Zeta potential measurements can be used to assess the charge stability of a disperse system, and can aid in the formulation of colloidally stable products, e.g. liposomal formulations. Interactions between particles are strongly affected by the zeta potential. Particles with a high zeta potential of the same charge sign, either positive or negative, will repel each other.
Zeta potential (ζ-potential) is defined as the difference in potential between the surface of the shear plane of particles (e.g., liposomes, virus-like particles) or biomolecules (e.g., proteins) and the electroneutral region of the solution. The shear plane, or slipping plane, is the limit of a region around the particle surface consisting of a tightly bound electrical double layer of ions and solvent molecules.
The zeta potential may or may not be related to the surface charge, and can even be of opposite charge sign to the surface charge. The zeta potential, usually expressed in mV, is very sensitive to the formulation. In particular, the concentration and type of ions and buffer species in solution, as well as the pH, strongly affect the zeta potential. Because of this, a zeta potential value without information about solution composition is meaningless.
At Coriolis, we perform zeta potential measurements by laser Doppler electrophoresis, using a Malvern ZetaSizer Nano ZS (also utilized for dynamic light scattering measurements). By applying an electric field across the liquid sample, particles 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, also known as laser Doppler velocimetry. The frequency shift of an incident laser beam caused by these moving particles is measured as particle mobility, and this is converted to the zeta potential according to the Henry equation.
Zeta Potential, image Source: Wikipedia