What is the defoaming mechanism of defoamers?
As the understanding of foam stability factors continues to grow, so does the understanding of the defoaming mechanism. Foam stability factors are diverse, and so are the defoaming mechanisms.
Reducing local surface tension
Some people have studied the defoaming process of polysiloxane in oily systems: the silicone oil defoamer droplets reach the bubble film, causing the bubble film surface to break up and merge into large bubbles. This objectively explains why a drop of defoamer causes a series of bubbles to burst.
When the defoamer attaches to the bubble film, i.e. when it is immersed in the bubble film liquid, it significantly reduces the surface tension at that point. Because the active ingredient of the defoamer has low solubility in water in an aqueous system, the reduction in surface tension is only localised at the bubble film, while the surface tension around the bubble film remains almost unchanged. The part of the surface tension that is reduced is strongly pulled and stretched in all directions, which ultimately leads to the bubble breaking.
Destroying the elasticity of the membrane
The surface tension is reduced due to the adsorption of surfactant on the surface of the foam membrane. Therefore, when subjected to local pressure, the foam membrane thins locally in that area, while the surface tension increases due to the thinning of the surfactant in that area. It is precisely because of the difference in surface tension between the new surface and the original surface that the foam membrane has elastic recovery force when it is thinned by external impact, so that the foam membrane does not break and the foam is stabilised.
If we manage to destroy this elasticity, we can destroy the stability of the foam. Some experts believe that the function of a defoamer is to destroy the elasticity of the foam film. When a defoamer is added to a foam system, it will diffuse to the gas-liquid interface, making it difficult for surfactants with foam stabilising properties to restore the elasticity of the film.
Promoting liquid film drainage
Defoaming can be achieved by adding substances (defoamers) that accelerate the drainage of the foam film. If the foam film is thick, the drainage process to 30–40 nm is long, and the self-healing effect of the foam film is strong, and the foam film is elastic, so the foam life of the thick foam film is long, and the drainage rate of the foam film can reflect the stability of the foam. The fast drainage rate of the liquid film thins the liquid film, and when it becomes thin enough, the foam collapses.
Hydrophobic solid particle defoamers
In foaming aqueous systems, defoamers containing silicone attract the hydrophobic end of the surfactant to the surface of the bubble, causing the hydrophobic particles to become hydrophilic and enter the aqueous phase, thus defoaming.
This type of defoamer partially immerses itself in the bubble surface, reducing the local surface tension of the bubble. This, combined with the effect of the solid particles, which act like a needle tip inserted into the bubble, accelerates the bursting of the bubble. Therefore, defoamers containing silicone generally have a better defoaming effect.
