Thin films emulsifier-stabilized

Before determining the degree of stability of an emulsion and the reason lor this stability, the mechanisms of this destabilization should be considered. When an emulsion starts to separate, an oil layer appears on top. and an aqueous layer appears on the bottom. This separation is the final slate of the destabilization of the emulsion the initial two processes are called flocculation and coalescence. In flocculation, two droplets become attached to each other but are still separated by a thin film of the liquid. When more droplets are added, an aggregate is funned, in which the individual droplets cluster but retain the thin liquid films between them. The emulsifier molecules remain at the surface of the individual droplets during this process. 

The common nonionic surfactants are often soluble in both water and oil phases. In the practice of emulsion preparation, the surfactant (the emulsifier) is initially dissolved in one of the liquid phases and then the emulsion is prepared by homogenization. In such a case, the initial distribution of the surfactant between the two phases of the emulsion is not in equilibrium therefore, surfactant diffusion fluxes appear across the surfaces of the emulsion droplets. The process of surfactant redistribution usually lasts from many hours to several days, until finally equilibrium distribution is established. The diffusion fluxes across the interfaces, directed either from the continuous phase toward the droplets or the reverse, are found to stabilize both thin films and emulsions. In particular, even films, which are thermodynamically unstable, may exist several days because of the diffusion surfactant transfer however, they rupture immediately after the diffusive equilibrium has been established. Experimentally, this effect manifests itself in phenomena called cyclic dimpling and osmotic swelling. These two phenomena, as well as the equilibration of two phases across a film,568.569 3j.g described and interpreted below. 

Emulsifiers have long been used to stabilize O/W and water-in-oil (W/O) emulsions, because they play the role of decreasing the interfacial tension between the phases, which facilitates the separation of one phase in the form of small droplets [14]. To stabilize the emulsion, an emulsifier forms a thin film (interface) between the internal and external phases. The nature of the thin films formed at the interface of the emulsion droplets is controlled by the type of emulsifier and affects the stability of the emulsion interface [15-17]. For example, the addition of Tween 20 or Tween 60 results in thin interfaces around the oil... 

Figure 8 Air bubbles in ice cream (a). Interface (arrows) between a large air bubble (A) and water phase (W) in an ice cream sample without emulsifier. There is very little adsorption of fat globules to the air-water interface, which is stabilized by a thin protein film only, (b) Corresponding structures in an ice cream with emulsifier (saturated mono-diglycerides). Fat globules interact strongly with the air-water interface. Reprinted from reference 23, p 242, courtesy of Marcel Dekker Inc.

The "electrical double layer" effect, i.e. the orientation of electrical charge on each film surface due to the use of ionic emulsifiers, is generally more important in aqueous foams than in organic polymeric foams. The stability effect arises from the repulsion of the electrical charges as the two surfaces approach each other, thus limiting the thinning of the film (cell walls) (3). 

Oil Configurations in Foams. In the presence of oil, the mechanisms of foam stability are more complex than without oil. Solubilized oil decreases the stability by accelerating the stepwise foam film thinning, as shown in the previous section. The effect of emulsified oil on foams is closely connected with the configuration of oil relative to the aqueous and gas phases. This configuration can be, in most cases, one of the following (Figure 22) ... 

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. 

A foam is a dispersion of gas bubbles in a relatively small volume of a liquid or solid continuous phase. Liquid foams consist of gas bubbles separated by thin liquid films. It is not possible to make a foam from pure water the bubbles disappear as soon as they are created. However, if surface active molecules, such as soap, emulsifiers or certain proteins, are present they adsorb to the gas-liquid interfaces and stabilize the bubbles. Solid foams, e.g. bread, sponge cake or lava, have solid walls between the gas bubbles. Liquid foams have unusual macroscopic properties that arise from the physical chemistry of bubble interfaces and the structure formed by the packing of the gas bubbles. For small, gentle deformations they behave like an elastic solid and, when deformed more, they can flow like a liquid. When the pressure or temperature is changed, their volume changes approximately according to the ideal gas law (PF/r= constant). Thus, foams exhibit features of all three fundamental states of matter. In ice cream, the gas phase volume is relatively low for a foam (about 50%), so the bubbles do not come into contact, and therefore are spherical. Some foams, for example bubble bath. 

Lobo and co-workers (11) investigated the influence of oil (in the micellar environment) on the stability of foam. Two different types of emulsified oil systems were studied, i.e. (a) a microemulsion (solubilized within the micelle), and (b) a macroemulsion system. It was found that in each case, the foam stability was affected by a completely different mechanism. In the case of (a), where the foam films containing oil is solubilized within the micelle to form a microemulsion, the normal micellar interactions are changed. It had been earlier demonstrated that micellar structuring causes a step-wise thinning due to layer-by-layer expulsion of the micelles and such as effect was found to inhibit drainage and thus increase the foam stability. 

The absolute values of the interfacial tensions varied between different amphi-philes and solvents (Table 1). AOT, which is well known in the literature for the formation of microemulsions, showed the lowest surface tension at the interface of both solvents. The other nonionic snrfactants mentioned here. Span 80 and Brij 72 showed shghtly higher valnes. This was also observed for Lecithine, but this lipid precipitated partly during the spinning-drop measurements. Due to this phenomenon, it was not possible to measure accurate data for this emulsifying compound. The interfacial tension had also some influence on the mean size of the emulsion droplets and on the stability of the vesicles (Table 3). In addition to the stationary values of the surface tension, dynamic processes as the surfactant diffusion represented another important factor for the process of stimulated vesicle formation. If an aqueous droplet passed across the fluid interface it carried-over a thin layer of emulsifiers and thereby lowered the local surfactant concentration in the vicinity of the oil-water interface. In the short time span, before the next water droplet approached the interface, the surfactant films should entirely reform and this only occurred, if the surfactant diffusion was fast enough. 

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