Dispersant Dispersion Mechanism

one
Double layer principle - The dispersants used in water-based coatings must be water-soluble and selectively adsorbed onto the interface between the powder and water. Currently, the commonly used type is anionic, which ionizes in water to form anions and has certain surface activity, which is adsorbed on the surface of powders. After the surface of powder particles adsorbs dispersants, a double layer of electricity is formed, and anions are tightly adsorbed on the surface of the particles, known as surface ions. Ions with opposite charges in the medium are called counter ions. They are adsorbed by surface ions through electrostatic adsorption, and some of the counter ions are tightly bound to particles and surface ions, which are called bound counter ions. They become a moving whole in the medium, carrying negative charges, while another part of the counter ions surround them, forming a diffusion layer called free counter ions. This forms a double layer between surface ions and counter ions.
two
Electrokinetic potential - The negative charge carried by particles and the positive charge carried by the diffusion layer form a double layer, known as electrokinetic potential Thermoelectric potential: The double layer formed between all anions and cations, corresponding to the potential.
It is the electrokinetic potential that plays a dispersing role, rather than the thermodynamic potential. The electrokinetic potential is imbalanced with charges and there is a phenomenon of charge repulsion, while the thermodynamic potential belongs to the phenomenon of charge balance. If the concentration of counter ions in the medium increases, and the free counter ions in the diffusion layer are forced to enter the bound counter ion layer due to electrostatic repulsion, the double electric layer is compressed, and the electrokinetic potential decreases. When all free counter ions become bound counter ions, the electrokinetic potential is zero, which is called the isoelectric point. Without charge repulsion, the system lacks stability and undergoes flocculation.
three
Steric hindrance effect - The formation of a stable dispersion system not only utilizes electrostatic repulsion, which means that negative charges adsorbed on the surface of particles repel each other to prevent adsorption/aggregation between particles and ultimately form large particles for layering/sedimentation, but also utilizes the theory of spatial steric hindrance effect, which means that when particles with negative charges adsorbed approach each other, they slide and stagger each other, These surfactants with steric hindrance are generally non ionic surfactants. By flexibly applying the theory of electrostatic repulsion combined with spatial hindrance, a highly stable dispersion system can be formed.
The polymer adsorption layer has a certain thickness, which can effectively block the mutual adsorption of particles, mainly relying on the solvation layer of the polymer. When the surface adsorption layer of the powder reaches 8-9nm, the repulsive force between them can protect the particles from flocculation. So polymer dispersants are better than ordinary surfactants.