Zeta potential

Colloidal particles dispersed in a solution are electrically charged due to their ionic characteristics and dipolar attributes. The development of a net charge at the particle surface affects the distribution of ions in the neighboring interfacial region, resulting in an increased concentration of counter ions (ion of charge opposite to that of the particles) close to the surface.
Each particle dispersed in a solution is surrounded by oppositely charged ions called fixed layer. Outside the fixed layer, there are varying compositions of ions of opposite polarities, forming a cloud-like area.
Thus an electrical double layer is formed in the region of the particle-liquid interface.

This double layer may be considered to consist of two parts: a inner region which includes ions bound relatively strongly to the surface and an outer, or diffuse region in which the ion distribution is determined by a balance of electrostatic forces and random thermal motion. The potential in this region, therefore, decays with the distance from the surface, until at a certance distance it becomes zero (see the graph on the left).

Zeta potential is a function of the surface charge of a particle, any adsorbed layer at the interface and the nature and composition of the surrounding medium in which the particle is suspended.
Zeta potential can be calculated with the following Smoluchowski's formula:

The principle of determining zeta potential is very simple.
A controlled electric field is applied via electrodes immersed in a sample suspension and this causes the charged particles to move towards the electrode of opposite polarity. Viscous forces acting upon the moving particle tend to oppose this motion and an equilibrium is rapidly established between the effects of the electrostatic attraction and the viscosity drag. The particle therefore reach a constant terminal velocity.

Source: http://nition.com/en/products/zeecom_s.htm

Zeta potential in filters

Most materials when immersed in water exhibit a zeta potential. The majority of demineralized water contaminants, including most colloids, particles, bacteria, and pyrogens (bacterial fragments), are negatively charged. Filter media can be chemically modified to give them a positive zeta potential.

Positive zeta potential elements provide an important advantage: they additionally remove very fine negatively charged organisms and particles, well below their micron rating. The removal mechanism is electrostatic attraction, and is effective in water over the typical demineralized water pH range (pH 5-8).

As the active sites become occupied by the collected particles, the removal efficiency by electrostatic attraction decreases. However, actual efficiency will not drop below the removal rating of the filter. Because of the highly porous nature of the membrane, actual total membrane area containing positive zeta potential sites is several orders of magnitude greater than the effective filtration area. Thus, the capacity for electrostatic adsorption of fine particles is very large.

Related topics

Beta ratio

Bubble point

Absolute Vs. nominal rating for filters

Sediment filters

Oil filtration and Oil-Block filters