Microscopy, Microtechnology & Image Analysis


New ISO Standards for Zeta Potential Analysis Mark Bumiller, Horiba Scientific


Zeta potential is a measure of the charge on a particle surface in a specific liquid medium. With growing interest in nanotechnology and protein research, zeta potential analysis has become an increasingly important analytical technique. Since few chemists posses an extensive background in zeta potential theory or practice, an ISO working group was formed to create new several new standards on this subject. These new ISO standards [1, 2] provide excellent background material and advice to chemists new to the analysis of zeta potential.


What is zeta potential?


Particle surfaces pick up an electric charge due to several phenomenon including ionisation, ion adsorption, and ion dissolution. The charge on the surface of a particle influences the ionic environment in the region close to the particle surface. This ionic environment is typically described using a double layer model – the Stern layer of ions firmly attached adjacent to the particle surface, and the diffuse layer further away from the particle surface, but still attracted to the particle such that these ions will move with the particle. Beyond the double layer the ions are in equilibrium with the solution. The boundary between the electric double layer and the ions in equilibrium in the solution is called the slipping plane, as shown in Figure 1. Zeta potential is defined as the potential measured in mV at the slipping plane distance from the particle surface.


Particle Surface Slipping plane


Equation 1 is used to calculated the electrophoretic mobility (µ) using the measured frequency


shift (ω) and the applied electric field strength (E). The other variables include the refractive index of the medium (n) the laser wavelength λ, and the angular light scattering information contained in θ’.


(1)


Stern Layer


After the electrophoretic mobility is determined using equation 1, the zeta potential (ζ) is calculated using equation 2 where the other variables include the dielectric permittivity (ε ) of the medium, the viscosity (ηo), and a function f(κr) that describes the relationship between the ratio of particle radius and the distance from the particle surface to the slipping plane.


(2)


Applications


There are many applications for the information derived from zeta potential measurements, the most common ones being related to predicting dispersion stability and determining the surface chemistry conditions that create a state of zero zeta potential – the iso-electric point (IEP).


Figure 1. Zeta potential definition Dispersion Stability How is it measured?


There are microscopic, optical and acoustic techniques for measuring the zeta potential of charged particles. Optical techniques are typically based on electrophoretic light scattering and acoustic techniques are based on either electrokinetic sonic amplitude (ESA) or colloid vibration current (CVI). This article will focus on the more popular electrophoretic light scattering technique.


Most zeta potential measurements are fairly easy in practice. A small quantity of sample is injected into a cell containing two electrodes that are used to create an induced electric field. Once the electric field is applied the particles move toward either the anode or cathode depending on whether the surfaces are positively or negatively charged. The direction of the motion indicates positive vs. negative charge. The speed of the particle motion is used to calculate the magnitude of the charge. Factors influencing the particle motion include the temperature, viscosity, and dielectric constant of the dispersing medium, the electric field strength, and the zeta potential of the particles. The particle motion is measured using laser Doppler electrophoresis, basically detecting the Doppler shift from the particle motion as they pass through a laser beam within the applied electric field.


Solid in liquid suspensions and oil in water emulsions can be stabilised by increasing the charge on the particle surfaces, increasing the repulsion forces that inhibit aggregation. Many chemists working in new product formulation use zeta potential as a predictive value, altering surface chemistry conditions to maximise zeta potential. Chemistry conditions that can be altered to manipulate the zeta potential for a formulation include pH, salt concentration, type of surfactant and surfactant concentration. Higher zeta potential magnitudes (independent of whether the sign is positive or negative) typically indicate that a given chemical formulation will be more stable over time.


Conversely, in other applications the goal is to find the conditions that lead to near zero zeta potential. This is where the system will likely destabilise and aggregate, making it easier to separate the particles through processes such as filtration. A typical example of this approach is water treatment where coagulant concentration as a function of zeta potential is studied to find the optimum conditions to promote aggregation prior to filtration.


Figure 2 shows a plot of zeta potential as a function of coagulant (alum and gypsum) concentration from a study looking for ideal conditions to filter clay from a wastewater steam.


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