Hydrophobic particles, antifoaming

In contrast to the simplicity of using fatty acids or soaps, hydrophobic oil-hydrophobic particle antifoams or their precursors (e.g., substances which transform into hydrophobic particles in situ) must be incorporated as stable emulsions where coalescence, creaming, or sedimentation upon storage in the detergent liquid is... 

Despite a reputation for causing paint film defects, even polydimethylsiloxane-hydrophobed silica antifoams can also apparently find use in waterborne latex paints if certain preventive measures are applied (see, e.g., reference [28] and Section 9.3.2). It is tempting then to infer that, despite the complexity of waterborne latex paint formulations, the conventional oil-hydrophobed particle antifoams listed in Chapter 4 (Tables 4.A1 through 4.A3) find ready application in fhis context, albeit with additional components such as emulsifiers designed fo ease dispersal or structurants to ensure stability of the stored antifoam against sedimentation. Practical experience then implies that the rupture of foam films confaining high volume fractions of latex polymer particles by conventional antifoams is readily achieved. 

Although silicone oils by themselves or hydrophobic particles (e.g., specially treated silica) are effective antifoams, combinations of silicone oils with hydrophobic silica particles are most effective and commonly used. The mechanism of film destruction has been studied with the use of surface and interfacial tensions, measurements, contact angles, oil-spreading rates, and globule-entering characteristics for PDMS-based antifoams in a variety of surfactant solutions.490 A very recent study of the effect of surfactant composition and structure on foam-control performance has been reported.380 The science and technology of silicone antifoams have recently been reviewed.491... 

Precipitated and pyrogenic silicas are widely employed in antifoams used in industry. To be effective, the silica surface must be rendered hydrophobic by reaction with an agent such as polydime thylsiloxane. The hydrophobic particles are dispersed in a carrier fluid such as mineral or silicone... 

Foams can occur in the intestines where they may cause indigestion, pain, and/or a sense of being full, collectively known as dyspepsia [884]. Pharmaceuticals designed to combat dyspepsia usually contain antifoams, antacids, and possibly enzymes [884]. The most common antifoams seem to be polymethylsiloxanes (referred to as dimethicone in pharmaceutical literature [884]) mixed with hydrophobic particles, such as hydrophobic silica. The formulation is delivered as a tablet, suspension, or emulsion (simethicone tablets simethicone oral suspensions simethicone emulsions). Foams can also be used to administer drugs, such as in contraceptive foams. 

The results about spreading of antifoams as well as the inhibition of black spot formation in microscopic foam films, permit to draw a conclusion about the existence of a special mechanism of heterogeneous defoaming, based on the stability analysis [14,25,48]. The details of this mechanism are given later [18,55,56]. For the defoaming action of hydrophobic solid particles in heterogeneous system another (bridge) mechanism has been proposed [57]. Later it was applied on drops, mixtures of hydrocarbon oil and hydrophobic particles [19,20,53,54]. 

In this Section instability of asymmetric films is explained by decrease in the surfactant adsorption. Another reason for this instability can be the presence of solid particles at the water-oil interface. Such a heterogeneous defoaming is created when a foam is broken down by the antifoam drops that contain solid hydrophobic particles. The mechanism of action of such types of antifoams will be discussed in Section 9.4. 

One of the most striking aspects of antifoam behaviour is the synergy shown by the mixtures of hydrophobic particles and apolar oils dispersed in a foaming solution. A list of such mixtures is given in [19]. The effect of hydrophobic particles on the defoaming ability of oils is illustrated in Fig. 9.10. 

Garrett came to the conclusion that most important for the synergy action of an oil-particle antifoam seems to be the ability of the particles to facilitate the appearance of oil droplets into the air/water surface. However, the sizes of the antifoam oil/particle composites should be sufficiently small to ensure a high probability of presence in a given foam film, but not so small to slow down the film drainage and suppress antifoaming effect. It order to possess such properties the particles should be hydrophobic but not completely wetted by the oil. The contact angle 9ow at the oil/water interface should satisfy the condition [20]... 

