Introduction to the Principles and Measurement Methods of Zeta Potential

Electrical Double Layer

The determination of final charge on the particle surface influences the distribution of nearby contact area ions due to an increase in the density of counter ions (having opposite charge close to the surface). Thus, an electrical double layer is distributed around each particle.

Zeta Potential

There are two liquid layers around the particle; the inner region (Stern layer: electron layer) where ions are strongly bound, and the outer region (diffuse) which is loosely bound. The outer region is the theoretical boundary where ions and particles stably exist. For instance, when a particle moves (influenced by gravity), ions move within the boundary. Ions outside the boundary coexist with massive dispersants. The potential of this boundary (the hydrodynamic shear stress on the surface) is the zeta potential. (figure 7).

• pH

In aqueous solutions, the pH of the sample acts as the most significant factor affecting zeta potential. Let us assume there are negatively charged particles in the suspension.
If alkaline is added to the suspension, the particles become increasingly negatively charged. Adding acid to the suspension will reach a point where the charge becomes neutral. Continuous addition of acid will make the particles positively charged.

In this case, the zeta potential curve according to pH will have a positive value at low pH and a negative value at high pH. When the zeta potential according to pH is zero, this is called the isoelectric point, which is an important value for understanding particles. Generally, in colloidal systems, the isoelectric point should be higher to maintain minimal stability.

• Conductivity

The thickness of the double layer (κ-1) is determined by the ion concentration in the solution, which can be calculated based on the ion strength in the solution. If the ion strength is high, a more compressed double layer can form. Ions also affect the thickness of the double layer.
Aluminum ions like Al3+ form a thicker double layer compared to monovalent ions like Na+.
Inorganic ions can affect the surface charge in one of two ways. i) If ion adsorption that does not affect the isoelectric point does not occur, or ii) If specific ion adsorption occurs that can affect the isoelectric point. Specific ion adsorption on the particle surface significantly affects the zeta potential of the particle dispersion, even at low concentrations. In some cases, specific ion adsorption can even cause charge reversal.