Zeta potential is a measurement used to understand the behavior of tiny particles suspended in a liquid, known as a colloidal dispersion. It quantifies the effective electrical charge on the particle surface at the boundary where it separates from the surrounding liquid. Expressed in millivolts (mV), this measurement provides insight into the degree of electrostatic repulsion or attraction between particles. The magnitude of the zeta potential is a direct indicator of a dispersion’s stability, relevant across industries from pharmaceuticals to industrial slurries. A high positive or negative value suggests strong repulsive forces, meaning the particles resist clumping and the formulation is stable.
The Principle of Electrophoresis
The physical mechanism used to measure zeta potential is electrophoresis, which describes the movement of charged particles in a stationary liquid under the influence of an electric field. When voltage is applied across a sample, charged particles migrate toward the electrode of the opposite polarity. The speed and direction of this migration depend on the particle’s charge, the strength of the electric field, and the properties of the liquid medium.
A charged particle in a liquid is surrounded by a cloud of counter-ions, forming an electrical double layer. When the particle moves, a thin layer of liquid and some ions remain attached to its surface, defining the shear plane or slipping plane. The electrical potential measured at this shear plane is the zeta potential.
Instruments do not directly measure the zeta potential, but rather the speed at which the particle moves through the liquid. This velocity, divided by the applied electric field strength, is termed the electrophoretic mobility. Mobility is the measurable physical quantity that is mathematically converted into the zeta potential value using established electrokinetic theories. This relationship between the electric field strength and the resulting particle velocity forms the basis of modern zeta potential measurements.
Electrophoretic Light Scattering Technology
The most common method for determining electrophoretic mobility is Electrophoretic Light Scattering (ELS), also called Laser Doppler Velocimetry. This technique combines electrophoresis with advanced optical detection based on the Doppler effect. The colloidal sample is placed in a cell with electrodes, and a voltage is applied to initiate the movement of the charged particles.
A laser beam is directed through the sample, illuminating the moving particles. As the particles travel toward the electrode of the opposite charge, the scattered light shifts in frequency. This Doppler frequency shift is directly proportional to the velocity of the particles.
The instrument detects this change in the scattered light’s frequency by mixing it with a reference beam, allowing it to precisely calculate the particle velocity. This measurement is repeated numerous times to ensure an accurate determination of the electrophoretic mobility. Once mobility is known, a calculation converts this mobility into the zeta potential value.
This conversion uses physical models such as the Smoluchowski or Henry equations, which incorporate specific properties of the liquid, including its viscosity and dielectric constant. By measuring the particle’s movement under a known electric field and factoring in the liquid’s characteristics, ELS technology provides a reliable method for quantifying the zeta potential.
Environmental Factors Affecting Measurement
The measured zeta potential value depends heavily on the sample’s environment, requiring several factors to be controlled and reported for meaningful data.
pH
The pH of the liquid is a significant external variable, as changes in acidity or alkalinity directly influence the ionization state of functional groups on the particle surface. Adjusting the pH can alter the particle’s net surface charge, often leading to the isoelectric point—the state where the zeta potential is zero and stability is lowest.
Ionic Strength
Ionic strength, or conductivity, represents the concentration of dissolved ions or salts in the liquid. A high concentration of ions compresses the electrical double layer surrounding the particle, screening its surface charge from the bulk liquid. This screening effect reduces the magnitude of the measured zeta potential, potentially making a high-salt solution appear less stable than a low-salt one.
Temperature
Temperature is a consideration because it affects two parameters used in the final mathematical conversion: viscosity and the dielectric constant of the liquid. These are input values in the Smoluchowski or Henry equations. A change in temperature will change the calculated zeta potential because it affects the speed at which the particle moves through the liquid. Accurate results require the documentation of these environmental parameters alongside the final measured value.