Zeta Potential

Almost all particulate or macroscopic materials in contact with a liquid acquire an electronic charge on their surfaces. Zeta potential is an important and useful indicator of this charge which can be used to predict and control the stability of colloidal suspensions or emulsions.

What is Zeta Potential?

A Particle with a formal surface charge for Zeta Potential
A particle with a formal surface charge will attract small ions of opposite charge resulting in the formation of the electric double layer.

The Zeta potential (ζ) of a colloid is controlled by the screened electrostatic potential of a colloidal particle, it reflects the magnitude of the electrostatic charge that develops at the interface between a solid surface and its liquid medium.

The inner layer is referred to as the Stern layer. The net charge at the particle surface impacts the distribution of ions with the concentration of counterions increasing as you move closer to the surface of the colloidal particle.

The layer typically referred to as the slipping plane serves as the boundary between strongly bound ions and bulk solution. The potential measured at this boundary is defined as the zeta potential. The zeta potential, as commonly measured by light scattering, reflects the motion of the particle in response to an electric field.  The motion of the particle is controlled by the potential at the slipping plane, and as such the zeta potential will always be proportional to, but lower in magnitude than, the true surface potential. The zeta potential is typically reported in millivolts (mV).

Several factors can control the zeta potential of a given colloidal particle, the most direct of which is the surface charge, as generated by the dissociation of ion-carrying side chains (ex, titratable groups such as carboxyl groups, primary and secondary amines, and any other pH or redox-sensitive groups), or by the presence of permanent charge carriers (ex. sulfonates, quaternary amines, etc.).

Why is it important?

Almost all nanomaterials in contact with a liquid carry a formal charge on their surfaces. Zeta potential is an important and useful indicator of the magnitude of this surface charge. It is used to predict and control the stability of colloidal suspensions and emulsions as well as to predict the tendency of particles to aggregate or self-assemble into larger, potentially unstable, structures.

Negatively Charged Particle for Zeta Potential
The stability of a charged particle in a continuum is based on the fact that like-charged particles with a sufficiently high surface charge will repel one another.

This tendency for charged particles to resist attractive forces in this way is what is usually referred to as colloidal stability. This balance between long-range repulsion and short-range attraction is what allows colloids to remain stably dispersed. 

There are a wide range of industries that require their final product to have this kind of colloidal stability, essentially stability against aggregation or self-association. Whether paints, pigments, cosmetics, or food manufacturing, they all can, or will, use zeta potential as an indicator for the stability of their final product to maintain a desired consistency.

Uncharged or reduced charge particles for Zeta Potential
When surface charge is reduced this has the potential to compromise the colloidal stability of a collection of particles.

As the zeta potential approaches zero, the electrostatic barrier between particles is reduced, and these particles will begin to cluster together, which often leads to open-ended aggregation or agglomeration as particles adhere to one another.

A common way to leverage this loss of colloidal stability is with the use of coagulants, which are used to induce agglomeration. This is commonly done in water, wastewater and the pulp and paper markets. In the treatment of surface water coagulants are used to create a floc from particles that are 5 µm or smaller that would ordinarily not settle out without treatment. The formation of a floc allows for the particulate to be removed from the water so that it meets the EPA guidelines for the maximum turbidity level for drinking water.

Stable and Instable chart for Zeta Potential

In summary, the absolute value of the zeta potential is a predictor of the stability/instability of a suspension or emulsion. As values approach 0 mV there will be a tendency for particles to adhere to each other while at greater values > +/-20 mV the particles will be stabilized and remain in suspension.

How is Zeta Potential Measured?

Measuring Zeta Potential

Brookhaven Instruments provide zeta potential instrumentation based on the principles of electrophoretic light scattering. We offer two methodologies that use similar instrument designs. The methodologies are based on Laser Doppler Electrophoresis (LDE), for historical reasons referred to here as Electrophoretic Light Scattering (ELS) and Phase Analysis Light Scattering (PALS).

To measure zeta potential an electrode assembly is placed into a cuvette that contains the sample of interest. The electrode is made with two palladium plates to which a voltage is applied to create an electric field.  When the voltage is applied to the electrode the particles will move towards the pole of the opposite charge. The electrophoretic velocity or movement of the particle is determined by using a laser light source and measuring scattered light using an Avalanche Photodiode (APD) positioned at a 15o scattering angle (forward scatter). Sample scattering along with reference scattering are optically mixed, and the resulting signal is used to determine the electrophoretic velocity.

Zeta Potential block diagram

The diagram below shows the overall block diagram of an instrument designed to measure zeta potential. It is seen that the laser light source is split into a sample beam and a reference beam. The sample beam will pass through the cuvette with the zeta potential electrode.  The reference beam is oscillated at a known frequency by a piezoelectric transducer.

Electrophoretic Light Scattering (ELS)

Here, ELS refers specifically to laser doppler electrophoresis, the most general form of electrophoretic light scattering.  With ELS, zeta potential is determined by measuring the doppler shift that occurs when a particle moves towards a given electrode.  This electrophoretic velocity is what is used to calculate zeta potential.

ELS can resolve simple multimodal distributions and offers high accuracy in low salt aqueous media.

Electrophoretic Light Scattering (ELS) chart

Phase Analysis Light Scattering (PALS)

With PALS the zeta potential is determined by measuring a phase shift between reference and sample signals.

Phase Analysis Light Scattering (PALS) Chart

The PALS method is 1000x more sensitive than ELS.  Having a higher sensitivity makes it ideal for particles with low mobilities caused by high salt, organic solvents, or viscous media. It is also ideal for proteins, small peptides, antibodies, oligonucleotides, and other biological samples due to the lower voltage being applied to the electrode, which minimizes the denaturing effects of higher voltages used with other LDE technologies.

Brookhaven Instruments offer multiple instruments for measuring zeta potential. We offer four models to choose from:

1. Economical Zeta Plus that measures zeta potential by using ELS

2. ZetaPALS offers both ELS and PALS technologies

3. 90Plus PALS that offers both ELS and PALS technologies for zeta potential measurement and also measures particle size by DLS using 15o or 90o  scattering angles.

4. Omni that has all of the same specifications as the 90Plus PALS but includes particle size by a 173o detection angle that is ideal for smaller nanoparticles of < 50 nm

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