Mechanism of Antifoam agents

When a system foams violently, adding antifoam agents can cause the foam to disappear almost immediately. This rapid action occurs because the role of these agents is to counteract the stabilizing effect of foam-promoting substances. Pure liquids, like water, might foam when agitated, but this foam is inherently unstable and collapses quickly once agitation stops. However, when foam-promoting substances (surfactants, proteins, etc.) are present, they stabilize the foam bubbles, making them difficult to burst. Therefore, the key to foam control lies in disrupting this stability. Below, we will delve into the specific antifoam mechanism (often synonymous with the defoamer mechanism) by which antifoam agents achieve this.

In 1941, Robinson and Woods proposed the concept of the spreading coefficient (often denoted as S) to help explain part of this mechanism: S = γm – γint – γa— Formula (1)

In formula (1): γm represents the surface tension of the foaming medium (N/m); γint is the interfacial tension between the defoaming agent and the foaming medium (N/m); and γa is the surface tension of the defoaming agent itself (N/m).

If the spreading coefficient S is positive, the antifoaming agent can spread across the surface of the bubble film. If S is negative, spreading is difficult. Therefore, a larger positive S value indicates easier spreading for the anti foaming agent on the bubble film surface. Formula (1) shows that a lower surface tension (γa) of the defoaming agent contributes to a larger S value, promoting diffusion and better defoaming effect.

In 1948, the concept of the entering coefficient E was proposed: E = γm + γint – γa (formula 2).

Similarly, the sign of the entering coefficient E determines whether the antifoam chemical can penetrate the surface of the bubble film. Again, a lower surface tension (γa) for the anti-foam agents is generally desirable. Note the difference between formulas (1) and (2): the sign before γint is negative in (1) and positive in (2).

American colloidal chemist Ross, through experiments with various surfactants, explored the relationship between the solubility of antifoaming agents in the foaming liquid and their defoaming effectiveness. Based on earlier work, he hypothesized that: in a solution, dissolved solutes tend to be foam stabilizers, while insoluble solutes act as antifoam agents if both the spreading coefficient (S) and entering coefficient (E) are positive. Only these insoluble defoaming agents can aggregate into molecular clusters capable of disrupting foam.

According to Ross’s hypothesis, as soon as these molecular clusters of the anti foaming agent contact the bubble film (assuming S>0 and E>0), they first enter (E>0) and then spread (S>0) across the bubble film surface. This action locally thins the bubble film, causing it to rupture or merge with other bubbles. If both S and E were negative, the droplet would neither enter nor spread. If E were positive but S negative, the droplet might enter but wouldn’t spread effectively. Thus, Ross concluded that both coefficients must be positive for a substance to function well as an antifoam agent.

S<0 e=””>0 (Assuming this notation refers to the conditions where it doesn’t work or only partially works)

This hypothesis laid a foundation for understanding the action mechanism of antifoaming agents and was widely adopted. However, Japanese expert Tsunetaka Sasaki later pointed out that the Ross hypothesis wasn’t fully comprehensive. He argued that the defoaming effect encompasses both foam suppression (preventing formation) and foam breaking (destroying existing foam). Sasaki suggested that droplets of an anti-foam agent might disrupt the surfactant layer on the bubble film surface and replace it with an unstable film, without necessarily embedding fully as Ross described. Therefore, multiple defoaming mechanisms likely exist. While most effective antifoaming agents are indeed largely insoluble, Sasaki noted that some foam control can occur with soluble additives, challenging the idea that antifoam agents must be absolutely insoluble substances.

As soon as the molecular clusters of the anti-foaming agent touch the bubble film, because S<0 E<0 s=””>0 E>0 E>0 is immersed first; S>0 expands on the bubble film; then the bubble film changes locally Thin and broken. This causes the bubbles to merge or burst. And when the immersion coefficient S and the expansion coefficient E are both negative, the droplet neither immerses nor expands; when the immersion coefficient E is positive and the expansion coefficient S is negative, the droplet is immersed in a prism shape and can be immersed. But it does not expand; only when both are positive, can it be a antifoam agents.

