The principle of double electric layers
Time：
2025-06-05
The dispersants used in water-based coatings must be water-soluble and they are selectively adsorbed onto the interface between the powder and water. The commonly used type is anionic. They ionize in water to form anions and have certain surface activity, which are adsorbed by the surface of the powder. After the powder particles adsorb the dispersant on their surface, a double electric layer is formed. The anions are closely adsorbed by the particle surface and are called surface ions. Ions with opposite charges in a medium are called counterions. They are electrostatically adsorbed by surface ions. Some of the counterions are relatively tightly bound to the particles and surface ions, and they are called bound counterions. They become a moving whole in the medium, carrying a negative charge, while another part of the counterions surround them. They are called free counterions and form a diffusion layer. In this way, a double electric layer is formed between the surface ions and the counterions.
Electrokinetic potential: The double electric layer formed by the negative charge carried by the particle and the positive charge carried by the diffusion layer is called the electrokinetic potential. Thermal potential: The corresponding potential of the double electric layer formed between all anions and cations.
It is the electrokinetic potential rather than the thermal potential that plays a dispersing role. The electrokinetic potential has an unbalanced charge and a charge repulsion phenomenon, while the thermal potential belongs to a charge balance phenomenon. If the concentration of counterions in the medium increases, the free counterions in the diffusion layer will be forced into the bound counterion layer due to electrostatic repulsion. As a result, the double electric layer is compressed and the electromotive potential drops. When all the free counterions become bound counterions, the electromotive potential becomes zero, which is called the isoelectric point. Without charge repulsion, the system has no stability and flocculation occurs.
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