Discussion Examples for Zeta Potential Measurement | Relationship Between Dispersion Stability and Aggregation

What Is Zeta Potential?

Zeta potential is the electric potential near the boundary between the liquid layer that moves together with a particle and the outer liquid when a particle dispersed in a solution moves.
This boundary is called the slipping plane.
Although zeta potential does not directly represent the potential of the particle surface itself, it is an important indicator that reflects the charge state of the particle surface and the distribution of surrounding ions.

Zeta potential is related to the strength of electrostatic repulsion between particles.
When the absolute value of the zeta potential is large, particles carrying charges of the same sign repel one another strongly and the dispersed state tends to be maintained.
On the other hand, when the absolute value of the zeta potential is small, repulsion between particles becomes weaker and aggregation becomes more likely.

Main Items to Include in the Results

In the results of zeta potential measurement, organize the sample name, particle concentration, dispersion medium, pH, electrolyte concentration, measurement temperature, average particle size, zeta potential, number of measurements, standard deviation, particle-size distribution, appearance of the dispersed state, and other information.
Because zeta potential is strongly affected by pH and salt concentration, the measurement conditions must always be recorded.

Main Items to Include in the Results

• Type of particle or colloid measured
• Type of dispersion medium
• Particle concentration
• pH
• Type of electrolyte
• Electrolyte concentration
• Measurement temperature
• Average particle size
• Particle-size distribution
• Average zeta potential
• Standard deviation of zeta potential
• Number of measurements
• Appearance of the dispersion
• Presence or absence of aggregation or sedimentation
• Presence or absence of an isoelectric point
• Causes of error and points for improvement

Reference Experimental Values and Calculation Examples for Zeta Potential Measurement

Here, changes in zeta potential and dispersion stability when the pH or electrolyte concentration of a dispersion is changed are organized using reference experimental values.

Zeta potential is an indicator that reflects the electrical state formed near the surface of dispersed particles.
In general, the larger the absolute value of the zeta potential, the more strongly the particles tend to repel one another and the higher the dispersion stability is considered to be.
On the other hand, as the zeta potential approaches 0 mV, electrostatic repulsion between particles weakens and aggregation becomes more likely.

Reference Experimental Conditions

Zeta Potential When pH Is Changed

First, the zeta potential is compared when the pH of the dispersion is changed.
Because the charge on the particle surface changes with pH, the zeta potential may differ under acidic, nearly neutral, and alkaline conditions.

Example Calculation of Average Zeta Potential

For Sample A, suppose the measured zeta potentials were +31.8 mV, +33.2 mV, and +32.5 mV.
The average value is calculated as follows.

Average zeta potential = (31.8 + 33.2 + 32.5) ÷ 3

Average zeta potential = 97.5 ÷ 3 = 32.5 mV

Therefore, the average zeta potential of Sample A is +32.5 mV.
Because it is a positive value with a relatively large absolute value, electrostatic repulsion between the particles is considered to act effectively, making it easier to maintain the dispersed state.

Relationship Between the Absolute Value of Zeta Potential and Dispersion Stability

For zeta potential, it is important to consider not only the sign but also the magnitude of the absolute value.
Whether positive or negative, a large absolute value tends to result in greater electrostatic repulsion between particles and higher dispersion stability.

Concept of the Isoelectric Point

The pH at which the zeta potential becomes close to 0 mV is called the isoelectric point.
Near the isoelectric point, the apparent charge on the particle surface becomes small and electrostatic repulsion between particles weakens, making aggregation more likely.

In this reference example, the zeta potential is +8.4 mV at pH 5.0 and -6.1 mV at pH 7.0, and the sign of the zeta potential changes from positive to negative within this range.
Therefore, the isoelectric point is considered to lie around pH 5 to 7.

Aggregation and sedimentation were also observed after 24 hours in the samples at pH 5.0 and pH 7.0, so it can be explained that dispersion stability decreased under conditions where the zeta potential was close to 0 mV.

Zeta Potential When Salt Concentration Is Changed

Next, the zeta potential is compared when the NaCl concentration is varied under the same pH conditions.
As the electrolyte concentration increases, the electrical double layer on the particle surface may be compressed and the electrostatic repulsion between particles may weaken.

