Pseudoemulsion films

When an oil drop in an aqueous phase rises to the surface of the solution or an oil drop approaches a bubble inside a foam an asymmetrical, oil/water/oil film, the so-called pseudoemulsion film forms between the oil and air phases (Figure 8.) The importance of... 

Figure 8. Formation of a pseudoemulsion film drop between an oil drop and air...

When two fluid interfaces have a high radius of curvature, such as in the pseudoemulsion film, the distance between the interference patterns is too small to be measured by common reflected light interferometry. In this case, differential interferometry can be used for imaging the interface profile [40-45]. (Another technique for studying curved films is the controlled drop tensiometer, as was shown in section 2.)... 

Figure 10. Photomicrograph of the differential interference pattern of a pseudoemulsion film (octane drop in 4 wt% C1215AE30 solution).

Figure 11. Thinning pseudoemulsion film, a) Thick film with dimple, b) Film undergoing stratification - two thickness transitions - and, c) Enhanced image of film undergoing stratification. The film has three discrete thicknesses resulting from the first two transitions.

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. 

This type of films (wetting films, films on a substrate and pseudoemulsion films) represent thin films covering the surface of another liquid. They can be formed when a liquid flows on the surface of another liquid-substrate (insoluble in the former), when liquid drops approach the surface of another liquid, when a gas bubble approaches the interface of two immiscible liquids as well as at the adsorption of gas on liquid substrates. 

During the process of three phase foam thinning, three distinct films may occur foam films (water film between air bubbles), emulsion films (water between oil droplets) and pseudoemulsion films (water film between air and oil droplets) (Figure 1). To study the behavior of these films and particularly the oil droplet-droplet, oil droplet-air bubble and oil droplet-foam frame interactions it is necessary to utilize numerous microscopic techniques, including transmitted light, microinterferometric, differential interferometric and cinemicrographic microscopy. 

Figure 1. Pseudoemulsion film between oil droplet and air/water surface.

Study of Oil Attachment to the Surface. The process of oil droplet penetration to the air/water surface and the spreading of the oil depends upon the rupture of the pseudoemulsion film which separates the oil (Figure 1). 

The study of the behavior of this pseudoemulsion film with curvature is essential for understanding the role of spreading phenomena in three phase foam stability. 

Three different configurations of oil droplets were observed by using differential (DI) interferometry (18,32) (Figure 7). Firstly there are oil droplets separated with a thick film from the air-water surface. This configuration of oil was very common for C AOS approximately 60% of the oil for this surfactant was present in this configuration, separated by the pseudoemulsion film. For C-.AOS this oil amount was approximately 40% and less than 20% for C AOS. 

A second and third configuration of the oil phase at the surface is a droplet separated by a thin pseudoemulsion film (Figure 7b,c). 

Figure 7. Three oil droplet configurations at an air/water surface a) thick pseudoemulsion film b) thin pseudoemulsion film c) oil droplet spread on surface.

This new DI technique therefore presents a method for determining and relating the pseudoemulsion film tension to spreading... 

As has been shown in the previous section, the stability of foam, emulsion and pseudoemulsion films manifest stratification phenomena, curvature phenomena and M arangoni phenomena. We will first discuss the microstructure of the thinning films due to micellar interactions, which we have observed through stratification phenomena. We will then discuss the observed behavior of the pseudoemulsion films with curvature and finally the role of Marangoni effects in the stabilization of the foam structure in the presence of oil. 

Pseudoemulsion Film with Curvature. We observed previously that two types of pseudoemulsion films (thick and thin) may occur when oil droplets attach to an air/water surface in the presence of 1 wt % electrolyte (see Figure 7 and Table I). An explanation of the two film types may be given on the basis of the DLVO theory. 

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. 

Finally we should comment that it is necessary to employ in the calculation of the spreading coefficient (which is often used as a stability criterion) accurately measured values of the various tensions operative in the pseudoemulsion film to determine whether oil is spreading or nonspreading in the three phase foam structure. 

The interactions between foams and emulsified oil drops are discussed in the second part of this chapter. In the presence of emulsified oil, the mechanisms of foam stability are more complex than without oil. The mechanism of foam stability in the presence of oil drops is shown to be determined by the stability of the pseudoemulsion film. When the pseudoemulsion film is stable, the oil drops enhance the foam stability when the film is unstable, the oil acts as an antifoam (defoamer). In... 

Figure 22. Configurations of oil at the gas—aqueous interface. Key a, oil drop inside the solution b, oil drop at the surface separated by a pseudoemulsion film from the gas phase c, oil lens and d, spread oil layer at the solution surface.

The observation that the oil preferred the pseudoemulsion film configuration in the C16AOS solution indicated that this film is stable in this system, which prevented the entering of oil (i.e., lens formation) (Figure 22b and 22c). However, in the C12AOS solution in which the lens configuration was preferred, the pseudoemulsion film was unstable. 

In contrast, Figure 25 shows frames with the C16AOS solution. The oil drops drain from the films into the Plateau borders without entering or spreading, and the foam does not break. This observation was also in accordance with the observation that the typical oil configuration on the surface of C16AOS solution (Figure 23) was stable (thick or thin) pseudoemulsion film. These experiments clearly showed that the foam stability in the presence of emulsified oil is controlled by the stability of the pseudoemulsion film. 

Krugljakov (79) suggested that the asymmetrical aqueous film between an antifoam drop and the air phase (i.e., the pseudoemulsion film) should be unstable enough for the drop to break the foam. He... 

Schramm and Novosad (76), Manlowe and Radke (77), and Hanssen and Dalland (80) also concluded that the pseudoemulsion film stability is a controlling factor in the stability of three-phase foams within porous media. 

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