Zeta potential is an electrokinetic property used to characterize nanoparticles and other colloidal systems in solution. It provides information about the electrical environment at the particle-liquid interface and can help researchers assess electrostatic stability, compare surface modifications, and understand how particles respond to changes in pH, ionic strength, and formulation conditions.
Zeta potential is not a direct measurement of surface charge. The measured electrophoretic mobility of particles is converted to zeta potential using an appropriate electrokinetic model, so meaningful interpretation requires information about both the nanoparticle and the surrounding dispersant.
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On this page
- What Zeta Potential Measures
- How Zeta Potential Is Measured
- Interpreting Zeta Potential
- Effects of pH and Ionic Strength
- Sample Preparation
- Evaluating Data Quality and Phase Plots
- Troubleshooting Zeta Potential Measurements
What Does Zeta Potential Measure?
When a nanoparticle is dispersed in a liquid, charge at the particle surface influences the distribution of ions in the surrounding medium. Counterions become concentrated near the particle, forming an electrical double layer.
The inner region, often described as the Stern layer, contains ions that are relatively strongly associated with the particle surface. Beyond it lies a diffuse layer containing more loosely associated ions.
As the particle moves through the liquid, part of this interfacial layer moves with it. The boundary between liquid that moves with the particle and the bulk dispersing medium is called the slipping plane or shear plane. Zeta potential is the electrical potential at this plane relative to the bulk liquid.
Zeta potential represents the electrical potential at the slipping plane surrounding a dispersed particle.
Zeta Potential Is Not the Same as Surface Charge
Surface charge influences zeta potential, but the two are not equivalent. Zeta potential also depends on the composition of the surrounding solution, ion adsorption, electrical double-layer thickness, surface-bound molecules, and the location of the slipping plane.
Two measurements of the same nanoparticle can therefore produce different zeta potentials if the pH, ionic strength, buffer composition, solvent, or other measurement conditions differ. These conditions should be reported whenever zeta potential data are compared.
Electrostatic vs. Steric Stabilization
Zeta potential is particularly useful for understanding electrostatic stabilization. Particles with sufficiently large zeta potentials of the same sign experience electrostatic repulsion that can reduce particle-particle association.
Colloidal stability can also arise from steric stabilization. Polymers, surfactants, ligands, or other surface-bound molecules can physically prevent particles from approaching closely enough to aggregate. A PEG-coated nanoparticle, for example, may remain highly stable even when its measured zeta-potential magnitude is relatively low.
For this reason, zeta potential should not be treated as a universal pass/fail test for nanoparticle stability.
How Is Zeta Potential Measured?
Most nanoparticle zeta potential measurements use electrophoretic light scattering. A nanoparticle dispersion is placed between electrodes and an electric field is applied. Charged particles move through the dispersing medium in response to the field, a phenomenon known as electrophoresis.
The instrument uses laser-based measurements to determine the particles' electrophoretic mobility, which describes their velocity relative to the applied electric field. Zeta potential is then calculated from the measured mobility using an appropriate electrokinetic model.
The Henry Equation
A common relationship between electrophoretic mobility and zeta potential is the Henry equation:
UE = 2εζf(κa) / 3η
where UE is electrophoretic mobility, ε is the dielectric permittivity of the dispersing medium, ζ is zeta potential, η is dispersant viscosity, and f(κa) is the Henry function. The parameter κa relates particle size to the thickness of the electrical double layer.
The theoretical model used for this conversion matters. The Smoluchowski approximation is commonly used when the electrical double layer is relatively thin compared with particle size, while the Hückel approximation applies when the double layer is relatively thick. More complete models may be appropriate for intermediate or more complex conditions.
Viscosity, dielectric properties, ionic environment, particle size, and the electrokinetic model used should therefore be considered when interpreting zeta potential results.
Phase Analysis Light Scattering
Malvern Zetasizer instruments use phase analysis light scattering (PALS) to sensitively measure electrophoretic mobility. The technique tracks changes in the phase of scattered light as particles move in an applied electric field.
Mixed-mode measurement approaches such as M3-PALS combine rapid and slower field reversals. The fast field reversal portion provides a robust measurement of mean electrophoretic mobility, while slower field reversal can provide additional information about the distribution of mobilities within the sample.
