Antifoaming mechanisms

Based on this low surface tension feature and the commonly observed insolubiUty of defoamers, two related antifoam mechanisms have been introduced (29) (/) The agent dispersed in the form of fine drops enters the Hquid film between bubbles and spreads as a duplex film. The tensions created by this Spreading lead to the mpture of the original Hquid film. (2) A droplet of the agent enters the Hquid film between bubbles, but rather than spreading produces a mixed monolayer on the surface. This monolayer, if of less coherence than the original film-stabilizing monolayer, causes destabilization of the film. 

Two related antifoam mechanisms have been proposed for low surface tensions of certain defoamer formulations ... 

Figure 7.45 A schematic drawing of the antifoaming mechanism of antifoam droplets (i) entering o foam lamella, (ii) bridging-de-wetting and (iii) rupturing the foam wall.

Below, in Sections 5.2 and 5.3, we consider effects related to the surface tension of surfactant solution and capillarity. In Section 5.4 we present a review of the surface forces due to intermo-lecular interactions. In Section 5.5 we describe the hydrodynamic interparticle forces originating from the effects of bulk and surface viscosity and related to surfactant diffusion. Section 5.6 is devoted to the kinetics of coagulation in dispersions. Section 5.7 regards foams containing oil drops and solid particulates in relation to the antifoaming mechanisms and the exhaustion of antifoams. Finally, Sections 5.8 and 5.9 address the electrokinetic and optical properties of dispersions. 

Antifoaming Mechanisms. Several theories can be found in the literature on the mechanisms of antifoaming by the three types of antifoams. 

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 37. Suggested antifoaming mechanisms for mixed-type antifoams. Key a, the oil drops containing solid particles collect in the Plateau borders b, they get trapped in the thinning Plateau border c, the pseudoemulsion film breaks, a drop enters and forms a solid phis oil lens d, the lens gets trapped in a later stage of thinning and e, the lens bridges the film at the Plateau border and the bridge ruptures.

Figure 38 summarizes our current knowledge on antifoaming mechanisms. 

Figure 38. Summary of antifoaming mechanisms for aqueous foams.

Solid-Phase Components. Dispersed sohds are vital ingredients in commercial antifoam formulations. Much of the cmrent theory on antifoaming mechanism ascribes the active defoaming action to this dispersed solid phase with the liquid phase primarily a carrier fluid, active only in the sense that it must be surface-active in order to carry the solid particles into the foam films and cause destabilization. For example, PDMS, despite its considerable effectiveness in nonaqueous systems, shows little foam-inhibiting activity in aqueous surfactant solutions. It is only when compounded with hydrophobic silica [7631-86-9] to give the so-called silicone antifoam compounds that highly effective aqueous defoamers result. The three main solid-phase component classes are hydrocarbons, silicones, and fluorocarbons. 

FIG. 29 Antifoaming mechanisms for the oil-type antifoams (a) and the solid-type antifoams (b). (From Refs. 205 and 208.)... 

Figure 8.11. The suggested antifoaming mechanism for a mixed-type antifoamer (a) oil drops (containing solid particles) collect in the Plateau border (b) the drops become trapped in the thinning border (c) the pseudo-emulsion film breaks and a drop enters and forms a solid plus oil lens (d) the lens becomes trapped during thinning (e) the lens bridges the film at the plateau border and the bridge (from ref (12)), reproduced by permission of Academic Press...

Much recent work on the mode of action of antifoams has been concerned with both establishing the importance of the stability of the pseudoemulsion films, which separate antifoam oil drops from gas-liquid surfaces and the necessity that the oil spread at that surface. As we will show in later chapters, the stability or otherwise of those films has been shown to be a key aspect of antifoam mechanism. However, it has also been firmly established that spreading is not a necessary property of the oil [15]. Nevertheless, the presence of spread oil layers at gas-liquid surfaces can have profound effects on the relative effectiveness of antifoams. 

Early speculations about the mode of action of PDMS-based antifoams assume that duplex film spreading from drops in foam films induces shear in the intralamellar liquid, which leads to foam film rupture. Clearly the rate of spreading would be a key aspect of that mechanism. However, as we describe in Chapter 4, this view of antifoam mechanism is now somewhat discredited. Nevertheless, other aspects of antifoam action, such as the effect of antifoam viscosity on deactivation during prolonged interaction with foam generation (see Chapter 5), could be determined by spreading rates. It is therefore appropriate to briefly review this topic here. 

Theories of antifoam mechanism appear to fall into two broad categories those that concern modification of surface tension gradients and those that concern formation of capillary instabilities in foam films. Theories that concern surface tension gradients appear to be rather speculative. Some of these theories attribute antifoam action to the generation of a surface tension gradient that supposedly drives fluid... 

We are therefore left without an entirely satisfactory mathematical treatment of the antifoam mechanism proposed by Ewers and Sutherland [38]. Clearly the problem is difficult. However, a solution would permit more serious assessment of the significance of the mechanism particularly with regard to the effect of the magnitude of S, or Ao w Such a solution should take account of the effect of compression of the surfactant monolayer at the foaming liquid-air surface by the spreading front. This... 

There appears to be a consensus that necessary properties of antifoam oils for aqueous foams include insolubility in surfactant solution and a tendency to emerge (or enter ) into the air-water surfaces of foam films. Here we review early work concerning the antifoam behavior of neat oils possessing these properties. Of particular interest is the interpretation of the observed behavior using the theories of antifoam mechanism outlined in Sections 4.4 and 4.5. 

Robinson and Woods [9] produced perhaps one of the earliest experimental studies of antifoam mechanism. The study concerned the effect of various undissolved oils on the foam behavior of both aqueous and non-aqueous solutions of surfactant The oils included alkyl phosphates, alcohols (including diols), fatty add esters, and PDMS. The solutions were of aerosol OT (AOT or sodium diethylhexyl sulfosucci-nate) in either ethylene glycol or triethanolamine and sodium alkylbenzene sulfonate in water. Many quoted entry and spreading coefficients, however, violate Equations 3.11 and 3.12, which implies that these coefficients were non-equilibrium (i.e., initial) values where the relevant liquids are not mutually saturated. Robinson and Woods [9] observed that for these systems, wherever < 0, no antifoam effect is found. This then represents some evidence that a positive value of the initial entry coefficient is necessary for antifoam action. 

Another rare study of the antifoam mechanism of PDMS oils in a non-aqueous medium was presented 25 years later by Callaghan et al. [114]. In contradiction of... 

INERT HYDROPHOBIC PARTICLES AND CAPILLARY THEORIES OF ANTIFOAM MECHANISM FOR AQUEOUS SYSTEMS... 

It is clear from the patent literature that an antifoam mechanism invoking Marangoni spreading of the oil (see Figure 4.10) underlies some of the thinking... 

Bergeron, V. PDMS based antifoams mechanisms and performance, in Les Mousses Moussage et Demoussage (Lagerge, S., ed.), EDP Sciences, Cahiers de Formulation, Paris, 2000, Vol 9, p 116. 

Since most eommercially effective antifoams are oil based, a chapter is devoted to the entry and spreading behavior of oils and the role of thin film forces in determining that behavior. The book reviews the mode of action of antifoams, including theories of antifoam mechanisms and the role of bridging foam films by particles and oil drops. It also addresses issues related to the effect of antifoam concentration on foam formation by air entrainment and the process of deactivation of mixed oil-particle antifoams during dispersal and foam generation. 

There are many contributions to account for antifoam mechanisms [65-67] but none of them is generally accepted for all systems. Yet, it is generally approved that antifoams have to meet at least the following requirements ... 

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