Foaming and Antifoaming by Aqueous Solutions of Surfactants

The pressure difference across a curved interface due to the surface or interfacial tension of the solution is given by the Laplace equation 

Absolutely pure liquids do not foam. Foam is also not pronounced in mixtures of similar types of materials (e.g., aqueous solutions of hydrophilic substances). Bubbles of gas introduced beneath the surface of an absolutely pure liquid rupture immediately on contacting each other or escape from the liquid as fast as the liquid can drain away from them. For true foaming to occur, the presence of a solute capable of being adsorbed at the L/G interface is required. The presence of this surface-active solute produces lamellae between the gas cells of the foam that have 

Surfactants and Interfacial Phenomena, Third Edition. Milton J. Rosen ISBN 0-471-47818-0 2004 John Wiley Sons, Inc. 

FIGURE 7-1 Plateau border at point of meeting of three bubbles. 

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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... 

Foamed emulsions are disperse systems with two disperse phases (gas and liquid) in the disperse medium (surfactant solution). Water foamed emulsions are formed when foams or aqueous surfactant solutions are used to clean up oil deteriorated surfaces, in the process of oil flotation of waste waters, in firefighting when the foam contacts various organic liquids and in the processes of chemical defoaming (foam destruction by antifoams). Individual foamed emulsions can have practical importance e.g. a foamed emulsion of bitumen is used in road coating foamed emulsions from liquid fuels are used as explosives. 

By virtue of their structure, surfactants perform several functions in aqueous solution. Often, however, there are processes in which one uses a surfactant for a desired property and does not want the other properties inherent in the surfactant. For example, one may want detergency without foam. Modihcation of the surfactant molecule offers minimal relief. Consequently, antifoam compounds... 

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. 

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