Zeta Potential Video Guides
For additional guidance, watch our videos on zeta potential fundamentals and zeta potential measurement and interpretation.
How to Interpret Zeta Potential
The magnitude and sign of zeta potential can provide useful information about nanoparticle electrostatic stability and surface behavior, but the result should always be interpreted in the context of the formulation.
| Zeta Potential Magnitude | General Electrostatic Interpretation |
|---|---|
| 0 to ±10 mV | Little electrostatic repulsion |
| ±10 to 20 mV | Limited electrostatic stabilization |
| ±20 to 30 mV | Moderate electrostatic stabilization |
| > ±30 mV | Stronger electrostatic stabilization |
These ranges are general rules of thumb for primarily electrostatically stabilized systems, not universal stability criteria. Steric stabilization, particle concentration, ionic strength, surface chemistry, and other formulation variables can substantially affect colloidal stability.
Monitoring Surface Modification
Zeta potential can also serve as a secondary indicator of changes in nanoparticle surface chemistry. For example, replacing a strongly negative citrate surface with a relatively neutral polymer such as PEG commonly decreases the magnitude of the measured zeta potential. Changing from a negatively charged surface to a positively charged amine-containing surface may cause the sign of the zeta potential to change.
Because solution conditions also influence zeta potential, a change in zeta potential alone does not prove that a surface modification was successful. Whenever possible, pair it with complementary characterization methods appropriate to the surface chemistry.
How pH and Ionic Strength Affect Zeta Potential
A zeta potential value without information about the dispersing medium has limited meaning. Two of the most important variables are pH and ionic strength.
pH and the Isoelectric Point
Changing pH alters the protonation state of many particle surfaces and surface-bound molecules. Decreasing pH generally shifts ionizable surfaces toward a more positive state, while increasing pH often shifts them toward a more negative state.
The isoelectric point (IEP) is the pH at which electrophoretic mobility and the corresponding zeta potential cross zero. Near the IEP, electrostatic repulsion is minimized, so particles that rely primarily on charge stabilization may become particularly susceptible to aggregation or agglomeration.
Measuring zeta potential across a range of pH values can therefore help identify the IEP and define formulation conditions where electrostatic stabilization is stronger or weaker.
The isoelectric point is the pH at which the measured zeta potential crosses zero.
Ionic Strength and Electrical Double-Layer Compression
Dissolved ions screen the electrical potential surrounding a charged particle. At relatively low ionic strength, the electrical double layer can extend farther into solution. Increasing ionic strength compresses the double layer and reduces the distance over which electrostatic interactions act.
As salt concentration increases, the measured zeta-potential magnitude often decreases. At sufficiently high ionic strength, electrostatic repulsion may no longer prevent particle-particle interactions.
The effect depends on ion identity, concentration, nanoparticle surface chemistry, pH, and other formulation variables. Measurements made in high-purity water therefore should not automatically be assumed to predict nanoparticle behavior in buffers, cell culture media, physiological solutions, or other high-ionic-strength environments.
Sample Preparation for Zeta Potential Measurements
Sample preparation can strongly influence zeta potential. The objective should be to obtain a reliable measurement while preserving the chemical environment relevant to the scientific question.
Sample Volume and Concentration
For conventional folded-capillary measurements, approximately 1 mL of sample is convenient, although lower-volume cells and specialized measurement approaches can reduce the required volume. When submitting samples for analysis, 1–2 mL generally provides sufficient material for measurement and repeat testing.
The sample must contain enough particles to generate a suitable scattering signal, but excessive concentration can reduce transmitted light, increase multiple scattering, or introduce particle-particle interactions.
The following values provide broad starting points for particle dispersions with optical properties and density similar to common reference particles:
| Particle Size | Approximate Minimum Concentration | Approximate Maximum Concentration |
|---|---|---|
| <10 nm | 0.5 mg/mL | Material-dependent |
| 10–100 nm | 0.1 mg/mL | ~5% mass |
| 100 nm–1 µm | 0.01 mg/mL | ~1% mass |
| >1 µm | 0.1 mg/mL | ~1% mass |
These concentrations are approximate starting ranges. The useful concentration range depends on particle size, density, refractive index, absorption, dispersant, and the specific instrument configuration and should be established experimentally for each material.
