Antifoam Synergy

This antifoam synergy appears to be quite general for all manner of hydrophobic particles and oils. For example, intrinsically hydrophobic organic particulates, such as precipitates of polyvalent metals with long-chain alkyl phosphates or carboxy-lates, may be combined with either hydrocarbons [43,71, 211, 212] or PDMSs [213] to produce synergistic antifoam behavior. Moreover, antifoam synergy has also been 

---

Mechanistic studies of oil-particle antifoam synergy did not appear in the scientific literature until the mid-seventies [53, 67, 71, 207, 209] of the last century. Indeed only a few references to it appeared before 1970 [39, 106]. However, descriptions of many examples of mixtures of hydrophobic particles and oils have been appearing in the patent literature since the early fifties of that century. Examples of those early patents, filed before 1980, are listed in Appendix 4.1 as Tables 4.A1 through 4.A3. Here they are categorized as mixtures of hydrophobed mineral particles and silicone oils (Table 4.A1), mixtures of hydrophobed mineral particles and organic liquids (Table 4. A2), and mixtures of intrinsically hydrophobic organic particles and organic liquids (Table 4. A3). 

No experimental study of the antifoam effect of mixing hydrophobed crystals of weU-characterized habit with oils has been reported. Experimental confirmation of the expectation of synergy only under restricted circumstances is therefore not available. However, Frye and Berg [75] have shown evidence of only weak antifoam synergy when hydrophobed ground-glass particles, which possess sharp edges (and are therefore relatively effective antifoams when used alone), are mixed with hexadecane. 

Examination of Table 4.11 reveals that, for example, if O w < 15° in the absence of an oil layer, then none of the configurations j = 3-5 will result in pseudoemulsion film rupture. The presence of the oil layer would, however, result in pseudoemulsion film rupture in all cases provided Oqw > 90° (where of course we must have Oqw > 135° if configuration 7 = 4 is to occur at all). Moreover, if 0 < 15°, then as we have seen, there exists no configuration at the air-water surface that would permit rupture of air-water-air foam films (see Table 4.6). The presence of a spread oil layer is therefore seen to expand the circumstances in which oil-particle antifoam synergy is to be expected relative to those expected in the absence of a spread layer. 

Garrett et al. [108] and later Ashcroft et al. [109] have claimed that a rather novel application of the antifoam synergy found with hydrocarbon-calcium soap mixtures can be applied in this context. Consider then a cleaning liquid where both the hydrocarbon and a sodium soap (or fatty acid) are solubilized in a concentrated aqueous micellar solution of another surfactant. If use of the product involves dilution in hard water, then both the solubilized hydrocarbon and soap will be precipitated as the concentration of micelles decreases and the water hardness increases. It is claimed [108,109] that synergistic foam control is then observed as exemplified by the results shown in Table 8.2. Possibly precipitation of bulk phase hydrocarbon is nucleated on the calcium soap particles so that a hydrocarbon-calcium soap antifoam entity could be formed in situ giving rise to that synergy. 

A similar comparison of values of F with pseudoemulsion film stability is shown in Table 9.2, but using mixtures of hexadecane and hydrophobed silica. The usual antifoam synergy is observed regardless of the presence or absence of the latex polymer. No evidence of enhanced pseudoemulsion stability, due to the presence of the... 

One of the most striking aspects of antifoam behaviour is the synergy shown by the mixtures of hydrophobic particles and apolar oils dispersed in a foaming solution. A list of such mixtures is given in [19]. The effect of hydrophobic particles on the defoaming ability of oils is illustrated in Fig. 9.10. 

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]... 

One of the most striking aspects of antifoam behavior is the synergy shown by mixtures of insoluble hydrophobic particles and hydrophobic oils when dispersed in aqueous media. We illustrate this behavior in Figure 4.69 with a plot of the ratio F (defined by Equation 4.71) against antifoam composition for a mixture of methyl-silane hydrophobed silica and liquid paraffin. Foam from a solution of 0.5 g dm ( 1.4 X 10 M) commercial sodium (C10-C14) alkylbenzene sulfonate was generated by cylinder shaking [43]. The liquid paraffin is seen to be virtually without effect on... 

In what follows we explain why particles are usually surface active at liquid interfaces. We also discuss mechanisms by which particles can rupture thin films in foams and emulsions. Because antifoam formulations are frequently dispersions of particle-containing oil droplets in an aqueous phase, we also allude to the way in which oil droplets can break foam films and to the synergy that exists between the oil and particles in this process. Finally, we illustrate some of our own work on the behavior of particle monolayers at (mainly) oil/water interfaces. Particularly important is the observation that very long range electrical repulsion between charge-bearing particles can occur through nonpolar oils at the oil/water interface. 

---