There is no unified understanding of the mechanism of action of defoamers, and according to the mechanisms of defoamers proposed by predecessors, there are roughly the following:
It has a generalized defoaming mechanism
Typical generalized defoaming mechanisms are Robinson's defoaming mechanism and Ross hypothesis. Among them, Robinson's mechanism is the basis of the Roth hypothesis, which mainly emphasizes the destruction of foam drainage by defoamers and the Marangoni effect to achieve defoaming. The Ross hypothesis is based on the fact that the defoamer particles are non-soluble droplets, but in fact, some defoamers produce defoaming in the dissolved state, so the defoaming mechanism of the Ross hypothesis is not comprehensive.
Mechanism of action of polysiloxane defoamer
The representative polysiloxane defoaming mechanisms mainly include the "bridging-spreading" mechanism, the "bridging-dehumidification" mechanism, and the "spreading-liquid entrainment" mechanism. The "bridging-spreading" mechanism is mainly based on the basic point that "the tension of polysiloxane itself is relatively low and it is easy to spread on the liquid film", which emphasizes that the droplets of the defoamer are easy to deform, but this theory cannot explain the difference between polysiloxane alone and polysiloxane and solid ion mixture when used as a defoamer. The "bridging-dehumidification" mechanism is mainly from the perspective of the hydrophobicity of polysiloxane itself, but the defoaming effect of polysiloxane with high viscosity cannot be well explained. The "spreading-liquid entrainment" mechanism has not been proven, as some facts have shown that polysiloxanes sometimes do not spread on the surface of the bubble film, but can also break the foam.
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 surfactant in the foam film and promoting the foam bursting. This defoaming mechanism cannot explain the mechanism of action of other defoamers, and is too one-sided. In addition, some foam bursts are caused by the impact of the defoamer's propagation, the solubilization of the surfactant, and the bursting of the electric double layer on the surface of the electrolyte disintegration solution. From the above defoaming mechanisms, it can be seen that each defoamer has a different focus on different foam systems, but it is achieved by destroying the stabilizing factors of the foam.
Defoaming mechanism of polyether modified silicone oil
For the explanation of the defoaming process of polyether modified silicone oil as a defoamer, the most complete defoaming mechanism is "bridging-tensile" mechanism and "bridging-dehumidification" mechanism.
"Bridging-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 penetrate on the surface of the liquid film, and the local liquid film of the foam continues to thin, and finally forms a bridge between the oil in the middle of the water, the surface tension of the oil phase and the water phase is very different, and the oil phase is elongated and thinned under the continuous traction of the surrounding water phase, and after the deformation exceeds a certain range, the liquid film is destroyed, resulting in the bursting of the foam.
"Bridging-dehumidification" mechanism: after adding solid hydrophobic particle defoamer to the foaming liquid, the defoamer 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 have sufficient hydrophobic angle between the liquid film, the solid particles have the opposite contact surface with the surrounding liquid film, which becomes the bridging between the surrounding liquid films, and finally can penetrate the foam liquid film and enter the foam.
Based on the unique low surface tension and easy spreading characteristics of silicone oil, the "bridging-tensile" mechanism 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. The "bridging-dehumidification" mechanism is based on the lipophilicity of silicone oil, which can explain the working principle of low-viscosity polyether modified silicone oil. Therefore, polyether-modified silicone oil defoamer has three characteristics of defoaming: first, it is basically insoluble in foaming solution (most of the dissolved ones help foaming); secondly, the surface tension should be lower than that of the foaming liquid; Finally, it can be quickly dispersed in the foaming solution. Only substances with small solubility and large dispersion can become defoamers with good foam breaking and foam inhibition ability, so as to maximize their dispersibility and achieve two-pronged foam inhibition and foam breaking.
United States colloidal chemist Ross, S. once clearly said: no one defoaming mechanism can cover all defoaming phenomena, and a variety of complex defoamers can correspond to a variety of defoaming mechanisms.
