Zeta potential analysis may be carried out by Microtrac instruments that operate on the basis of Dynamic Light Scattering (DLS). This product family consists of analyzers that provide information on particle size, zeta potential, concentration and molecular weight in a single system. Microtrac is a pioneer of particle size analysis and has been developing DLS analyzers for over 30 years.
When particles, droplets or colloids are present in a liquid, an electrical double layer is usually formed, consisting of ions in the liquid. This happens because particle surfaces usually carry a surface charge that is attractive to these ions. If the particle moves in the liquid, the electrical double layer moves with it, along the so-called slipping plane, i.e. the interface of the electrical double layer with the surrounding liquid. The electric potential at this slipping plane is the zeta potential. Zeta potential is specified in millivolts and is usually in the range between -200 mV and + 200 mV.
| Layer | Potential | |
| 1. | Surface charge (negative) | Surface potential |
| 2. | Stern layer | Stern potential |
| 3. | Slipping plane (shear plane) | ζ potential (zeta potential) |
When particles carry a strongly positive or strongly negative zeta potential, there is also a strongly repulsive electrostatic interaction between the particles. This prevents the particles from coming close to each other and forming agglomerates.
According to DLVO theory, when the particles are close to each other, van der Waals forces come into effect, which are based on dipole-dipole interactions. These forces have an attractive effect. At a zeta potential close to zero, the repulsive effect of the electrical double layer is small, and coagulation is more likely to occur.
The zeta potential is not a direct measurement of the stability of a dispersion, but it provides a good prediction of stability. Because analysis of the zeta potential is much easier and faster to perform than a stability measurement, the zeta potential is often used to assess the quality of the dispersion.
Changing the electrolyte composition and concentration leads to a shift of the zeta potental.
The following graph visualizes the effect with five examplatory samples:
(1) -20.6 mV (2) -16.8 mV (3) -9.9 mV (4) +13.9 mV (5) +15.1 mV
With the increasing addition of a positive polyelectrolyte, the particle size of the sample also changes:
(1) red, (2) green, (3) yellow, (4) blue, (5) purple
Zeta potential measurement is on the one hand based on particle properties, i.e. type of material and surface condition. On the other hand, it strongly depends on the dispersion liquid. Here, the type and concentration of the electrolytes (dissolved ions) play a decisive role.
Very often, the zeta potential is determined at different pH values, and a significant shift is observed depending on the material. In many cases, the zeta potential changes from positive to negative values as the pH increases. The pH at which the zeta potential is zero is also called the isoelectric point. Here, it is very likely that flocculation or agglomeration will occur because the electrical double layer is practically neutralized here.
Therefore, zeta potential measurement is often performed in combination with titration at different pH values.
There are various ways to analyze the Zeta Potential. The most prevalent technique is the so-called Laser Doppler Electrophoresis, which is also used in Microtrac particle analyzers. Microtrac analyzers for the measurement of zeta potential operate on Dynamic Light Scattering (DLS) technology and use the same power spectrum methodology that is applied to measure nanoparticles.
The laser-enhanced detection signals are detected in backscatter, as in the size measurement, and the rapid change in applied electric fields prevents electroosmosis. Two probes are used, one to determine the polarity of the particle charge (electrode) and one to measure the mobility of the particles in an electric field (optical probe).
In the sample cell, cationic (positive) particles are drawn to the optical probe and anionic (negative) particles to the electrode. The analysis is based on determining the mobility of charged particles in an alternating electric field.
1. excitation source | 2. Teflon zeta cell | 3. Backplate electrode | 4. optical probe
Zeta potential is thus determined by modulated power spectrum analysis of the combined Brownian motion and electric field driven motion (particle velocity). Zeta potential is proportional to mobility. To convert electrophoretic mobility to zeta potential, the following parameters must be considered: Dielectric constant and Henry coefficient.
Literature values are available for the dielectric constant. The Henry coefficient is based on the ratio of the thickness of the electric double layer to the particle size. Different models or approximations are used for this, depending on the type of dispersion. For aqueous systems this would be the Smoluchowski approximation, for non-polar systems the Hueckel approximation.
Both models are stored in the evaluation program of according Microtrac particle size analyzers.
Accurate particle size zeta potential analyzer, ideal for characterizing nanoparticles
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Zeta potential is the electrical potential at the shear plane of nanoparticles, droplets, or colloids. Dispersed nanoparticles in a liquid medium form a charge on the surface, the so-called double layer. This is compensated by the addition of counterions to the surface charge. If a particle moves in solution, the ions move with it, and a potential drop occurs between the different layers. This difference is called the zeta potential.
The zeta potential is measured indirectly via the electrophoretic mobility of particles. There are various ways to analyze the Zeta Potential, mostly Laser Doppler Electrophoresis is used. During a measurement, positive particles are drawn to the anode and negative particles to the cathode which determine the mobility of charged particles in an alternating electric field. The zeta potential is calculated from the mobility via the Henry or Smoluchowski equation.
Zeta potential can be indicator for the stability of a dispersion or emulsion. In general, the higher the magnitude of the potential, the better the stability of the dispersion or emulsion. For the stability it does not matter which sign (positive or negative) the dispersion has. However, the sign of the dispersion can have a huge impact in the application of the dispersion.
The zeta potential can be influenced by many factors like pH value or conductivity. Both play a key role on the magnitude and sign of the zeta potential. Polyelectrolytes can have a similar influence. If the sign changes, it will pass the isoelectric point (pH) or the zero point of charge (Polyelectrolytes). At this points the zeta potential is ±0. Strong dilution can also occur this effect.