In these results, the absolute value of the zeta potential became smaller as the NaCl concentration increased, and the dispersion became more prone to aggregation and sedimentation.
This was considered to result from shielding of the effect of the particle-surface charge by the electrolyte, which weakened the repulsion between particles.

Observation Results for Dispersion Stability

The zeta potential measurement results are easier to discuss when evaluated together with observations of the actual appearance of the dispersion.
Here, the appearance immediately after measurement and after 24 hours is compared.

Example of How to Write the Results

When the zeta potential was measured while varying the pH, it was +32.5 mV at pH 3.0, +8.4 mV at pH 5.0, -6.1 mV at pH 7.0, -25.2 mV at pH 9.0, and -38.8 mV at pH 11.0.
Because the sign of the zeta potential changed from positive to negative between pH 5.0 and pH 7.0, the isoelectric point of these particles was considered to be around pH 5 to 7.

In addition, little sedimentation occurred in the samples at pH 3.0 and pH 11.0, where the absolute value of the zeta potential was large.
On the other hand, aggregation and sedimentation were observed after 24 hours in the samples at pH 5.0 and pH 7.0, where the zeta potential was close to 0 mV.
These results suggest that under conditions where the absolute value of the zeta potential is small, electrostatic repulsion between particles becomes weak and dispersion stability decreases.

Points for Connecting the Results to the Discussion

In a discussion of zeta potential measurement, it is important to explain not only whether the value is positive or negative but also the magnitude of the absolute value together with the appearance of the dispersion.

• Was the dispersion stable under conditions where the absolute value of the zeta potential was large?
• Did aggregation or sedimentation occur under conditions where the zeta potential was close to 0 mV?
• Can the surface charge state of the particles be considered to have changed because of a change in pH?
• Can the pH near which the sign of the zeta potential changes be explained as the isoelectric point?
• Can the increase in salt concentration be considered to have compressed the electrical double layer and reduced dispersion stability?
• Was the standard deviation of the measured values small and was the measurement reproducible?
• Could particle concentration, pH adjustment, bubbles, sedimentation, or contamination with aggregates have affected the measured values?

Example Discussion

In this experiment, the zeta potential changed greatly depending on the pH of the dispersion.
The average zeta potential was +32.5 mV at pH 3.0, whereas it was -38.8 mV at pH 11.0, showing a tendency for the particles to be positively charged under acidic conditions and negatively charged under alkaline conditions.
In addition, because the sign of the zeta potential changed between pH 5.0 and pH 7.0, the isoelectric point of these particles was considered to be around pH 5 to 7.

Regarding dispersion stability, the samples at pH 3.0 and pH 11.0, where the absolute value of the zeta potential was large, showed little sedimentation even after 24 hours and maintained a relatively stable dispersed state.
On the other hand, at pH 5.0 and pH 7.0, the zeta potential was close to 0 mV and aggregation and sedimentation were confirmed.
This was considered to result from weakened electrostatic repulsion between the particles, making it easier for them to approach one another.

In addition, when the NaCl concentration was increased, the absolute value of the zeta potential became smaller and the dispersion stability also decreased.
This was considered to result from compression of the electrical double layer on the particle surface as the electrolyte concentration increased, weakening the electrostatic repulsion between particles.
Therefore, to maintain a stable dispersion, it is necessary to appropriately control not only the pH but also the electrolyte concentration.

However, zeta potential may be affected by particle concentration, the accuracy of pH adjustment, bubbles in the measurement cell, the presence of aggregates, and other factors.
Therefore, it is important to sufficiently homogenize the dispersion before measurement and perform multiple measurements to confirm the average value and variation.

Summary

In zeta potential measurement, dispersion stability and the tendency for aggregation can be evaluated by investigating the surface charge state of dispersed particles.
In this reference example, the dispersion was stable under conditions where the absolute value of the zeta potential was large, whereas aggregation and sedimentation were more likely under conditions close to 0 mV.

