Silicone surfactants antifoaming mechanisms

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. 

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... 

Hydrophobic antifoams, the second group, attract surfactant from the foam bubbles and lead to film, rupture dispersion of hydrophobed silica particles in silicone oil is believed to occur following this mechanism [49]. Another hypothetical mechanism for a silicone defoamer is proposed by Dippenaar [50], Garrett [51], and Berg [52], in which the hydrophobic particles bridge the foam lamella, dewetting occurs at the surface of the particle, and, consequently, the foam breaks. 

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