Antifoaming effect

Fig. 9.10. Antifoam effectiveness F of a dispersion of hydrophobed silica/mineral oil antifoam as a...

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]... 

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. 

The utilisation of a PO with EO mixture (e.g., with 17-18% EO) strongly diminishes the antifoaming effect of the very high molecular weight polyether formed. 

Foaming occurred in this absorber which used crude oil as solvent. Foaming and antifoam effectiveness were successfully tested in a foam test apparatus. Prob-l n was solved by intomittent antifoam iiqection. 

Figure 8.7. Antifoam effect of polytetrafluorethylene particles ( 5 pm) as a function of the receding contact angle (from ref. (3))...

Here we briefly review some of the more important experimental techniques that have been used in the study of antifoam action. Many of the techniques are common to the study of both antifoam action and foam in the absence of antifoam. That is hardly surprising because establishing antifoam effectiveness requires knowledge of the latter. 

As we describe in detail elsewhere in this book, effective antifoams are usually both oil-based and insoluble in the medium to be defoamed. In order to function, it is necessary that the oil exhibit the property of emerging into the air-liquid surface of that medium. In turn this requires that the film of foaming liquid separating the oil from the air be unstable. The stability of these so-called pseudoemulsion films [1] is therefore a key issue in determining antifoam effectiveness. We therefore review the methods for studying such films. 

One of the earliest generalizations concerning antifoams states that they must be present as undissolved particles (or drops) in the liquid to be defoamed [2-4]. Indeed the presence of antifoam materials at concentrations lower than the solubility limit can even enhance foamability [5, 6]. One weU-known example concerns the foamenhancing effect of dissolved polydimethylsiloxanes (PDMSs) on hydrocarbon lube oils [5]. Amaudov et al. [7] report a similar, but small, effect for solubilized 2-butyl octanol on the foamability of saline aqueous micellar sodium dodecylbenzene sulfonate solutions where the oil has a significant antifoam effect on the stability of foam when present at concentrations above the solubility limit. Another example concerns the effect of dodecanol on the foam of aqueous micellar anionic surfactant solutions. According to Amaudov et al. [7] drops of dodecanol in excess of the solubility limit function as weak antifoams—at least in the case of saline micellar solutions of sodium dodecylbenzene sulfonate. By contrast, Patist et al. [8] find that solubilized... 

The generalization that antifoams must be present as undissolved entities has, however, occasionally been challenged [6,9,10]. A number of authors in fact report experimental results that purport to show antifoam effects due to additives that are solubilized in the foaming solution [11-13]. Thus, Ross and Haak [11], for example, identify two types of antifoam behavior associated with the effect of oils like tributyl phosphate and methyl isobutyl carbinol on the foam behavior of aqueous micellar solutions of surfactants such as sodium dodecylsulfate and sodium oleate. Wherever the oil concentration exceeds the solubility limit, emulsified drops of oil contribute to an effective antifoam action. However, it is claimed [11,14] that a weak antifoam effect is associated with the presence of such oils even when solubilized in micelles. The consequences of all this behavior are revealed if, for example, tributyl phosphate is added to micellar solutions of sodium oleate [11] at concentrations below the solubilization limit. A marked decrease in foamability is found immediately after dispersing the oil. As the oil becomes slowly solubilized, the foamability increases. However, even after the oil is completely solubilized, the foamability is still apparently less than that intrinsic to the uncontaminated surfactant solution [11]. By contrast, Arnaudov et al. [7] have more recently shown that the significant antifoam effect of n-heptanol on aqueous micellar solutions of sodium dodecylbenzene sulfonate (in the presence of NaCl) is almost completely eliminated after solubilization. 

There would appear then to be only limited evidence that oils which exhibit antifoam effects, when present as emulsified bulk phase, can also produce antifoam effects when present only as solubilizates in aqueous micellar solutions of surfactants. In many instances, alternative explanations for supposed observations of the latter are possible, which do invoke the presence of the oils as bulk phase. However some of the observations described here are difficult to dismiss. Of particular interest in this context are the findings of Koczo et al. [15], Lobo et al. [21], and Binks et al. [16] concerning the effect of solubilized alkanes on the foam stability of aqueous micellar solutions of various surfactants. Attempts to explain such effects by recourse to dynamic surface tension behavior after the manner of Ross and Haak [11] would appear to be unconvincing (see reference [22]). It is, however, possible that it may concern the effect of the solubilized oil on the relevant disjoining pressure isotherm. Wasan and coworkers [15,21] have suggested that the phenomenon is a consequence of the effect of solubilization of alkanes on intermicellar interactions. Lobo et al. [21] find that the instability of the foams formed from certain ethoxyl-ated alcohols in the presence of solubilized alkanes depends on the magnitude of the micellar second virial coefficient describing those interactions. Reduction of the... 

We may conclude by noting that, in general, antifoam effects with oils appear to require that the antifoam be undissolved in the foaming medium. Solubilization of the antifoam oil in micelles largely restores the foamability. However, there is some evidence to suggest that the some solubilized antifoam oils may have a weak adverse effect on the foamability or foam stability of some aqueous micellar surfactant solutions. We should note, however, that dissolved antifoam can even enhance foamability [5,6], at least in the absence of surfactant micelles. 

This approach of Ewers and Sutherland [38] was advanced with some claim to wide generality. However, as we will show here (see Sections 4.5,4.7, and 4.8), many substances yield antifoam effects without causing destabilizing surface tension gradients [9, 12, 39, 40-44], which confounds exclusive generality. Moreover, a difficulty with the mechanism concerns the implicit assumption that the antifoam is always to be found in the thinnest and most vulnerable part of the foam. Thus, if the antifoam entity spreads from the Plateau border, this will drag liquid into the adjacent foam films, which will stabilize those films by increasing their thicknesses. 

Another limitation of the approach of Ewers and Sutherland [38] is that it is essentially qualitative. However, there have been some attempts to put it on a quantitative basis [5, 45, 46]. The first such attempt was due to Shearer and Akers [5], who were concerned with the antifoam effect of PDMSs on lube oils. Here the effect of antifoam on a monolayer raft of bubbles is modeled theoretically and the results compared with experiment. 

The crude nature of the model presented by Prins [45] gives rise to an awkward feature in Equation 4.11. Thus, if S = 0, then we appear to have 8 = . Equation 4.11 in fact means that antifoam effectiveness is predicted to increase as S decreases. Thus, Prins [45] presents a model of antifoam action, based on spreading driven by surface tension forces, which predicts increasing antifoam effectiveness as those forces become weaker. Intuition would suggest an opposite conclusion (which is stated by Ewers and Sutherland [38] and deduced by Shearer and Akers [5]). 

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