In a report, presenting the average zeta potential, standard deviation, relationships with pH and salt concentration, and observations of the appearance of the dispersion together makes it easier to explain “why the dispersion was stable” and “why aggregation occurred.”

Discussion of Particle Surface Charge

Colloidal particles may carry surface charges because of dissociation of surface functional groups, adsorption of ions, defects in the crystal lattice, pH conditions, and other factors.
For example, on oxide particles, hydroxyl groups on the surface may accept or release H+, causing the surface to become positively or negatively charged.
In polymer particles and protein particles as well, the surface charge changes through ionization of functional groups.

Particle surface charge is directly related to repulsion and aggregation between particles.
Particles carrying charges of the same sign electrostatically repel one another and therefore tend to maintain the dispersed state.
When the surface charge becomes smaller, the repulsive force weakens and particles can approach one another and aggregate more easily.

What Is the Electrical Double Layer?

The electrical double layer is a structure in which ions with the opposite charge gather around a charged particle surface and the charge is distributed in layers.
The particle surface has a surface charge, and ions with the opposite sign gather strongly near it.
Further outside is a diffuse layer spread by thermal motion.

When the electrical double layer is thick, the electrical double layers of two approaching particles overlap and strong repulsion occurs.
Therefore, the particles are less likely to aggregate.
As the electrolyte concentration increases, the number of ions in the solution increases and the electrical double layer is compressed.
As a result, repulsion between particles becomes weaker.

The Slipping Plane and Zeta Potential

When a particle moves in an electric field, part of the liquid close to the particle surface moves together with the particle.
However, liquid located a certain distance away from the particle does not move together with it.
The boundary between these regions is called the slipping plane.
Zeta potential is treated as the potential at this slipping plane.

Therefore, zeta potential is not the potential of the particle surface itself.
It changes depending on surface charge, adsorbed ions, the viscosity of the dispersion medium, electrolyte concentration, pH, and other factors.
Because zeta potential is related to the electrophoretic behavior actually shown by particles in solution, it is a practical indicator of dispersion stability.

Meaning of the Sign of Zeta Potential

When the zeta potential is positive, the particle near the slipping plane is considered to be positively charged.
When the zeta potential is negative, the particle near the slipping plane is considered to be negatively charged.
The sign changes depending on the functional groups on the particle surface, adsorbed ions, pH, and conditions of the dispersion medium.

Particles with zeta potentials of the same sign electrostatically repel one another.
However, when considering dispersion stability, the magnitude of the absolute value is often more important than the sign itself.
Whether positive or negative, a large absolute value is considered to produce strong repulsion, while a small absolute value makes aggregation more likely.

Absolute Value of Zeta Potential and Dispersion Stability

The larger the absolute value of the zeta potential, the stronger the electrostatic repulsion between particles.
Therefore, particles become more difficult to approach one another and the dispersed state tends to be more stable.
In general, dispersions with zeta potentials having large absolute values tend to be less prone to sedimentation or aggregation.

On the other hand, when the zeta potential is close to 0, the electrostatic repulsion between particles becomes weak.
In this state, attractive forces such as van der Waals forces become relatively stronger, making it easier for particles to approach one another and aggregate.
Therefore, the absolute value of the zeta potential serves as a guideline for evaluating dispersion stability.

Why Aggregation Occurs

Van der Waals forces act between colloidal particles and attract them to one another.
On the other hand, when particles carry charges of the same sign, electrostatic repulsive forces act between them.
Whether a dispersion is stable is determined by the balance between these attractive and repulsive forces.

When the absolute value of the zeta potential is large, the repulsive force exceeds the attractive force, making aggregation less likely.
However, when the zeta potential becomes small, the repulsive force weakens and the particles become more likely to aggregate through attractive forces when they approach one another.
Aggregated particles have a larger apparent particle size and may be observed as sedimentation or changes in turbidity.

Relationship with DLVO Theory

DLVO theory describes interactions between colloidal particles as the sum of electrostatic repulsive forces and van der Waals attractive forces.
When there is a sufficiently large repulsive-energy barrier between particles, the particles are difficult to approach and the dispersion remains stable.
Conversely, when this energy barrier becomes small, the particles become more likely to aggregate because of attractive forces.

