Foam film destabilization

Surface viscosity is reduced (thus destabilizing the foam films). 

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. 

The stability of emulsion and foam films have also been found dependent upon the micellar microstructure within the film. Electrolyte concentration, and surfactant type and concentration have been shown to directly influence this microstructure stabilizing mechanism. The effect of oil solubilization has also been discussed. The preceding stabilizing/destabilizing mechanisms for three phase foam systems have been shown to predict the effectiveness of aqueous foam systems for displacing oil in enhanced oil recovery experiments in Berea Sandstone cores. 

Another possible mechanism of foam-breaking by hydrophobic particles is that the particles can spread on the foam film surface and destabilize the film by decreasing the surface tension of the solution (96). The experiments of Aveyard et al. (86) show that this effect is probably not significant. 

Solid-Phase Components. Dispersed sohds are vital ingredients in commercial antifoam formulations. Much of the cmrent theory on antifoaming mechanism ascribes the active defoaming action to this dispersed solid phase with the liquid phase primarily a carrier fluid, active only in the sense that it must be surface-active in order to carry the solid particles into the foam films and cause destabilization. For example, PDMS, despite its considerable effectiveness in nonaqueous systems, shows little foam-inhibiting activity in aqueous surfactant solutions. It is only when compounded with hydrophobic silica [7631-86-9] to give the so-called silicone antifoam compounds that highly effective aqueous defoamers result. The three main solid-phase component classes are hydrocarbons, silicones, and fluorocarbons. 

This approach of Ewers and Sutherland [38] was advanced with some claim to wide generality. However, as we will show here (see Sections 4.5,4.7, and 4.8), many substances yield antifoam effects without causing destabilizing surface tension gradients [9, 12, 39, 40-44], which confounds exclusive generality. Moreover, a difficulty with the mechanism concerns the implicit assumption that the antifoam is always to be found in the thinnest and most vulnerable part of the foam. Thus, if the antifoam entity spreads from the Plateau border, this will drag liquid into the adjacent foam films, which will stabilize those films by increasing their thicknesses. 

Van der Waals attraction destabilize liquid foam films. As a result, if we do not take care to add a repulsive force component, liquid films become highly unstable and rupture immediately. The first and most common approach is to add charged surfactants such as sodium dodecyl sulfate (SDS). With dissociated surfactants present, the air-water interface changes. This leads to an electrostatic doublelayer repulsion between the two interfaces. Since the system is symmetric with respect to the two interfaces, electrostatic forces are repulsive. The double-layer repulsion for low surface potentials and thick films h > Xd) can be expressed as (Eq. (4.57))... 

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]. 

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. 

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... 

---