Zeta potential measurement using laser Doppler electrophoresis (LDE)
Zeta potential characterisation of particulate dispersions, emulsions and biomolecules
Sometimes thought of as a ‘charge’ measurement, measurement of zeta potential is used to assess the charge stability of a disperse system, and assist in the formulation of stable products. Zeta potential may be related to the surface charge in a simple system, but equally well may not. The zeta potential can even be of opposite charge sign to the surface charge. One of the most important lessons is that it is the zeta potential that controls charge interactions, not the charge at the surface.
Schematic representation of zeta potential
What is zeta potential?
Most particles dispersed in an aqueous system will acquire a surface charge, principally either by ionization of surface groups, or adsorption of charged species. These surface charges modify the distribution of the surrounding ions, resulting in a layer around the particle that is different to the bulk solution. If the particle moves, under Brownian motion for example, this layer moves as part of the particle. The zeta potential is the potential at the point in this layer where it moves past the bulk solution. This is usually called the slipping plane. The charge at this plane will be very sensitive to the concentration and type of ions in solution.
Zeta potential is one of the main forces that mediate interparticle interactions. Particles with a high zeta potential of the same charge sign, either positive or negative, will repel each other. Conventionally a high zeta potential can be high in a positive or negative sense, i.e. <-30mV and >+30mV would both be considered as high zeta potentials. For molecules and particles that are small enough, and of low enough density to remain in suspension, a high zeta potential will confer stability, i.e. the solution or dispersion will resist aggregation. The Faraday gold sol made in the 1850’s now in the science museum in London, is still a stable dispersion, particle aggregation being slowed to an imperceptible rate due to it’s high zeta potential.
Typical applications are in the formulation of particulate dispersions. Zeta potential can be used to assess the effect of each additive in the formulation. Additives can have surprising effects; some materials sold as dispersion agents have been known to reduce the zeta potential in particular formulations. It is not always possible to predict the effect or the magnitude of the effect of an additive. The Zeta potential can also be used to increase shelf life by assessing the impact of product changes during storage, e.g. hydrolysis or gas ingress.
Principle of zeta potential measurement
Zeta potential is measured by applying an electric field across the dispersion. Particles within the dispersion with a zeta potential will migrate toward the electrode of opposite charge with a velocity proportional to the magnitude of the zeta potential.
This velocity is measured using the technique of laser Doppler anemometry. The frequency shift or phase shift of an incident laser beam caused by these moving particles is measured as the particle mobility, and this mobility is converted to the zeta potential by inputting the dispersant viscosity, and the application of the Smoluchowski or Huckel theories. These theories are approximations useful for most applications. More recent models are available which can give a more exact conversion, but require more knowledge of the chemistry of the dispersion.
Advantages of recent technology introductions
One of the biggest practical issues when making zeta potential measurements is that of contamination. If any part of the system has been in contact with a previous sample then the zeta potential, being so sensitive to small changes in the environment can be affected.
The disposable capillary cell available for the Zetasizer Nano series is the only cell with entirely disposable cuvette and electrodes, that will therefore eliminate this problem.
The Zetasizer Nano series uses second generation PALS (Phase Analysis Light Scattering), called M3PALS to measure the particle velocity. Using phase analysis rather than frequency analysis is up to 1000 times as sensitive to changes in particle mobility. This is particularly important when measuring samples at high ionic concentration, e.g. isotonic saline, or in low dielectric constant dispersants such as hexane.