Do you understand the principle of dispersion of ultrafine particles?
1. The principle of dispersion of solid particles
1 Wetting principle (polarity similarity principle)
The granules must be wetted or wetted by the medium or binder so that they are well dispersed in the medium or binder.
2 Surface force principle
The total surface force between the particles must be a large positive value so that there is sufficient mutual repulsive force between the particles to prevent their bonding or agglomeration.
2. Wetting of solid particles
The dispersion of solid particles in the liquid is first wetted. The wettability of the solid particles in the liquid is usually measured by the wetting contact angle π>θ>π/2, indicating that the surface is completely non-wetting; π/2>θ>0, indicating partial wet or limited wetting of the solid surface; θ = 0 or no contact angle, indicating that the solid surface is completely wetted. The wetting process can also be expressed by the spreading factor S1/s: S1/s = γ1/g (cos θ - 1).
When θ = 0, the liquid can spread and wet on the solid surface;
When θ > 0, the wetting of the liquid on the surface of the solid particles is hindered.
As long as there is a wet contact angle, it means that the liquid does not completely wet the solid surface and spreads on its surface. In order to meet the wetting principle, it is necessary to add a wetting or wetting agent, or to surface-treat the powder to minimize the wetting contact angle, preferably to zero or absent, to create a condition of complete wetting.
3. Dispersion/agglomeration state of ultrafine particle suspension
When solid particles are immersed in a liquid medium, the particle group exists in two different states: one is that the particles attract each other to form agglomerates, and the single particles form "secondary" large particles; the other case is particles. They are mutually exclusive and can move freely in a liquid medium to form a stable dispersion system.
In some cases, dispersed particles and agglomerates are present in the system, and the suspension exhibits unstable dispersion characteristics. At this time, if the dispersion behavior predominates, the suspension system exhibits dispersion stability characteristics for a certain period of time. However, the two states of the suspension system are not static and can be changed or transformed under certain conditions.
Criteria for solid particle dispersion
In particle-based floatation systems, the stability of particle dispersion depends on the total energy of interaction between particles, ie, depending on the particle's van der Waals interaction energy (UA), electrostatic repulsion energy (Uel), and the steric hindrance of the adsorption layer. The relative relationship between energy (Ust), hydrophobic interaction energy (UH) and solvation energy (Us).
The theoretical criterion for the dispersion between particles and agglomeration is the total interaction energy Ur between particles, which can be expressed by the following formula:
Ur= UA + Uel + Ust + UH + Us
It can be seen that when the repulsion between the particles is greater than the mutual attraction energy, the particles are in a stable dispersion state; otherwise, the particles are agglomerated. Obviously, the various action energies acting between the particles vary with conditions. The addition of dispersants or surfactants plays an important role in the wetting of the particles in the liquid phase and the dispersion and suspension of the suspension.
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