Destabilization of foams

All foams are thermodynamically unstable in accordance with the following equation that we have seen in earlier chapters (e.g. Chapters 3 and 12)  

There are several mechanisms of foam destabilization gas diffusion, drainage and coalescence. It is important to remember that these processes are typically strongly coupled, which makes it difficult to analyse and optimize a foam in a given situation. It is the rate (kinetics) of the various processes fliat determines whether these are important. Each of these destabilization processes will now be discussed in more detail. 

Some foams collapse due to disturbances, whereas others may go through the full number of stages. The rate at which they undergo these processes may differ greatly from one foam system to another. External disturbances can be vibrations or draughts, organic 

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Cooper, D. J. Husband, F. A. Mills, E. N. C. Wilde, P. J. Role of Beer Lipid-Binding Proteins in Preventing Lipid Destabilization of Foam. J. Agric. Pood Chem. 2002, 50, 7645-7650. 

The role of surfactants in stabilization/destabilization of foam (air/liquid dispersions) is similar to that for emulsions. This is due to the fact that foam stability/instability is determined by the surface forces operative in liquid films between air bubbles. In many industrial applications, it is essential to stabilize foams against collapse, e.g., with many food products, foam in beer, fire-fighting foam, and polyurethane foams that are used for furniture and insulation. In other applications, it is essential to have an effective way of breaking the foam, e.g., in distillation... 

Cooper, D.J., Husband, F.A., Mills, E.N.C., and Wilde, P.J. Role of beer lipid-binding proteins in preventing lipid destabilization of foam, /. Agric. Food Chem., 50, 7645, 2002. 

Barker, D.K., Wilde, P.J., and Clark, D.C. Control of surfactant-induced destabilization of foams through polyphenol-mediated protein-protein interactions, /. Agric. 

Influence of Unstable Pseudoemulsion Film Antifoaming and Defoaming Action. When the pseudoemulsion films are unstable in the three-phase foam, the oil generally destabilizes the foam. Destabilizing of foam has important applications as antifoaming or defoaming in several industrial processes (83, 84). Foam inhibitors are also used in the petroleum industry, such as in gas—oil separation, in natural gas processing, and in distillation processes (85). 

Destabilization of foams is a dynamic process that includes disproportionation, coalescence in addition to drainage of the thin film between bubbles. In addition to the bulk phase viscosity, all of these processes involve interfacial film properties [2, 4, 7, 10, 11]. The greater stabilizing effect may be attributed to a greater enhancement of the local viscosity in a foam lamellae which tends to inhibit film drainage, as well as to increased thickness of the mixed adsorbed layer which tends to enhance steric stabilization and inhibit bubble coalescence [3],... 

The importance of the thin film between the mineral particle and the air bubble has been discussed in a review by Pugh and Manev [74]. In this paper, modem studies of thin films via SFA and interferometry are discussed. These film effects come into play in the stability of foams and froths. Johansson and Pugh have studied the stability of a froth with particles. Small (30-/ m), moderately hydrophobic 6c = 65°) quartz particles stabilized a froth, while more hydrophobic particles destabilized it and larger particles had less influence [75]. 

Where required, silicones are usually added to the feedline at concentrations of 0.1 to 0.5% w/w. They are of long-lasting durability and generally applied to prevent foams developing, rather than to destabilize existing foams. 

Thirdly, (Type C) oil will enter the surface of foam lamellae and then spread over the lamellae surfaces if both E and S are greater than zero. This latter behavior, typically, will destabilize the foam. 

Often, dispersed solids are active in defoaming in suitable formulations. Some liquid defoamers are believed to be active only in the presence of a solid. It is believed that a surface-active agent present in the system will carry the solid particles in the region of the interface and the solid will cause a destabilization of the foam. 

The foam-negative effects of lipids can also be counteracted with the addition of a lipid-binding protein, wheat puroindoline (PIN), to beer. The PIN may bind the residual free lipids in such a way that they can no longer destabilize the foam. (Adapted from Cooper et ah, 2002)... 

Among the many available defoamers, crude oil has been used to prevent the formation of foams, or destroy foams already generated, in a variety of industrial processes [43,46,327]. Crude oil can also destabilize foams applied in petroleum reservoirs, i.e., foams in porous media [3,306,328-331] (see Section 11.2.2). Although crude oils tend to act as defoamers, foams actually exhibit a wide range of sensitivities to the presence of oils, and some foams are very resistant to oil [3,332,333]. Many system variables influence the oil tolerance of a given foam and many attempts have been made to correlate foam-oil sensitivity with physical parameters [307,332-337]. These have met with mixed success [114,338],... 

Foams stabilized with proteins, such as egg white, can be quite sensitive to the presence of oil (fat) droplets (see also Section 5.6.7). Just as is the case with foam sensitivity to oil in other industries, the presence of even small amounts of oil (0.03 mass % in foods [814]) can destabilize a foam. Oils such as lipids are thought to interfere with foaming by displacing proteins from the air-aqueous interface. One approach to improving the foam stability involves combining acidic proteins, such as whey or serum albumin, with basic proteins [814]. 

Raterman, K.T. An Investigation of Oil Destabilization of Nitrogen Foams in Porous Media in Proceedings, 64th. Ann. Tech. Conference, Society of Petroleum Engineers Richardson, TX, 1989, paper SPE 19692. 

Substance used to destabilize, or inhibit the formation of foam. Commercial antifoams are usually some form of poly-(dimethylsiloxane) polymer, but short chain alcohols are frequently effective. 

Anderson, M., Brooker, B.E. 1988. Dairy foams. In Advances in Food Emulsions and Foams (E. Dickinson, G. Stainsby, eds.), pp. 221-256, Elsevier Applied Science, London,. Anderson, M., Brooker, B.E., Needs, E.C. 1987. The role of proteins in the stabilization/ destabilization of dairy foams. In Food Emulsions and Foams (E. Dickinson, ed.), pp. 100-109, Royal Society of Chemistry, London. 

Foam stability is most commonly monitored by following its collapse and liquid drainage. Both are macroscopic properties and can easily be measured. These properties are not directly related to microscopic events such as drainage occurring from lamellae, but they allow a description of foam behaviour over time, and, in conjunction with the transient development of bubble size distributions (BSD), yield a complete description of foam destabilization mechanisms (Hailing 1981 Bisperink et al. 1992 Patino 1995 Lau and Dickinson 2005). The common parameter characterising foam stability is half-life time for liquid drainage or foam collapse (Deeth and Smith... 

The above description of foam destabilization mechanisms is only applicable to liquid foods. In solid dispersions, these mechanisms are significantly slowed or even eliminated. As mentioned earlier, solid matrixes such as aerated confectionery wiU hold air for up to years. Structure stabilization occurs for example in chocolate foams when the product is cooled to solidify and trap the bubbles formed within. The baking... 

Fillers affect foams in several ways they induce nucleation of gas bubbles and polymer crystallization, they destabilize liquid foam, they interfere with rate of chemical reactions and gas generation processes, they influence the viscosity of liquid premixes, they improve mechanical properties of foam, and foam durability. Care must be used in the selection of fillers for foams. They are used only sparingly in spite of the potential gains in foam performance which might be expected. 

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