Antifoam Effects due to Solubilized Oils

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. 

FIG U RE 4.1 Effect of saturated decane vapor of foam volume generated by sparging aqueous micellar solutions of CijEOj for 10 min. (After Binks, B.P. et al., Colloids Surf. A, 216, 1, 2003.) 

FIGURE 4.2 Effect of relative partial pressure of 0.2 of decane on stability of foam generated by sparging an aqueous solution of 0.5 mM C12EO5 for 10 min. (Reprinted from Colloids Surf. A., 216, Binks, B.P., Fletcher, P.D.I., Haynes, M., 1. Copyright 2003, with permission 

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 

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