Zeta potential provides a clue for considering the magnitude of electrostatic repulsion in DLVO theory.
The larger the absolute value of the zeta potential, the stronger the repulsive force and the higher the energy barrier is considered to be.
When the electrolyte concentration increases, the electrical double layer is compressed and the repulsive force weakens, making aggregation more likely.

Effect of pH

pH greatly affects the ionization state of functional groups on particle surfaces.
For example, oxide particles have -OH groups on their surfaces, which tend to accept H+ and become positively charged under acidic conditions and lose H+ and become negatively charged under basic conditions.
Therefore, changing the pH changes the sign and magnitude of the zeta potential.

When the pH is changed, the zeta potential may change from positive to negative or from negative to positive.
At a pH where the zeta potential approaches 0, repulsion between particles weakens and aggregation becomes more likely.
The pH near this point may be called the isoelectric point.

Acidic side: The surface readily accepts H+ and tends to become positively charged.

Basic side: The surface readily loses H+ and tends to become negatively charged.

Discussion of the Isoelectric Point

The isoelectric point refers to the pH at which the zeta potential of particles becomes close to 0 or to conditions where the positive and negative charges of the particles appear to balance.
Near the isoelectric point, electrostatic repulsion between particles becomes extremely weak.
Therefore, particles become more likely to aggregate, and sedimentation or an increase in particle size may be observed.

For proteins, metal oxides, mineral particles, and other materials, the isoelectric point is strongly related to dispersion stability and separation operations.
If the zeta potential crosses 0 in an experiment where pH is varied, that pH can be discussed as the isoelectric point.
At pH values away from the isoelectric point, particles may be strongly positively or negatively charged and dispersion stability may increase.

Effect of Electrolyte Concentration

Increasing the electrolyte concentration increases the number of ions in the solution.
This may compress the electrical double layer around the particles and reduce the potential at the slipping plane, that is, the absolute value of the zeta potential.
As a result, electrostatic repulsion between particles weakens and aggregation becomes more likely.

At low electrolyte concentrations, the electrical double layer is relatively thick and strong repulsion acts when particles approach one another.
At high electrolyte concentrations, the electrical double layer becomes thinner and particles can approach one another more easily.
Therefore, if particle size increases or sedimentation occurs under conditions where salt is added, a decrease in zeta potential caused by the electrolyte should be considered.

Effect of Ionic Valence

The valence of electrolyte ions also affects zeta potential and aggregation.
Multivalent ions with a charge opposite to that of the particle surface may strongly neutralize the particle-surface charge and greatly compress the electrical double layer.
Therefore, divalent ions may cause aggregation more readily than monovalent ions, and trivalent ions more readily than divalent ions, even in small amounts.

This is also related to the Schulze-Hardy rule known in colloid coagulation.
However, in practice, ion hydration, specific adsorption, pH changes, and functional groups on the particle surface also have effects.
If the zeta potential changes greatly when multivalent ions are added, not only ionic strength but also adsorption onto the surface must be considered.

Relationship Between Particle Size and Zeta Potential

Zeta potential is an indicator of the electrical state near the particle surface and does not directly represent particle size itself.
However, when particles aggregate, the apparent particle size increases, and this often occurs under conditions where the absolute value of the zeta potential is small.
Therefore, combining particle-size measurement with zeta potential measurement allows dispersion stability to be evaluated in greater detail.

If the absolute value of the zeta potential is small and the average particle size is large, the particles may be aggregated.
On the other hand, if the absolute value of the zeta potential is large and the particle-size distribution is narrow, the particles may be relatively well dispersed.
However, particle size is also affected by particle shape, concentration, measurement method, and sedimentation.

Relationship with the Appearance of the Dispersion

Changes in zeta potential may also affect the visible appearance of a dispersion.
When the dispersion is stable, particles remain finely and uniformly dispersed, and sedimentation or large aggregates are less likely to be observed.
On the other hand, as aggregation progresses, changes in turbidity, precipitation, clarification of the supernatant, and particle sedimentation may be observed.

