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Discussion on the Defoaming Mechanism of Defoamers
The mechanism of defoamers remains an area of ongoing research, and there is no unified understanding of how defoamers work. However, several theories and mechanisms have been proposed by researchers over the years, with the most common ones outlined below.
1. General Defoaming Mechanism
Robinson Defoaming Mechanism and Ross Hypothesis: The Robinson defoaming mechanism is considered the basis of the Ross hypothesis. The Robinson mechanism suggests that defoamers break down foam by disrupting the foam drainage process and utilizing the Marangoni effect (the movement of liquid due to surface tension differences). This theory emphasizes the physical properties of the defoamer and its interaction with the foam structure.
The Ross hypothesis, on the other hand, assumes that defoamers work as non-soluble droplets. While this mechanism holds for some defoamers, it does not cover all cases, especially those where defoamers act in dissolved states. Thus, the Ross hypothesis is not comprehensive, as it does not explain the behaviour of defoamers that perform effectively while dissolved in the foam system.
2. Polysiloxane Defoamer Mechanism
Polysiloxane-based defoamers are commonly used and have several proposed mechanisms, including:
Bridging - Spreading Mechanism: Polysiloxane has low surface tension, allowing it to spread easily across the foam liquid film. This spreading thins the liquid film and eventually leads to bubble rupture. However, this theory does not explain why a mixture of polysiloxane and solid ions may perform differently from pure polysiloxane.
Bridging - Dehumidification Mechanism: This theory suggests that polysiloxane’s hydrophobicity plays a role in breaking down foam by reducing moisture content within the foam structure. This mechanism does not adequately explain the defoaming effect of high-viscosity polysiloxanes.
Spreading - Liquid Entraining Mechanism: This theory posits that polysiloxane can cause the foam to collapse by entrapping liquid. However, this mechanism has not been conclusively proven, as there are instances where polysiloxane can break foam without visibly spreading over the foam surface.
3. Defoaming Mechanism of Hydrophobic Solid Particles
Hydrophobic solid particles in a foam system can attract the hydrophobic ends of surfactants, transforming the particles into hydrophilic surfaces. This reduces the surfactant concentration in the foam film, ultimately causing foam instability and rupture. However, this explanation is limited and does not apply universally across all defoamers. Other defoaming mechanisms, such as the expansion of the defoamer and its interaction with electrolytes, may also play a role in foam destruction.
4. Polyether Modified Silicone Oil Defoaming Mechanism
Polyether modified silicone oil is an effective defoamer, and two primary mechanisms explain its action:
Bridging - Stretching Mechanism: The surface tension of polyether modified silicone oil is much lower than that of the liquid film. This difference in surface tension allows the defoamer to spread across the foam, gradually thinning the foam film until it ruptures. The mechanism explains the action of low-viscosity silicone oils but is less effective in explaining the behaviour of silicone pastes.
Bridging - Dehumidification Mechanism: This mechanism explains how hydrophobic particles in silicone oils disrupt foam by interacting with the foam film’s structure. The particles form a bridge between the liquid films of the bubbles, causing them to break.
Polyether modified silicone oils typically exhibit three essential characteristics: they are largely insoluble in the foaming solution (with any dissolved material potentially aiding foam formation), they have low surface tension relative to the foaming solution, and they disperse rapidly throughout the foam system. These qualities allow polyether modified silicone oils to effectively control both foam formation and stability.
5. Conclusion: The Complexity of Defoaming Mechanisms
As American colloidal chemist Ross (S.) famously stated, "No defoaming mechanism can cover all defoaming phenomena." Different types of defoamers correspond to various defoaming mechanisms depending on the system in which they are used. While theories like the Robinson mechanism and Ross hypothesis are useful for understanding some defoamers, the wide range of defoamer types and the specific foam systems they target necessitate multiple, complementary explanations for defoaming actions.
