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 the Zetasizer Nano ZS (Malvern Panalytical), which calculates zeta potential using the principle of electrophoresis. During a zeta potential analysis, charged colloidal dispersions are placed into a zeta cell (Figure 1) 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 to the left 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
Important factors to consider when analyzing zeta potential:
pH – H+ can cause the buildup of positive charge on a particle’s surface while OH- will add negative charge thus altering the zeta potential. Functional groups at the particle surface may become protonated/deprotonated.
Conductivity – conductivity is inversely proportional to electrophoretic mobility. An increase in carrier conductivity in an applied electric field will yield a decrease in average drift velocity. Zeta cells often experience tarnishing when sample conductivity is too high. To account for this the applied voltage can be decreased.
Carrier viscosity – viscous mediums inhibit electrophoretic mobility, which is proportional to zeta potential.
Temperature – affects kinetic energy and carrier viscosity.3
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.