Action of Antifoams

This chapter adonpts a complete review of the various mechanisms proposed for the action of antifoams over the past seven or so decades. It is a feature of this subject that some proposed mechanisms, although plausible, have been speculative. Thus, unequivocal experimental evidence has sometimes been lacking. Indeed, the full theoretical implications of proposed mechanisms have also often not been fully developed. In the main, aU of this derives from the extreme complexity of the relevant phenomena. As we have seen, foam is itself extranely complex, consisting of (usually) polydisperse gas bubbles separated by draining films. These films exhibit complicated hydrodynamics involving the distinct rheology of air-liquid surfaces and, for thin films, colloidal interaction forces. The nature of the foam film collapse processes that are intrinsic to foam are stUl imperfectly understood. 

Antifoams for aqueous systems are, for example, usually hydrophobic, finely divided, insoluble materials. Their preseuce therefore further complicates the complexities associated with foam. Indeed commercial antifoams for aqueous solutions usually consist of hydrophobic particles dispersed in hydrophobic oils. The action of such antifoams concerns the effect of a dispersion (of antifoam in foaming liquid) of a dispersion (the antifoam) on yet a third dispersion (the foam). 

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 

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The mode of action of antifoams is usually explained by assuming that they either force surfactant molecules away from the interfaces or they penetrate interfaces that are already occupied by surfactants, thereby creating defects. These defects weaken the mechanical strength of the foam lamellae and cause their rupture. 

As a rule the choice of antifoams is empirical and is imposed by the manufacture processes and conditions. However, the mechanism of action of antifoams remains unclear. 

Detailed analysis of these approaches is presented by Garrett [19]. As far as these processes are valid for homogeneous defoaming, the impossibility for a complete inhibition of foam formation in many systems, even when the solutions are saturated with antifoams, indicates that these kinetic phenomena are not sufficient for heterogeneous defoaming. Therefore, none of the considered points of view gives a satisfactory explanation of the mechanism of action of antifoams in heterogeneous systems. 

Garrett, P.R., Ed., Defoaming Theory and Industrial Applications, Marcel Dekker, New York, 1993. Garrett, P.R., The mode of action of antifoams, in Defoaming Theory and Industrial Applications, Marcel Dekker, New York, 1993, chap. 1. 

The main subject in this paragraph is foam breakdown mechanisms by antifoam particles. Various mechanisms proposed for the action of antifoams are described in many superior reviews [112,180-184]. A number of investigations about the breakdown of foams by dispersed insoluble oils [185-202], hydrophobic solid particles [203-214], and oils and particles in combination [215-220] were reported as antifoams. The mixed-type antifoams exhibit excellent defoaming performance thus they are widely used in many industries. Several theories have been presented in the literatures on the mechanisms of defoaming through the use of three types of antifoams [216,217]. A... 

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. 

To frame a context for describing the mode of action of antifoams in later chapters, we are concerned here with the criteria that determine whether oils will emerge into gas-liquid surfaces and whether they will spread at those surfaces. Those criteria are not confined to consideration of thermodynamic stability alone. In reality metastable states can exist that have a profound effect on whether antifoam action occurs. Complexity also arises because spreading oil molecules can be partially solubilized in the foam-stabilizing surfactant monolayers present at the relevant gas-liquid surfaces. All of these aspects are reviewed here, thermodynamic criteria being... 

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