Zeta Potential

Zeta potential is an abbreviation for electrokinetic potential in colloidal systems.

 

In the colloidal chemistry literature, it is usually denoted using the Greek letter zeta, hence ζ-potential. From a theoretical viewpoint, zeta potential is electric potential in the interfacial double layer (DL) at the location of the slipping plane versus a point in the bulk fluid away from the interface. In other words, zeta potential is the potential difference between the dispersion medium and the stationary layer of fluid attached to the dispersed particle. 

 

Consider a negatively charged surface of a particle (anionic). Such a surface immediately attracts counterions or gegenions at the boundary of its surface. This first layer has an opposite, cationic charge that attracts other anions and repels cations. Adding layer on layer, and with Brownian motion that tends to even out the distribution, there will be a boundary at a particular distance from the surface that is electrically neutral. This boundary is the so called "shear plane" or "slipping plane". Beyond that plane, all ions are less tightly bound: this is the diffuse layer. So in fact, there is a bilayer electrical system surrounding any charged surface. The zeta potential now measures the difference in milliVolts (mV) in electrokinetic potential between the tightly bound layer around the surface and the distant zone of electroneutrality. 

A value of 25 mV (positive or negative) can be taken as the arbitrary value that separates low-charged surfaces from highly-charged surfaces. 

 

The significance of zeta potential is that its value can be related to the stability of colloidal dispersions (e.g. a multivitamin syrup). The zeta potential indicates the degree of repulsion between adjacent, similarly charged particles (the vitamins) in a dispersion. For molecules and particles that are small enough, and of low enough density to remain in suspension, a high zeta potential will confer stability, i.e. the solution or dispersion will resist aggregation. When the potential is low, attraction exceeds repulsion and the dispersion will break and flocculate. So, colloids with high zeta potential (negative or positive) are electrically stabilized while colloids with low zeta potentials tend to coagulate or flocculate as outlined in the table[1]. 

 

Zeta Potential [mV] 

Stability behavior of the colloid 

from 0 to ±5, 

Rapid coagulation or flocculation 

from ±10 to ±30 

Incipient instability 

from ±30 to ±40 

Moderate stability 

from ±40 to ±60 

Good stability 

more than ±61 

Excellent stability 

 

Zeta potential is widely used for quantification of the magnitude of the electrical charge at the double layer. However, zeta potential is not equal to the Stern potential or electric surface potential in the double layer. Such assumptions of equality should be applied with caution. Nevertheless, zeta potential is often the only available path for characterization of double-layer properties. Zeta potential should not be confused with electrode potential or electrochemical potential (because electrochemical reactions are generally not involved in the development of zeta potential). 

 

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