Small or Weakly Scattering Particles
Very small nanoparticles can be challenging to measure because they scatter much less light than larger particles. This is especially important for small particles that strongly absorb the measurement wavelength.
Weak scattering may require a higher particle concentration or longer acquisition. Measurement quality should be evaluated from the actual signal and repeatability rather than by applying a universal particle-size cutoff.
Particulate Contamination
Dust and unintended particulates can degrade measurement quality. Use clean labware, appropriate high-purity dispersants, and carefully prepared measurement cells.
Filtering the dispersant before preparing the nanoparticle sample can help minimize contamination. Filtering the nanoparticle suspension itself should be done only when removal of larger material is scientifically appropriate, because filtration may change the particle population being characterized.
Dilution
Dilution can improve measurement quality when a sample is too concentrated or highly absorbing. However, dilution can also change the zeta potential by altering pH, ionic strength, ligand concentration, or other equilibria at the particle surface.
Whenever possible, dilute with a matched dispersing medium rather than pure water. Record the dilution and measure or confirm relevant parameters such as pH and conductivity after dilution.
If reducing conductivity is intentionally part of the experimental question, dilution with lower-ionic-strength media can be useful, but the resulting zeta potential describes the diluted formulation, not necessarily the original sample.
High-Conductivity Samples
Conductive samples can present additional measurement challenges. High current during the applied electric field can contribute to Joule heating, electrode polarization, bubble formation, and electrode degradation.
Older folded-capillary systems may become increasingly challenging at conductivities above several mS/cm, although the practical limit depends strongly on the instrument, cell, applied field, and measurement protocol. High conductivity does not automatically make a measurement invalid.
When the goal is to characterize the material under application-relevant conditions, avoid lowering conductivity solely to produce a cleaner measurement. Instead, use an appropriate measurement method and evaluate whether heating, polarization, or electrode effects are influencing the result.
Aqueous and Nonaqueous Dispersants
Zeta potential measurements are commonly performed in aqueous dispersions, but suitable nonaqueous systems can also be measured when the dispersant has sufficient dielectric properties to support electrophoresis. Examples may include short-chain alcohols, THF, and other compatible solvents.
Cell compatibility should be confirmed before measuring nonaqueous formulations. Some solvents require specialized measurement cells rather than standard disposable folded-capillary cells. The dispersant viscosity and dielectric properties must also be known or appropriately estimated for calculation of zeta potential.
Folded capillary cells contain electrodes used to apply the electric field during electrophoretic zeta potential measurements.
Repeated measurements in highly conductive media can increase electrode polarization, heating, and electrode degradation. Cell performance should be monitored and periodically checked using an appropriate reference material.
Centrifugation and Reconstitution
Centrifuging nanoparticles and reconstituting them in a defined dispersant can sometimes remove residual synthesis components or create a controlled medium for comparative measurements.
This procedure also changes the chemical environment surrounding the nanoparticle and can alter surface equilibria, aggregation state, pH, and zeta potential. It should therefore be used as an intentional sample-conditioning step rather than a generic method for improving data quality.
Temperature
Temperature affects dispersant viscosity and can influence electrophoretic mobility. Allow samples to equilibrate to the measurement temperature and maintain consistent conditions when comparing formulations or monitoring changes over time.
Evaluating Zeta Potential Data Quality
A reliable zeta potential result should not be judged from the reported mean value alone. Evaluate repeatability, phase-plot quality, scattering signal, conductivity, sample behavior, and instrument quality indicators together.
Repeat Measurements
Collect multiple measurements whenever possible. Three repeat measurements provide a useful minimum, while five repeats can provide greater confidence for unfamiliar or challenging samples.
For a stable, high-quality sample, mean zeta potential values from repeated measurements should generally remain close to one another. A systematic trend across measurements can indicate sample instability, Joule heating, electrode polarization, or another time-dependent effect.
Allowing an appropriate pause between measurements can reduce heating and polarization effects in conductive samples.
How to Read a Phase Plot
On Malvern instruments using M3-PALS, the phase plot provides a useful view of measurement quality. It shows the change in phase between the measured scattering signal and a reference signal as a function of time.
During the fast field reversal (FFR) portion of the measurement, the applied electric field is rapidly reversed. A high-quality measurement produces clearly defined alternating phase slopes. These data are used to determine mean electrophoretic mobility and mean zeta potential.