According to Koczo et al. [86] antifoam drops containing solid hydrophobic particles flow out from the foam films into Plateau borders and are trapped there. In the borders these drops form unstable asymmetric films, whose rupture leads to formation of lenses. During the process of diminishing of the border size a bridge is formed through which the hydrophobic particle destabilises the film between the lens and the opposite surface of the border. 

The synergetic antifoaming effect of mixtures of insoluble hydrophobic particles and hydrophobic oils, when dispersed in an aqueous medium, has been weU established in the patent Hterature. These mixed antifoamers are very effective at very low concentrations (10-100 ppm). The hydrophobic particles may be hydrophobised silica and the oil is PDMS. 

Mixture of insoluble oil and hydrophobic particles. The oil phase can be silicone or hydrocarbon oil, the solid phase is any of class 2. The solid content of the mixture is 1—20%. These mixed-type antifoams are very effective, even at very low concentrations (10—1000 ppm) thus they are widely applied for the inhibition of aqueous foams (83—85, 87, 88). 

A different antifoaming mechanism was suggested by Kulkarni et al. (96). They found that surfactants adsorb on the surface of hydrophobic particles during antifoaming, and this adsorption results in deactivation of the particles. On the basis of this observation, they postulated that the adsorption of surfactants onto the hydrophobic particles is so fast that it results in surfactant depletion around the particle in a foam film, and this effect breaks the film. However, no direct proof was presented on this theory. Moreover, depletion of surfactant would cause the film liquid to flow toward the particle because of the increased surface tension (Gibbs— Marangoni effect), and thus cause a stabilizing effect. 

Figure 33. Pseudoemulsion film formed between mixed antifoam (oil with hydrophobic particles) and air, on the tip of a glass capillary (schematic). The film area is increased by pushing the antifoam out of the capillary. The particles destabilize the pseudoemulsion film by partially submerging into the aqueous phase.

Figure 35. Contact angles at the hydrophobic particles of mixed antifoam.

The penetration depth of oil lenses (with or without particles) is generally deeper than that of the particles alone because the shape of the lens is controlled by the air—oil (crAO) and the oil—water (interfacial tensions. In practical systems, aAQ is much higher than aqueous phase (Figure 31). This penetration depth difference is a reason that the mixed-type antifoams are much more effective than the hydrophobic particles alone. 

At a given point in the foam drainage, the drops get trapped in the shrinking Plateau borders (Figures 37a and 37b). Consequently, the capillary pressure increases in the Plateau borders and destabilizes the pseudoemulsion film formed between the trapped drop and the bubble, thus allowing the antifoam oil with the particles to enter the foam surface and form a lens (Figure 37c). Without hydrophobic particles, the oil lens spreads at the water—air surface. The spread lens has a low penetration... 

Debate continues about the detailed mechanisms of antifoaming by these synergistic combinations of insoluble oils and hydrophobic particles. These dewetting ideas imply that the hydrophobic particles become detached from the carrier... 

Fig. 33 illustrates the SLM reflection features of the pseudoemulsion films from the model foaming solution formed on the top of the hydrophobic silica antifoams (C series) and the silicone resin antifoams (D series). Estimated images of the vertical sectional views are also shown under these SLM features. It is seen that the pseudoemulsion films were confirmed on the top of their particles when the thickness of the surfactant solutions was slightly... 

At present, the most effective and versatile chemical antifoamers are mixtures. In fact, modem commercial formulations often contain mixtures of silicone oils, silicone surfactants or silica gels, with possibly two or more types of dispersed hydrophobic aggregates (with sizes around 1 micron, specific gravity 1.0-1.3, with rough fractal shapes). These mixed-type antifoamers are very effective at low concentrations (10-1000 ppm) and are widely used. The (hydrophobic) particles may be hydrophobized silica or glass and are often referred to as the activator, with the hydrocarbon or poly-dimethylsiloxane (PDMS) liquid or oil being referred to as the carrier. However, less expensive single-system antifoamers are commonly used, and these exist in many different forms such as soluble liquids, insoluble... 

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