Deactivation, Emulsification, and Drop Sizes

Another problem with this argument of Koczo et al. [2] is that there is strong evidence that that hydrophobed silica-polydimethylsiloxane antifoams actually function by bridging foam films (see, e.g., reference [5] and Section 4.8). It is therefore possible that the relatively small dimensions of foam films could mean that diminution of drop sizes in the case of silica-polydimethylsiloxane antifoams could lead to enhanced antifoam activity simply because the concomitant increase in number concentration of drops should lead to an increase in the probability of their presence in foam films [23]. 

Bergeron et al. [4,26] also argue that another possible cause of diminished antifoam effectiveness with decreasing size could concern the likely critical rupture thickness of pseudoemulsion films. Thus, if the theory of Vrij and Overbeek [27] (see Section 1.3.3) can be applied to such films, then the smaller the radius of the films, and therefore drops, the smaller the critical rupture thickness, the longer the drainage time, and therefore the lower the frequency of rupture. However, in applying this theory in this context, we should remember that the stability of the pseudoemulsion film is 

4 DEACTIVATION OF HYDROPHOBED SILICA-POLYDIMETHYLSILOXANE ANTIFOAM BY DISPROPORTIONATION 

The polydimethylsiloxane oils used for antifoams usually spread on the air-water surfaces of surfactant solutions (see Section 3.6.2). At equilibrium, this process produces either complete wetting and duplex films for which 5 = 0 or pseudo-partial wetting and oil films in contact with lenses of bulk oil for which 5 0 (see Section 3.6.2.1). It has been shown by Racz et al. [3], and later confirmed by Denkov et al. [6, 7, 21], that deactivation of hydrophobed silica-polydimethylsiloxane antifoams correlates with the disappearance of this spread oil film. These studies used solutions of both anionic (SDS [3] and AOT [6,7]) and non-ionic surfactants (alkyl glu-copyranoside) [21]. Loss of the spread layer during deactivation is accompanied by an increase in surface tension to that of the pure surfactant solution [6]. It has also been directly observed using ellipsometry [21]. This finding is key to the understanding of deactivation because the presence of a spread layer of polydimethylsiloxane at the air-water surface is a clear indicator that oil has emerged into that surface. 

Emergence into the air-water surface is of course a necessary prerequisite for antifoam action. The presence of the spread layer, once formed, also actually facilitates both the emergence of oil drops into air-water surfaces [28] and the formation of the unstable oil bridges, which lead to foam film rupture [29, 30], 

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