Confused on what a zeta potential measurement is and when it’s useful? You are not alone. In this helpful Q and A, Laboratory Division Manager Ryan Keefer walks us through the basics of zeta potential, points out factors to consider when measuring, and takes us on a little bit deeper dive into the specifics of measurement. This quick read will help you to know what zeta potential is, how it’s measured and when to use it.
Q: What Is Zeta Potential?
Zeta potential per ISO 13099-1:2021(E)(2017) is defined as “difference in electric potential between that at the slipping plane and that of the bulk liquid.” Simply put, it is the charge that develops at the interface between a solid surface and its liquid medium. This interface is called the Slipping plane.
Q: Why Is Zeta Potential Important?
The importance of zeta potential is relative to specific industry goals:
For pharmaceuticals, it can be an indicator of stability of a colloidal suspension. Colloidal suspensions can have many different stabilizing mechanisms; one stabilization mechanism is charge stabilization. Since zeta potential is relative to the surface charge, a high magnitude zeta potential result can prevent particles interacting with each other and agglomerating/aggregating.
In water reclamations, a low zeta potential is often desired. To remove unwanted particulate from water, one step used in water purification is allowing particulate to settle in a tank; smaller particles may remain suspended and will not settle under normal conditions. Knowing the conditions to minimize the surface charge of the suspended particles can induce agglomeration/aggregation, which will then allow the agglomerates/aggregates to settle. Inducing agglomeration/aggregation through charge destabilization allows gravity to influence the aggregates/agglomerates opposed to the diffusional forces that had previously kept the particulate suspended. For water reclamations studies, isoelectric determination studies are recommended.
Q: How Is Zeta Potential Measured?
At Particle Technology Labs, we use electrophoretic mobility as the method for determining zeta potential. Other methods such as an acoustic method exist but will not be discussed in this FAQ.
To start, dynamic light scattering (DLS) is utilized as a base technique. DLS evaluates particle size of suspended material freely moving in Brownian motion. Thus, the sample has to meet all the requirements for DLS to be suitable for zeta potential by electrophoretic mobility.
In order to obtain a zeta potential result, the suspended material is exposed to alternating positive and negative currents (electrophoresis). The suspended particles are then monitored for movement (mobility) towards either the positive or negative pole. The direction of the movement relates to the sign (positive/negative) of the zeta potential, and the rate of the movement relates to the magnitude of the zeta potential.
It should be noted that zeta potential is not a directly measurable value, but a theoretical value (ISO 13099-1-2012(2017)). As such, replicate analyses are recommended in order to determine the variance to be expected with a material.
Q: What Is the Slipping Plane?
Slipping plane (or shear plane) per ISO 13099-1:2021(E)(2017) is defined as “abstract plane in the vicinity of the liquid/solid interface where liquid starts to slide relative to the surface under influence of a shear stress.”
In simpler terms, a double layer of ions surrounds a particle in an aqueous medium. This double layer of ions is proportional to the surface charge of the particle and moves as one with the particle. The particle plus the ionic double layer forms the hydrodynamic diameter of the particle. The zeta potential is the relative potential at the edge of this double layer (shear plane/slipping plane). This means the zeta potential is relative to the surface charge, but not the surface charge of the particle.
Q: What Is Isoelectric Point and Why Is It Important?
A: Isoelectric point is the point of zero zeta potential. This value is usually determined by analyzing the sample across a range of pH values.
Traditionally isoelectric point determinations are conducted using a pH titration. By changing the suspension pH, functional groups on the suspended material can ionize. A perfect example are proteins/amino acids with a carboxyl and amine functional group in their structure. As a simple demonstration, an amino acid structure is shown here:
As pH is increased, the amine functional group can release a H+ ion, imposing a negative charge onto the amino acid. Alternatively, as pH is decreased the carboxyl functional group can uptake a H+ ion, imposing a positive charge onto the amino acid. The uptake or release of a H+ ion is a part of the suspended material maintaining the solutions equilibrium.
These net changes in charge of the amino acid cause a change in the ionic double layer, impacting the overall zeta potential. Thus, the isoelectric point is the point in which the functional groups on the surface of the suspended material have a net neutral charge.
Q: Why Do My pH Titration Curves (Isoelectric Point Data) Not Overlay?
A: The generated sigmoidal curves generated from low to high pH and from high to low pH do not overlay. This overlay discrepancy is related to the increase in conductivity as acids and bases are added to the suspension. For more information on how conductivities effect on zeta potential, see “What are the Key Factors to Consider with Zeta Potential Measurement?” below.
Q: What are Key Factors to Consider with Zeta Potential Measurement?
There are many different factors that will impact your zeta potential measurement. Below are 5 of the most important factors:
As described under isoelectric point determination, depending on the functional groups exposed on the surface of the suspended material, pH can have a major impact on the zeta potential value. If the surface chemistry of a material has polar functional groups, then pH is the most critical factor affecting zeta potential.
Due to the impact of pH on zeta potential results, all zeta potential results should be accompanied with the pH of the sample during the analysis. A zeta potential without a correlating pH is in effect meaningless.
When diluting a sample for zeta potential analysis it is important to maintain the pH of the sample pre-dilution in order to not impact the zeta potential of the suspended material.
The conductivity of a solution is relative to the free ions within the solution. The concentration of available ions to form the double layer around a particle will affect the potential at the slip plane, which directly impacts the zeta potential value. The pH of the solution will have a larger impact on the zeta potential, but the conductivity will also play a role in the overall zeta potential.
Due to the impact of conductivity on zeta potential results, all zeta potential results should be accompanied with the conductivity of the sample during the analysis. A zeta potential without a correlating conductivity is in effect meaningless.
When diluting a sample for zeta potential analysis it is important to maintain the conductivity of the sample pre-dilution in order to not impact the zeta potential of the suspended material.
Zeta potential by electrophoretic mobility requires an understanding of any restrictions on the suspended materials mobility. Since the viscosity of the dispersion media will affect the mobility of the particles suspended, an accurate viscosity is required for appropriate calculation of the results.
Depending on the technology utilized and size of suspended material, there may be limitations on how viscous a suspension can be to obtain accurate results.
Zeta potential techniques have temperature controls as both the particle mobility and viscosity can be impacted by changes in temperature.
Concentration is important to zeta potential analyses as it is relative to the signal of the analysis. Analytical signal is a function of both particle size and concentration. An ideal analysis has a high enough concentration to generate sufficient signal to the detector to outweigh any background noise, as well as being dilute enough to prevent light interacting with more than one particle before reaching the detector. Ideally, a linear response study is recommended for any sample to be analyzed for zeta potential.
A linear response study determines the range of concentrations appropriate for your specific material that generate accurate results.
As can be seen based on the complexity of this FAQ, zeta potential is a useful physical characteristic that requires quite a bit of consideration to generate accurate results. The team at Particle Technology Labs has years of experience finding the appropriate testing conditions for materials appropriate for analysis by electrophoretic mobility. Please reach out with any questions about your testing needs!