The slow field reversal (SFR) portion uses a slower field sequence to obtain additional information about the distribution of electrophoretic mobilities. High-quality SFR data should show smooth, well-defined behavior rather than excessive noise or drift.
Noisy, poorly defined, or consistently trending phase data can indicate weak scattering, sample instability, excessive conductivity, bubbles, electrode problems, or other measurement limitations.
High-Conductivity Phase Data
High-conductivity samples may require reduced field strength or different measurement protocols to limit electrode and heating effects. Depending on the instrument and method, the measurement may rely more heavily on rapid field reversals rather than a full slow-field sequence.
Sharp, repeatable responses to each field reversal can still support a reliable mean zeta potential. Irregular, noisy, or drifting responses lower confidence in the result.
Automated Quality Indicators
Instrument software may provide an automated quality assessment based on phase data, signal-to-noise ratio, conductivity, repeatability, and other measurement parameters. These tools are useful, but they should support rather than replace inspection of the underlying data.
What to Report with Zeta Potential Data
A zeta potential value is most useful when the relevant measurement conditions are reported alongside it. Include:
- Mean zeta potential and measurement variability
- Dispersing medium and composition
- pH
- Conductivity or ionic strength when available
- Measurement temperature
- Particle concentration and any dilution
- Electrokinetic model or approximation used to convert mobility to zeta potential
- Relevant sample preparation or conditioning steps
Troubleshooting Zeta Potential Measurements
| Observation | Possible Causes and Next Steps |
|---|---|
| Weak or noisy phase data | Check particle concentration, scattering signal, cell cleanliness, bubbles, and optical absorption. A higher concentration may help weakly scattering samples. |
| Zeta potential drifts across repeated measurements | Evaluate Joule heating, electrode polarization, sample instability, temperature equilibration, or changing particle surface chemistry. Increase the pause between measurements when appropriate. |
| Large measurement-to-measurement variability | Check for aggregation, sedimentation, bubbles, particulate contamination, insufficient signal, or an unstable formulation. |
| High conductivity | Evaluate whether heating or electrode effects are occurring. Use an appropriate measurement protocol or cell rather than automatically diluting the sample if the original formulation conditions are important. |
| Unexpected result after dilution | Dilution may have changed pH, ionic strength, ligand concentration, or surface equilibria. Use a matched dispersant and document conditions. |
| Zeta potential near zero but dispersion remains stable | The material may rely on steric rather than electrostatic stabilization. Consider the surface chemistry and pair zeta potential with DLS or another stability measurement. |
| Visible bubbles near electrodes | Bubbles can interfere with electrophoretic measurements and may indicate electrochemical reactions. Remove bubbles and evaluate conductivity, applied voltage, and cell condition. |
| Electrode degradation or corrosion | High conductivity and repeated voltage application can accelerate electrode degradation. Check the measurement cell and verify instrument performance with an appropriate reference material. |
| Unexpected surface-charge trend | Confirm pH, ionic strength, dispersant composition, and surface treatment. Zeta potential is sensitive to both surface chemistry and solution conditions. |
A useful zeta potential measurement should answer a specific question about the material under defined conditions. Avoid altering the sample simply to produce a higher-magnitude zeta potential or cleaner phase plot if those changes make the measurement less representative of the system being studied.
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Selected References
- Bhattacharjee, S. DLS and zeta potential: What they are and what they are not? Journal of Controlled Release. 2016;235:337-351.
- Lowry, G.V., Hill, R.J., Harper, S., et al. Guidance to improve the scientific value of zeta-potential measurements in nanoEHS. Environmental Science: Nano. 2016;3:953-965.
- Lunardi, C.N., Gomes, A.J., Rocha, F.S., De Tommaso, J., and Patience, G.S. Experimental methods in chemical engineering: Zeta potential. Canadian Journal of Chemical Engineering. 2021;99:627-639.
- Clogston, J.D. and Vermilya, A. Measuring Zeta Potential of Nanoparticles. National Cancer Institute Nanotechnology Characterization Laboratory Assay Cascade Protocols.
- ISO 13099-2:2025: Colloidal systems — Methods for zeta-potential determination — Part 2: Optical methods. International Organization for Standardization.
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