The zeta potential is related to the net surface charge that nanoparticles have. It’s crucial for determining the colloidal stability of charged particles and understanding the performance of your system in a variety of conditions.
For more information on zeta potential testing and other analytical techniques, please visit our Characterization Services landing page. You can also see our zeta potential video series above for more information regarding sample preparation and results interpretation.
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Zeta Theory
Zeta potential is a physical property exhibited by all solid-liquid and liquid-liquid colloidal systems. Surrounding the surface of all dispersed particles is a thin layer of ions that have the opposite charge of the particle’s surface called the Stern layer. Further from the surface is an additional layer of more loosely-associated ions of opposite charge to the surface that move with the particle as it travels through a medium due to Brownian motion or sedimentation; this is called the double layer. The zeta potential is defined as the voltage at the edge of the slipping (shear) plane with respect to the bulk dispersing medium, where ions, molecules and other agents are no longer associated with a particle’s surface. If two adjacent particles have sufficiently high zeta potentials of the same sign, they will not agglomerate due to repulsive electrostatic forces between particles with like charges.
Another way to make particles stable is through steric hindrance. Nanoparticles can have naturally occurring or engineered ligands or surface groups that physically prevent the nanoparticles from contacting and agglomerating. However, coating nanoparticles with a surface that allows for steric hindrance can change the properties or function of the nanoparticle and is often not desirable. In many cases, Zeta potential is the primary mechanism for obtaining nanoparticle stability in aqueous environments.
Measurement Mechanics
At nanoComposix, we perform zeta potential measurements using a Malvern Zetasizer Nano ZS instrument equipped with a 632 nm HeNe laser operating at a 173 degree detector angle. In a zeta potential measurement, a sample is loaded into a disposable folded capillary cell. The cells have two conductive electrodes that make contact with the instrument’s applied voltage on the outside, and fold in to make contact with the liquid sample on the inside.
Charged particles inside the cell will move through the medium at a rate that is proportional to their zeta potential. Particles with a higher-magnitude zeta will move at a fast rate, while particles with low zeta potential will move more slowly. The particles are illuminated by a laser which indirectly measures the particle speed via a Doppler frequency shift of the scattered light. This frequency shift can be converted into a value of electrophoretic mobility. Zeta potential is calculated from electrophoretic mobility with solvent dielectric constant, viscosity and other constants using the Henry Equation.
While most zeta measurements will be taken in aqueous systems, any colloids dispersed in a solvent that has an appreciable dielectric constant will exhibit zeta potential. As long as a solvent is polarizable, ions will remain partially dissolved and associate with the surface, and an applied electrical potential will reach a particle’s double layer. This permits the measurement of dispersed colloids in solvents such as chloroform, THF and short chain alcohols. Measurements in these solvents require the use of a special zeta cell.
The folded capillary cells degrade by corrosion over time, especially when measurements are taken in high-salt or other conductive media. For this reason, it is important that the zeta cells be checked with a standard reference material to make sure that they are measuring appropriately. At nanoComposix, we calibrate cells daily using a polystyrene latex standard with a known zeta potential.
Importance of Zeta Potential
Knowledge of zeta potential can be used to help optimize formulation, resulting in more effective formulation development for suspensions, emulsions or nanoparticle dispersions.
Zeta can be used to predict the long-term stability of particles. For example, particles with zeta potentials larger than ±60 mV have excellent stability, where particles with zeta values between -10 mV and +10 mV, will experience rapid agglomeration unless they are sterically protected.
The sign and magnitude of zeta potential can be used as a secondary metric to determine surface chemistry changes as well. For example, when moving from a highly negative citrate capped nanoparticle dispersion to a neutral polymer like PEG, expect to see a decrease in the magnitude of the zeta potential. Similarly, when moving from citrate or another negatively-charged surface to bPEI or amine, expect to see the sign of the zeta potential change from negative to positive.
pH & Salt Dependence
A zeta potential value on its own without defining solution conditions is a virtually meaningless number. Zeta potential is strongly pH and salt-dependent and the solution pH needs to be measured and reported with every zeta potential measurement.
For example, when a solution containing nanoparticles is titrated with acid to decrease the pH, acidic protons associate with the electrical double layer, and the particle becomes more positive. The opposite is true with respect to base titration; adding base makes colloids more negative.
This universal pH dependence leads to an important characteristic trait of all colloidal materials – the Isoelectric Point, or IEP. The IEP is defined as the pH at which the zeta potential is zero. Certain classes of materials like plasmonic noble-metal nanoparticles and unfunctionalized silica exhibit very low IEPs – meaning that they tend to carry a negative ZP at all but the most acidic of pH conditions. The opposite is true with respect to aluminum oxide, cerium oxide and many other ceramics and metal oxides; they exhibit positive ZPs at most pH values due to their very high isoelectric points.
Knowing where you are with respect to a material’s isoelectric point can help assess stability and performance in final applications. Similarly, the zeta potential of a colloidal system also demonstrates an ionic dependence at a given pH. All colloidal systems show a Gaussian relationship with respect to salt content. In the limit of zero salt, there are few ionic species present to suppress the electrical double layer and the zeta potential has a large absolute value. As the salt content of the solution is increased, the electrical double layer is compressed and the zeta potential decreases. After a certain point, the electrical double layer will collapse and it becomes the same as the surrounding media, leaving the particles prone to agglomeration effects. The specific concentration of salts that lead to this behavior is a material-dependent function.