In summary, each defoamer type interacts with foam systems differently, and their mechanisms of action vary. These mechanisms are centred around disrupting the stability factors of foam, whether through changes in surface tension, moisture content, or the physical deformation of the foam structure.
1. General Defoaming Mechanism
Robinson Defoaming Mechanism and Ross Hypothesis: The Robinson defoaming mechanism is considered the basis of the Ross hypothesis. The Robinson mechanism suggests that defoamers break down foam by disrupting the foam drainage process and utilizing the Marangoni effect (the movement of liquid due to surface tension differences). This theory emphasizes the physical properties of the defoamer and its interaction with the foam structure.
The Ross hypothesis, on the other hand, assumes that defoamers work as non-soluble droplets. While this mechanism holds for some defoamers, it does not cover all cases, especially those where defoamers act in dissolved states. Thus, the Ross hypothesis is not comprehensive, as it does not explain the behaviour of defoamers that perform effectively while dissolved in the foam system.
2. Polysiloxane Defoamer Mechanism
Polysiloxane-based defoamers are commonly used and have several proposed mechanisms, including:
Bridging - Spreading Mechanism: Polysiloxane has low surface tension, allowing it to spread easily across the foam liquid film. This spreading thins the liquid film and eventually leads to bubble rupture. However, this theory does not explain why a mixture of polysiloxane and solid ions may perform differently from pure polysiloxane.
Bridging - Dehumidification Mechanism: This theory suggests that polysiloxane’s hydrophobicity plays a role in breaking down foam by reducing moisture content within the foam structure. This mechanism does not adequately explain the defoaming effect of high-viscosity polysiloxanes.
Spreading - Liquid Entraining Mechanism: This theory posits that polysiloxane can cause the foam to collapse by entrapping liquid. However, this mechanism has not been conclusively proven, as there are instances where polysiloxane can break foam without visibly spreading over the foam surface.
3. Defoaming Mechanism of Hydrophobic Solid Particles
Hydrophobic solid particles in a foam system can attract the hydrophobic ends of surfactants, transforming the particles into hydrophilic surfaces. This reduces the surfactant concentration in the foam film, ultimately causing foam instability and rupture. However, this explanation is limited and does not apply universally across all defoamers. Other defoaming mechanisms, such as the expansion of the defoamer and its interaction with electrolytes, may also play a role in foam destruction.
4. Polyether Modified Silicone Oil Defoaming Mechanism
Polyether modified silicone oil is an effective defoamer, and two primary mechanisms explain its action:
Bridging - Stretching Mechanism: The surface tension of polyether modified silicone oil is much lower than that of the liquid film. This difference in surface tension allows the defoamer to spread across the foam, gradually thinning the foam film until it ruptures. The mechanism explains the action of low-viscosity silicone oils but is less effective in explaining the behaviour of silicone pastes.
Bridging - Dehumidification Mechanism: This mechanism explains how hydrophobic particles in silicone oils disrupt foam by interacting with the foam film’s structure. The particles form a bridge between the liquid films of the bubbles, causing them to break.
Polyether modified silicone oils typically exhibit three essential characteristics: they are largely insoluble in the foaming solution (with any dissolved material potentially aiding foam formation), they have low surface tension relative to the foaming solution, and they disperse rapidly throughout the foam system. These qualities allow polyether modified silicone oils to effectively control both foam formation and stability.
5. Conclusion: The Complexity of Defoaming Mechanisms
As American colloidal chemist Ross (S.) famously stated, "No defoaming mechanism can cover all defoaming phenomena." Different types of defoamers correspond to various defoaming mechanisms depending on the system in which they are used. While theories like the Robinson mechanism and Ross hypothesis are useful for understanding some defoamers, the wide range of defoamer types and the specific foam systems they target necessitate multiple, complementary explanations for defoaming actions.
In summary, each defoamer type interacts with foam systems differently, and their mechanisms of action vary. These mechanisms are centred around disrupting the stability factors of foam, whether through changes in surface tension, moisture content, or the physical deformation of the foam structure.