The coagulation principle of polyaluminum chloride 01

1, compressed electric double layer

The structure of the micelle electric double layer determines the concentration of counter ions on the surface of the colloidal particles. As the distance from the surface of the colloidal particles increases, the concentration of counter ions decreases, and the ion concentration in the solution is equal.

When electrolyte is added to the solution to increase the ion concentration in the solution, the thickness of the diffusion layer is reduced.

When two colloidal particles are close to each other, because the thickness of the diffusion layer decreases and the zeta potential decreases, their mutual repulsive force is reduced, that is, the repulsive force between the colloids with high ion concentration in the solution is smaller than that with low ion concentration.

The suction force between the rubber particles is not affected by the composition of the water phase, but due to the thinning of the diffusion layer, the distance between them when they collide is reduced, so that the mutual suction force is greater.

It can be seen that the combined force of repulsion and attraction has changed from repulsive force to suction force (the repulsive potential energy disappears), and the colloidal particles can quickly condense.

This mechanism can better explain the sedimentation phenomenon in the harbor. When the fresh water enters the seawater, the salt increases, the ion concentration increases, and the stability of the fresh water entrained colloidal particles decreases, so clay and other colloidal particles are easy to deposit in the harbor.

According to this mechanism, when the added electrolyte in the solution exceeds the critical aggregation concentration for agglomeration, there will be no more excess counterions entering the diffusion layer, and it is impossible for the colloidal particles to change signs to stabilize the colloidal particles again.

This mechanism is to explain the effect of electrolyte on the destabilization of colloidal particles by simple electrostatic phenomenon, but it does not consider the effect of other properties in the destabilization process (such as adsorption), so it cannot explain other complex destabilization phenomena.

For example, if the amount of trivalent aluminum salt and iron salt used as coagulant is too much, the coagulation effect will decrease or even stabilize again; another example is the polymer or polymer organic matter with the same charge as the colloidal particles may have a good coagulation effect. The isoelectric state should have a good coagulation effect, but the coagulation is often very good when the ξ electric potential is greater than zero in practices.

 In fact, adding a coagulant to the aqueous solution to destabilize the colloidal particles involves the interaction of the colloidal particles and the coagulant, the colloidal particles and the aqueous solution, and the coagulant and the aqueous solution. It is a comprehensive phenomenon.