However, it may not always be possible to accurately judge dispersion stability from appearance alone.
If particles remain as fine aggregates, the dispersion may still look cloudy even though the particle size has increased.
Therefore, it is important to discuss zeta potential, particle size, and appearance together.

Effect of Particle Concentration

In zeta potential measurement, particle concentrations that are either too high or too low may affect the measured value.
If the particle concentration is too high, multiple scattering and particle-particle interactions may become strong and the measurement may become unstable.
Aggregation may also proceed more readily and affect the apparent zeta potential and particle size.

If the particle concentration is too low, the measurement signal may become weak and the variation in the data may become large.
Therefore, measurement must be performed within a concentration range suitable for the apparatus.
When the sample is diluted, care must be taken not to change the pH or electrolyte concentration.

Effects of Dispersion-Medium Viscosity and Dielectric Constant

Because zeta potential is often converted from electrophoretic mobility, the viscosity and dielectric constant of the dispersion medium are involved in the calculation.
When solvents other than water are used, or when polymers, sugars, salts, or other substances in the solution change the viscosity, the measured and converted values may be affected.

In a highly viscous dispersion medium, particle movement becomes slower.
Changes in dielectric constant also affect the electrical double layer and electrophoretic behavior.
If the dispersion-medium conditions entered into the measurement apparatus differ from the actual conditions, errors may occur in the calculated zeta potential.

Discussion of the Measurement Principle

Zeta potential is often measured by electrophoresis.
When an electric field is applied, charged particles move toward the electrode with the opposite sign.
The speed of this movement is measured as electrophoretic mobility and is converted to zeta potential using the viscosity and dielectric constant of the dispersion medium.

In other words, zeta potential measurement reflects how particles actually move in an electric field.
If particles have aggregated and become large or are sedimenting, the measured mobility may become unstable.
Understanding the measurement principle makes it easier to consider the causes of data variation and abnormal values.

Causes of Error in Zeta Potential Measurement

Causes of error in zeta potential measurement include inappropriate particle concentration, the presence of aggregated particles, bubbles, contamination of the measurement cell, errors in pH adjustment, deviations in electrolyte concentration, temperature changes, incorrect settings for dispersion-medium viscosity, sedimentation, and changes in the sample over time.
Because zeta potential is sensitive to the state of the dispersion, even small differences in sample preparation can change the value.

The results may also change depending on whether ultrasonic dispersion or stirring is performed before measurement.
Strong dispersion may break apart aggregates and change the zeta potential and particle size, while excessive treatment may alter the particle surface.
It is important to standardize the measurement conditions and confirm reproducibility by performing multiple measurements.

When the Results Can Be Considered Good

Zeta potential measurement can be considered to have produced good results when repeated measurements under the same conditions show little variation and consistent trends are obtained in response to changes in pH or electrolyte concentration.
For example, if the absolute value of the zeta potential decreases as the electrolyte concentration increases and aggregation or an increase in particle size is observed, the result can be explained as a decrease in dispersion stability caused by compression of the electrical double layer.

In an experiment where pH is varied, if the zeta potential changes from positive to negative and aggregation becomes more likely near the pH where the value is close to 0, the result can be considered reasonable and demonstrates the relationship between the isoelectric point and dispersion stability.
It is important that changes in zeta potential, particle size, and appearance correspond to one another.

Example Discussions When the Experiment Did Not Go Well

When zeta potential measurement does not go well, possible causes can be considered from results such as large variation in measured values, unstable positive and negative signs, poor reproducibility for the same sample, lack of correspondence with particle-size measurement, or absence of the expected pH dependence.
Organizing the causes according to sample preparation, particle concentration, pH, electrolyte concentration, aggregation, bubbles, cell contamination, and dispersion treatment makes the discussion easier.

How to Write Points for Improvement

In a discussion of zeta potential measurement, writing not only the causes of error but also points for improvement makes the report easier to organize.
Points for improvement can be organized according to sample preparation, pH and electrolyte conditions, measurement operation, and data analysis.

