ZetaPotential

Zeta potential, sometimes referred to as zetapotential, is the electrical potential at the slipping plane of a particle in an electrolyte. It reflects the surface charge and the surrounding electrical double layer. The magnitude and sign of the zeta potential are used as indicators of colloidal stability: larger absolute values generally indicate stronger electrostatic repulsion and lower aggregation tendency.

In colloids, each particle is surrounded by an electrical double layer consisting of adsorbed ions (Stern layer) and a diffuse layer. The slipping plane is the boundary within the diffuse layer where the fluid moves with the particle under an electric field; zeta potential is the potential at that plane rather than at the solid surface.

Measurement and theory: Zeta potential is inferred from electrophoretic mobility, measured by techniques such as electrophoretic light scattering or electroacoustic methods. The relation between mobility mu_e and zeta depends on the double layer: Smoluchowski equation mu_e = (epsilon * zeta) / eta (for thin double layers); thus zeta = (mu_e * eta) / epsilon. For thick double layers (Hückel regime) mu_e = (2/3) (epsilon * zeta) / eta, or zeta = (3 eta mu_e) / (2 epsilon). More advanced models include surface conductivity and Stern layer effects.

Factors and interpretation: Zeta potential depends on pH, ionic strength and electrolyte type, temperature, particle size and adsorption of ions or polymers. Values around ±30 mV or higher are often cited as indicating stable dispersions, but thresholds vary by system and DLVO interactions.

Applications and limitations: Zeta potential is widely used to predict and optimize dispersion stability in formulations, paints, ceramics, and nanomaterials, as well as in wastewater treatment and soil science. Measurements assume a mobile slip plane and moderate suspension conditions; in concentrated or non-Newtonian systems, the interpretation may be less straightforward.