note. MRK654-01. website https://caliscc.org/images/presentations/Morante_Zeta_Potential.pdf.
Advantages
- Zeta Potential is intended to give insight into the stability of a suspension.
Considerations
- Results are pH and conductivity dependent.
- High conductivity might not be suitable i.e. samples in isotonic saline or PBS.
- Carrier viscosity – viscous mediums inhibit electrophoretic mobility, which is proportional to zeta potential. A viscosity no higher than 10cP is ideal.
Sample Requirements
Ideal sample amount is 1 gram for solid samples, especially for new material, to allow for the determination of adequate preparation and instrument settings.
For liquid samples, the preferred amount is 20 mL if the sample is analyzed “as received.”
The above sample quantities are preferred. If sample availability is further limited, please contact us. Smaller sample quantities may be accommodated depending on sample properties, dispersion technique, and required instrument.
Detection Range
PTL’s Zeta Potential instruments can measure particulate from approximately 1 nanometer (nm) up to 1 micrometer (µm).
Data Reported
Zeta Potential (mV), Analysis Temperature, pH, and Conductivity.
Instrumentation
Particle Technology Labs has both Malvern Panalytical and Anton Paar instrumentation to ensure we find the correct instrumentation for your material.
Anton Paar Litesizer 500
