Foam destabilization

Foam destabilization is also a factor in the packing and orientation of mixed films, which can be determined from monolayer studies. It is worth mentioning that foam formation from monolayers of amphiphiles constitutes the most fundamental process in everyday life. The other assemblies, such as vesicles and BLM, are somewhat more complicated systems, which are also found to be in equilibrium with monolayers. 

The thin liquid films bounded by gas on one side and by oil on the other, denoted air/water/oil are referred to as pseudoemulsion films [301], They are important because the pseudoemulsion film can be metastable in a dynamic system even when the thermodynamic entering coefficient is greater than zero. Several groups [301,331,342] have interpreted foam destabilization by oils in terms of pseudoemulsion film stabilities [114]. This is done based on disjoining pressures in the films, which may be measured experimentally [330] or calculated from electrostatic and dispersion forces [331], The pseudoemulsion model has been applied to both bulk foams and to foams flowing in porous media. 

Care must be taken that finish systems not contain de-foamers or foam destabilizers. Polyvalent metal salts are often used as catalysts in resin systems and several of these, such as zinc nitrate,strongly inhibit foam generation. This is not dissimilar to the foam inhibiting action of hard water on fatty acid soaps. 

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... 

The interactions between an oil phase and foam lamellae are extremely complex. Foam destabilization in the presence of oil may not be a simple matter of oil droplets spreading upon foam film surfaces but may often involve the migration of emulsified oil droplets from the foam film lamellae into the Plateau borders where critical factors, such as the magnitude of the Marangoni effect in the pseudoemulsion film, the pseudoemulsion film tension, the droplet size and number of droplets may all contribute to destabilizing or stabilizing the three phase foam structure. 

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). 

Some possibilities for the mechanisms of foam destabilization by a given oil phase include... 

An outline of the fundamental steps involved in aqueous foam destabilization by oil, in porous media, is proposed as follows ... 

In some protein preparations, such as sunflower isolate and concentrate [16,70] and soybean protein [12], stable foams were obtained, although the extent of succinylation was high. In these cases, the other intrinsic and extrinsic parameters obviously overcome the charge repulsion. The investigation of foam destabilization kinetics... 

We therefore arrive at a depiction of foam destabilization by neat oil drops trapped in Plateau borders when the Plateau border capillary pressure exceeds a certain critical value that is approximated by the value p , measured by the FTT. Therefore, if... 

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. 

The main foam destabilization mechanism in freshly poured beer is vertical draining by gravity. The height of the fresh beer foam is measured to 5 cm and can be considered constant. It can be assumed that 25% of the cross-sectional area of the foam is used for vertical draining, in this initial phase, and that the foam structure remains constant and homogeneous. 

Features Low dynamic surf. tens. low use levels low pseudo-equilibrium surf, tens. exc. foam destabilization extremely low VOC (1.2%) easy to incorporate stable in hard water... 

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