Air-Water-Oil Pseudoemulsion Films

Measurement of Disjoining Pressure Isotherms EOR Air-Water-Oil Pseudoemulsion Films... 

The first and apparently only measurements of disjoining pressure isotherms for air-water-oil pseudoemulsion films have been made by Bergeron et al. [39,44,45]. These measurements used essentially the same approach as described here in Section... 

Drop in plateau border viewed perpendicularly from above one of air-water-oil pseudoemulsion films... 

As we will describe at length in Section 4.8, antifoam oils for aqueous solutions are usually mixed with hydrophobic particles where the main function of the latter concerns rupture of the air-water-oil pseudoemulsion films. Since the stability... 

FIGURE 4.79 Effect of spread PDMS oil layers on ease of rupture of air-water-oil pseudoemulsion film, even in presence of hydrophobed silica particles, as revealed by measurements of critical capillary pressure p. Effect is seen to be insensitive to changes in drop diameter. (Reprinted with permission from Denkov, N.D. Langmuir, 20, 9463. Copyright 2004 American Chemical Society.)... 

Another aspect of the role of particles in rupturing air-water-oil pseudoemulsion films concerns the extent to which the particles protrude from the oil-water... 

Air-Water-Oil Pseudoemulsion Film Rupture and Configurations at Oil-Water Surface of Particle with Several Edges... 

We therefore have two sets of conditions for the effectiveness of a rough particle in rupturing air-water-oil pseudoemulsion films while not rupturing air-water-air foam films. For partial wetting oils with no spread oil layer at the air-water surface. 

The mode of action of PDMS-hydrophobed silica antifoams in aqueous surfactant solutions has been extensively stndied by Denkov et al. [53] and reviewed in detail in Chapter 4. Essentially the hydrophobed silica particles rupture the so-called air-water-oil pseudoemulsion film, thereby enabling the oil to emerge into the air-water surface. It is known that once they emerge into the air-water surface, drops of PDMS oils usually initially spread over that snrface, exhibiting either complete wetting or pseudo-partial wetting behavior (see Section 3.6.2). This means that the oil spreads as either a thick duplex layer or spreads and breaks up into lenses in equilibrium with a thin oil layer. Since such behavior is ubiquitous with aqueous surfactant solntions, it is reasonable to expect similar behavior when PDMS oil drops are introduced into the gas-blood surface. It is not, however, known whether complete or pseudo-partial wetting behavior is to be expected. 

Oawo( ) Air-water oil pseudoemulsion film tension <7aow( ) Air-oil-water spread film tension Aa-F Classical spreading pressure of liquid at ij surface Sbik5 Foaminess as defined by Bikerman... 

The role of bridging foam films by particles and oil drops in the mode of action of antifoams was emphasized in the earlier book. Also included was the first evidence that the role of particles in synergistic oil-particle antifoam mixtures for defoaming aqueous foams concerns rupture of air-water-oil pseudoemulsion films. The past two decades have seen a general acceptance and further development of these aspects of the theory of antifoam action. Particularly noteworthy have been the insights provided by Professor Nikki Denkov and his coworkers at the University of Sofia. All of this is reviewed in a chapter specifically concerned with the mode of action of antifoams. 

The requirement that oils adhere to gas-liquid surfaces implies that entry coefficients be positive and that some agency be present that can destabilize any metastable pseudoemulsion films. In the case of air-water-oil pseudoanulsion films, the latter is usually represented by hydrophobic particles adhering to the oil-water surfaces as we describe in Section 4.8. In the case of solids, surface roughness appears to facilitate emergence into air-water surfaces [37], asperities perhaps helping to nucleate rupture of any metastable film separating the solid from the relevant gas-liquid surface (see Section 4.7.4). 

The absence of positive contributions to the disjoining pressures of gas-crude oil-antifoam pseudoemulsion films of an electrostatic origin suggests that these films will be unstable. The need for the presence of the particles required for effective rupture of air-water-antifoam pseudoemulsion films in the context of aqueous surfactant solution foams is apparently therefore obviated. However, there is apparently no knowledge at all of the behavior of, for example, gas-crude oil-PDMS pseudoanulsion films. 

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. 

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... 

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). 

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. 

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.

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 behavior of the pseudoemulsion film is controlled by mechanisms similar to foam films the interfadal rheological properties of the surfactant molecules at low surfactant concentrations and micellar ordering at high surfactant concentrations (i.e., much above the CMC). Not much above the CMC, both of these mechanisms can play a role in film thinning and stability. The stability of a thin pseudoemulsion film also depends on the van der Waals interactions between the phases at the two sides of the film, that is between air and oil, acting across the aqueous film. In a water pseudoemulsion film the Hamaker constant is generally negative, the van der Waals interactions are repulsive and stabilize the film. 

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