Zeta potential is a measure of the magnitude of the electrostatic or charge repulsion/attraction between particles, and is one of the fundamental parameters known to affect colloidal stability. It is typically measured using Electrophoretic Light Scattering (ELS).
When a charged particle is suspended in a liquid, ions of opposite charge will be attracted to the surface of the suspended particle. A negatively charged sample attracts positive ions from the liquid and conversely a positively charged sample attracts negative ions from the liquid. Ions close to the surface of the particle will be strongly bound while ions further away will be loosely bound forming a ‘diffuse’ layer. Within the diffuse layer there is a notional boundary and any ions within this boundary will move with the particle when it moves in the liquid; but any ions outside the boundary will stay where they are – this boundary is called the slipping plane.
A potential exists between the particle surface and the dispersing liquid which varies according to the distance from the particle surface – this potential at the slipping plane is called the zeta potential. The magnitude of the zeta potential gives an indication of the potential stability of the colloidal system.
ASTM E2865 (2012) Standard Guide for Measurement of Electrophoretic Mobility and Zeta Potential of Nanosized Biological Materials. ASTM International, West Conshohocken, PA. www.astm.org