On the surface of charged particles a layer of counter ions exists – the so-called Stern layer with the Stern potential. The counter ions are bound strongly to the surface of the charged particle. Another more diffuse layer of charged particles follows, in which a stable conformation forms. Within this boundary layer, the ions move with the movement of the whole particle in the same direction. Outside the boundary this directed movement does not exist. Hence the boundary is also the shear surface of the particle to the surrounding liquid. The potential at this boundary layer is called zeta potential. The described two layers are called the electrochemical double layer.
The measurement of the zeta potential is performed with the Zetasizer Nano indirectly by measuring the electrophoretic mobility by laser Doppler velocity measurements in a special cell. Again the light is scattered by the particles and fluctuations of the light intensity at the detector, which are a function of the particle velocity, get recorded.
Practically the measurement is carried out by applying a defined voltage to two electrodes in the cell leading to different speeds of the particles. Particles with higher potential have a higher speed in the electrical field.
The zeta potential can be calculated with the Henry function.