S<0 e=””>0 This hypothesis laid the foundation for the action mechanism of anti-foaming agents, and it was quickly spread and widely used. However, Japanese expert Tsunetaka Sasaki pointed out: The Rose hypothesis is not comprehensive enough. Because the defoaming effect includes two types of foam suppression and foam breaking. When the droplets of anti-foaming agent push open the surface-active breaker on the surface of the bubble film and replace it with an unstable film, they may not completely push the bubble film and embed it as Ross said. Therefore, it is believed that there are many defoaming mechanisms. Although most anti-foaming agents are insoluble, most of the relationship between expansion coefficient, immersion coefficient and defoaming effect of soluble additives has no defoaming effect. However, there is indeed a part of the defoaming effect is carried out in the dissolved state. In other words, anti-foaming agents are not absolutely insoluble substances.

With the continuous understanding of the foam stabilization factor, people’s understanding of the defoaming mechanism is also deepening. Foam stabilization factors are diverse, and the defoaming mechanism is also diverse.

It can reduce the local surface tension of the foam and cause the bubble to burst.

Someone studied the defoaming process of polysiloxane in an oil system. They took continuous pictures of the foam system at a speed of 1/1000 seconds. The picture was magnified by 100 times. The schematic diagram of the 4 pictures taken continuously can be seen. The silicone oil anti-foaming agent is small. When the drop reaches the bubble film, the surface of the bubble film breaks and merges into large bubbles. From 1 to 3, the bubbles are merged from 4 into one large bubble, and the bubble film becomes thinner. Finally, the gas and liquid separated rapidly, the large bubbles disappeared, and the bubble bursting ended at 4. This also objectively explains why a series of bubbles burst when a drop of anti-foaming agent goes down.

When the anti-foaming agent adheres to the bubble film, it is immersed in the bubble film liquid, which will significantly reduce the surface tension there. Because in the water system, the solubility of the active ingredients of the anti-foaming agent to water is small, the reduction of the surface tension is limited to the part of the bubble film, and the surface tension around the bubble film hardly changes. The part with reduced surface tension is intensively pulled and stretched around, and finally causes the bubble to burst.

As the surface of the bubble film adsorbs surfactant, the surface tension is reduced. Therefore, when the bubble film is locally thinned by local pressure, the surface tension will increase due to the thinning of the surfactant. It is precisely because of the difference in surface tension between the new surface and the original surface that when the bubble film is thinned by the external impact, an elastic restoring force is generated, and the bubble film can not be broken and played a role in stabilizing the bubble. If we manage to destroy this elasticity, we can destroy the stability of the foam. Some experts believe that the role of anti-foaming agent is to destroy the elasticity of the bubble film. When the anti-foaming agent is added to the foam system, it will diffuse to the gas-liquid interface, making it difficult for the foam-stabilizing surfactant to recover the elasticity of the film.

Adding a substance that accelerates the drainage of the bubble film (defoaming agent) can defoam. If the bubble film is thick, the process of draining to 30~40nm is long, and the self-healing effect of the bubble film is strong, and the bubble film has good elasticity, so the bubble life of the thick bubble film is long, and the bubble film drainage rate can reflect the bubble. The stability. The liquid film discharge rate is fast, so that the liquid film becomes thinner, and the bubble bursts when it is thin enough.

In the foaming water system, the hydrophobic silica (ie hydrophobic silica) particles are added separately to have defoaming effect. The hydrophobic silica particles attract the hydrophobic end of the surfactant on the surface of the bubbles, and the hydrophobic particles become hydrophilic particles and enter the water phase to complete the defoaming effect. The solid particles of hydrophobic silica are generally white carbon black particles treated with methyl silicone oil, silazane or DMC. It is partially immersed on the surface of the bubble to reduce the local surface tension of the bubble, plus the role of solid particles. Inserting a bubble like a needle tip accelerates the bursting of the bubble. Therefore, anti-foaming agents containing hydrophobic silica generally have better defoaming effects.