When particles are in suspension, they usually have a surface charge due to the chemistry at the particle surface. Counter ions will be attracted to the charged surface, enveloping the particle in a cloud of counter ions. These counter ions will be most strongly associated with the particle close to the particle surface, in such a way, that a relatively immobile layer of counter ions accumulates close to the particle surface, this layer being called the stern layer. Outside of the stern layer the concentration of counter ions will decrease proportionately with distance from the particle surface, and due to the higher mobility of ions within this region it is often termed the diffuse layer. The zeta potential can be described as the electrostatic potential which is present between the stern layer and diffuse layer (this is sometimes referred to as the slipping plane or boundary).
Measurement of zeta potential is a powerful tool in understanding the stability of a particulate suspension. If zeta potential is low, it indicates that the particles have a weak surface charge, and that repulsive forces between particles in that suspension will be weak. Particles with a low zeta potential will therefore be prone to aggregation and the suspension will not be stable. By contrast if the surface charge is high, there will be strong repulsive forces between particles and the suspension will be less susceptible to aggregation, and therefore more stable.
See also the WAVE series of instruments for porosity and pore zeta potential analysis.
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