Improvements to Sample Preparation

• Adjust the particle concentration to a range suitable for the apparatus
• Disperse the particles uniformly before measurement
• Remove large aggregates
• Do not change the pH or electrolyte concentration during dilution
• Use the same dispersion-treatment conditions
• Standardize the standing time before measurement

Improvements to pH and Electrolyte Conditions

• Measure pH accurately
• Allow sufficient equilibration time after pH adjustment
• Prepare the electrolyte concentration accurately
• Distinguish between pH changes and changes in ionic strength
• Keep the temperature constant
• Correctly set the viscosity and dielectric constant of the dispersion medium

Improvements to the Measurement Operation

• Thoroughly clean the measurement cell
• Avoid introducing bubbles
• Perform the measurement before sedimentation occurs
• Perform multiple measurements and calculate the average value and standard deviation
• Use particle-size measurement together with zeta potential measurement
• Record appearance observations as well

Difference Between a Superficial Discussion and a Good Discussion

In a discussion of zeta potential measurement, simply writing that “the value was large, so it was stable” or “it was close to 0, so aggregation occurred” results in a superficial discussion.
A good discussion relates particle surface charge, the electrical double layer, electrostatic repulsion, van der Waals forces, pH, electrolyte concentration, and particle-size changes.

Examples of Expressions That Can Be Used in Reports

The following expressions can be used when writing the results and discussion of zeta potential measurement.
Adjust the necessary parts according to your own experimental results.

• Zeta potential is the potential at the slipping plane surrounding a particle.
• Zeta potential reflects the particle-surface charge and the state of the electrical double layer.
• The larger the absolute value of the zeta potential, the stronger the electrostatic repulsion between particles.
• As the zeta potential approaches 0, particles become more likely to aggregate.
• As the electrolyte concentration increases, the electrical double layer may be compressed and the absolute value of the zeta potential may decrease.
• pH changes the ionization state of functional groups on the particle surface and affects the zeta potential.
• Near the isoelectric point, repulsion between particles is weak and aggregation or sedimentation is more likely.
• If a decrease in zeta potential corresponds to an increase in particle size, aggregation can be considered to have progressed.
• In DLVO theory, dispersion stability is explained by the balance between electrostatic repulsive forces and van der Waals attractive forces.
• Variation in measured values is affected by particle concentration, aggregation, pH, electrolyte concentration, bubbles, and cell contamination.

Points to Check When Discussing Zeta Potential Measurement

Checking the following points before writing the report makes the discussion easier to write.

• Is the definition of zeta potential explained?
• Is the concept of the slipping plane described?
• Are particle surface charge and the electrical double layer related?
• Are the sign and absolute value of the zeta potential distinguished?
• Is dispersion stability explained in terms of electrostatic repulsion?
• Is aggregation considered in terms of the balance with van der Waals forces?
• Is the change in zeta potential with pH explained?
• Are the isoelectric point and the tendency for aggregation related?
• Are electrolyte concentration and compression of the electrical double layer explained?
• Are changes in particle size and appearance related to zeta potential?
• Are measurement errors related to sample preparation and cell condition?
• Do the points for improvement correspond to the causes of error?

Summary

Zeta potential measurement is an important measurement for evaluating the dispersion stability of colloidal particles and fine particles.
Zeta potential is the potential at the slipping plane when a particle moves in solution and reflects the particle-surface charge and the state of the electrical double layer.
The larger the absolute value of the zeta potential, the stronger the electrostatic repulsion between particles and the more stable the dispersed state tends to be.

On the other hand, as the zeta potential approaches 0, repulsion between particles weakens and the particles become more likely to aggregate because of attractive forces such as van der Waals forces.
Changes in pH alter the ionization state of functional groups on the particle surface and change the sign and magnitude of the zeta potential.
In addition, as the electrolyte concentration increases, the electrical double layer may be compressed, the absolute value of the zeta potential may decrease, and aggregation may become more likely.

In a report, rather than simply writing that “the zeta potential was large” or “aggregation occurred,” organize and discuss particle surface charge, the electrical double layer, the slipping plane, pH, electrolyte concentration, the isoelectric point, DLVO theory, particle-size changes, appearance observations, causes of error, and points for improvement.
Zeta potential measurement is an important experiment for quantitatively understanding the relationship between colloidal dispersion stability and aggregation.