The Defoaming Mechanism of Defoamers

Oct 16, 2020

There is no unified understanding of the mechanism of defoaming agents. According to the mechanism of defoaming agents proposed by the predecessors, there are roughly the following:
A general defoaming mechanism
Typical general defoaming mechanisms are Robinson defoaming mechanism and Ross hypothesis. The Robinson mechanism is the basis of the Ross hypothesis. It mainly emphasizes the drainage of defoamers to destroy the foam and the Marangoni effect to achieve defoaming; the Ross hypothesis is carried out on the basis that the defoamer particles are insoluble droplets. In fact, some defoamers produce defoaming effect in the dissolved state, so the defoaming mechanism of Ross hypothesis is not comprehensive.
The mechanism of action of polysiloxane defoamers
Representative polysiloxane defoaming mechanisms mainly include "bridging-spreading" mechanism, "bridging-dehumidifying" mechanism, and "spreading-liquid entrainment" mechanism. The "bridging-spreading" mechanism mainly starts from the basic point of "polysiloxane has relatively low tension and is easy to spread on the liquid film". It emphasizes that the defoamer droplets are easy to deform, but this theory cannot explain The difference in defoaming between a single polysiloxane and a mixture of polysiloxane and solid ions as defoamers. The "bridging-dehumidification" mechanism is mainly based on the hydrophobicity of polysiloxane itself, but the defoaming effect of polysiloxane with high viscosity cannot be explained well. The "spreading-liquid entrainment" mechanism has not been proven, because some facts show that polysiloxanes are sometimes not spread on the surface of the bubble film, but they can also break the bubble.
The defoaming mechanism of hydrophobic solid particles
In the foam system, the hydrophobic solid particles will first attract the hydrophobic end of the surfactant, so that the hydrophobic solid particles become hydrophilic, thereby reducing the concentration of the surfactant in the bubble film and promoting the collapse of the foam. This defoaming mechanism cannot explain the action mechanism of other defoaming agents, it is too one-sided. There are also reasons for foam bursting due to the impact caused by the expansion of the defoamer, the solubilization of the surfactant, and the breakdown of the electric double layer on the surface of the electrolyte. It can be seen from the above defoaming mechanisms that each defoaming agent has a different focus on different foam systems, but they all achieve defoaming by destroying the stability of the foam.
The defoaming mechanism of polyether modified silicone oil
To explain the defoaming process of polyether modified silicone oil as a defoaming agent, the most complete defoaming mechanisms are: "bridging-stretching" mechanism and "bridging-dehumidifying" mechanism.
"Bridge-stretching" mechanism: The surface tension of the defoamer is far lower than the surface tension of the liquid film. The droplets of the defoamer can continue to spread and deepen on the surface of the liquid film, and the local liquid film of the foam will continue to thin, and finally A bridge between oil and water is formed. The surface tensions of the oil phase and the water phase are very different. The oil phase is continuously drawn by the surrounding water phase, elongated and thinned. After the deformation exceeds a certain range, the liquid film is destroyed, resulting in foam rupture.
"Bridging-moisture removal" mechanism: After adding solid hydrophobic particle defoamer to the foaming liquid, the defoaming agent is immediately distributed in the foam system, and the hydrophobic particles are fixed on the surface of the foam liquid film. When the solid particles and the liquid film There is a sufficient hydrophobic angle between them, and the solid particles have opposite contact surfaces with the surrounding liquid film, which becomes a bridge between the surrounding liquid film, and finally can penetrate the foam liquid film and enter the foam.
The "bridging-stretching" mechanism is based on the unique low surface tension and easy spreading characteristics of silicone oil. It points out that the defoamer droplets can produce different degrees of deformation, but this mechanism is difficult to explain the difference between silicone paste and pure silicone oil; "bridging- The mechanism of "moisture removal" is based on the lipophilicity of silicone oil, which can explain the working principle of low-viscosity polyether modified silicone oil. Therefore, the polyether modified silicone oil defoamer has three characteristics of defoaming: firstly, it is basically insoluble in the foaming liquid (mostly dissolved can help foam); secondly, the surface tension is lower than the foaming liquid; and finally, it can quickly Disperse in the foaming liquid. Only substances with low solubility and high dispersibility can become defoamers with good foam breaking and foam suppression capabilities to maximize their dispersibility and achieve both foam suppression and foam breaking.