The Electrokinetic Connection

Understanding zeta potential can help you to understand and control colloidal suspensions. Examples include complex biological systems such as blood, and functional ones like paint. Water, milk, wine, clay, inks, dyes, paper, and pharmaceuticals are good examples of useful colloidal systems. In many cases, the performance of a suspension can be improved by understanding the effects of colloidal behavior on such properties as viscosity, settling, and effective particle size.

Each colloid carries a "like" electrical charge which produces a force of mutual electrostatic repulsion between adjacent particles. If the charge is high enough, the colloids will remain discrete, dispersed, and in suspension. Reducing or eliminating the charge has the opposite effect - the colloids will steadily agglomerate and settle out of suspension or form an interconnected matrix.

We can often tailor the characteristics of a suspension by understanding how individual colloids interact with one another. At times we may want to maximize the repulsive forces between them in order to keep each particle discrete and prevent them from gathering into larger, faster settling agglomerates. Examples include pharmaceuticals and pastes. Sometimes we have the opposite goal and want to separate the colloids from the liquid. Removing the repulsive forces allows them to form large flocs that settle fast and filter easily. Viscosity is another property that can be modified by varying the balance between repulsion and attraction.

Each charged particle is surrounded by a diffuse layer of neutralizing ions. This causes an electrical potential on the order of millivolts to develop across the diffuse layer between the colloid and the bulk of the liquid. The magnitude of this potential is approximately proportional to the surface charge on the colloid, and it is called the zeta potential.  As a result, zeta potential an important measure of the electrokinetic forces involved.

This material is adapted from our booklet "Zeta Potential: A Complete Course in 5 Minutes."  The complete booklet is available for download, or from us by mail.