Liquid films emulsion film

As will become evident from the discussion that follows, essentially only the value of Aafh) in Equation 1.32 is of interest to us. For this reason, Aa h) can be viewed as a primary characteristic describing film properties and the interactions between surfaces. Unfortunately, for solid planar parallel surfaces, such measurements are nearly impossible the experimental integration of n(fi) between macroscopic surfaces requires that the surfaces remain flat and parallel to the precision of fractions of an angstrom in the course of measuring very small forces. While this is impossible for solid surfaces, such measurements are quite possible and indeed broadly utilized in the investigation of liquid films emulsion films, foam films, and wetting films. In all of these cases, a flat and parallel state can be maintained because of the high mobility of the interfaces. 

We distinguish different types of liquid films. Foam films are liquid lamellae between two gas phases. They form, for example, between two bubbles. Emulsion films are films of liquid A between two immiscible liquids B. This can, for example, be a water film between two oil drops. When a solid surface is involved, we have a liquid film on top of a solid substrate. Such films form, for example, when a particle collides with a bubble. 

The effects of particles on foam stability are usually discussed in terms of individual particles because a single particle can rupture a thin liquid film. Emulsion stabilization by particles alone presumably involves close-packed monolayers aroimd the emulsion drops. However, less than close-packed layers can, in principle, have important effects on emulsion stability in systems stabilized by smfactants, and this area warrants further study. 

The preceding treatment relates primarily to flocculation rates, while the irreversible aging of emulsions involves the coalescence of droplets, the prelude to which is the thinning of the liquid film separating the droplets. Similar theories were developed by Spielman [54] and by Honig and co-workers [55], which added hydrodynamic considerations to basic DLVO theory. A successful experimental test of these equations was made by Bernstein and co-workers [56] (see also Ref. 57). Coalescence leads eventually to separation of bulk oil phase, and a practical measure of emulsion stability is the rate of increase of the volume of this phase, V, as a function of time. A useful equation is... 

A foam can be considered as a type of emulsion in which the inner phase is a gas, and as with emulsions, it seems necessary to have some surfactant component present to give stability. The resemblance is particularly close in the case of foams consisting of nearly spherical bubbles separated by rather thick liquid films such foams have been given the name kugelschaum by Manegold [175]. 

Very finely disperse solids, which are adsorbed at the liquid/liquid interfaces, forming films of particles around the disperse globules. Certain powders can very effectively stabilize against coalescence. The solid s particle size must be very small compared with the emulsion droplet size and must exhibit an appropriate angle of contact at the three-phase (oil/water/solid) boundary [141]. 

Initially devised to measure interactions in single soap films (air/water/air) [8], the TFB technique has been progressively improved and its application has been broadened to emulsion films (oil/water/oil) [ 12] and asymmetric films (air/water/oil or air/water/solid) [13,14]. In a classical setup, a thin porous glass disk is fused on the side to a capillary tube and a small hole is drilled in the center of the disk. The liquid solution fills the disk, part of the capillary, and a thin horizontal film is formed across the hole. The disk is enclosed in a hermetically sealed box, with the capillary tube exposed to a constant reference pressure Pr. Under the effect of the pressure difference AP between the box and the reference, the... 

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. 

In the coalescence step, the thin liquid film hetween the droplets is destabilized, and a large droplel is formed Hence, the coalescing emulsion is characterized by a wide size distribution of the droplets, but no clusters are present. Finally, the droplets achieve such a size that they arc recognized by the naked eye as a separate phase. A fully separated emulsion consists of an oii layer and an aqueous layer. 

The traditional view of emulsion stability (1,2) was concerned with systems of two isotropic, Newtonian liquids of which one is dispersed in the other in the form of spherical droplets. The stabilization of such a system was achieved by adsorbed amphiphiles, which modify interfacial properties and to some extent the colloidal forces across a thin liquid film, after the hydrodynamic conditions of the latter had been taken into consideration. However, a laige number of emulsions, in fact, contain more than two phases. The importance of the third phase was recognized early (3) and the IUPAC definition of an emulsion included a third phase (4). With this relation in mind, this article deals with two-phase emulsions as an introduction. These systems are useful in discussing the details of formation and destabilization, because of their relative simplicity. The subsequent treatment focuses on three-phase emulsions, outlining three special cases. The presence of the third phase is shown in order to monitor the properties of the emulsion in a significant manner. 

Liquid crystals stabilize in several ways. The lamellar structure leads to a strong reduction of the van der Waals forces during the coalescence step. The mathematical treatment of this problem is fairly complex (28). A diagram of the van der Waals potential (Fig. 15) illustrates the phenomenon (29). Without the liquid crystalline phase, coalescence takes place over a thin liquid film in a distance range, where the slope of the van der Waals potential is steep, ie, there is a large van der Waals force. With the liquid crystal present, coalescence takes place over a thick film and the slope of the van der Waals potential is small. In addition, the liquid crystal is highly viscous, and two droplets separated by a viscous film of liquid crystal with only a small compressive force exhibit stability against coalescence. Finally, the network of liquid crystalline leaflets (30) hinders the free mobility of the emulsion droplets. 

A similar technique can be used to study the rheological properties of liquid films. Figure 4 shows the formation of a W/O/W emulsion film with two, identical aqueous phases (such as in water-in-oil emulsions) at the tip of the capillary. A pre-requisite of the experiment is that the surface of the capillary must be well wetted by the film phase, i.e., it should be hydrophobic in this case. First, an aqueous drop is formed inside the oil (film liquid) and the aqueous phase is in the bottom of the cuvette. Then, the level of the aqueous phase is slowly increased. As the oil/water interface passes the drop, a cap shaped oil film, bordered by a circular meniscus, covers the drop. This film can be studied in equilibrium and in dynamic conditions, similar to the single interfaces (See above). The technique can be used to study films from oil or aqueous phase which can be sandwiched between identical or different liquid or gas phases. 

The film is observed by a microscope using reflected light The film holder and the objective are immersed in air in the case of foam (i.e., air/liquid/air) film and in the oil phase, in the case of an O/W/O emulsion film, respectively. The film thickness can be determined by measuring the intensity of the light reflected from the film surfaces [9]. Further details of the technique will be discussed in Chapter 2. 

New experimental techniques and several of their applications were presented which help in the understanding of structure, texture and stability of food systems. For future research, the mechanism of film stability by the microlayering of colloid particles seems to be the most promising - especially in food emulsions and foams. Work is in progress in our laboratory to calculate the oscillatory disjoining pressure inside liquid films containing microlayers [30],... 

One of the central questions in the stability of foams is why are liquid films between two adjacent bubbles stable, at least for some time In fact, a film of a pure liquid is not stable at all and will rupture immediately. Formally this can be attributed to the van der Waals attraction between the two gas phases across the liquid. As for emulsions, surfactant has to be added to stabilize a liquid film. The surfactant adsorbs to the two surfaces and reduces the surface tension. The main effect, however, is that the surfactant has to cause a repulsive force between the two parallel gas-liquid interfaces. Different interactions can stabilize foam films [570], For example, if we take an ionic surfactant, the electrostatic double-layer repulsion will have a stabilizing effect. 

In concentrated emulsions and foams the thin liquid films that separate the droplets or bubbles from each other are very important in determining the overall stability of the dispersion. In order to be able to withstand deformations without rupturing, a thin liquid film must be somewhat elastic. The surface chemical explanation for thin film elasticity comes from Marangoni and Gibbs (see Ref. [199]). When a surfactant-stabilized film undergoes sudden expansion, then immediately the expanded... 

A dispersion of gas bubbles in a liquid, in which at least one dimension falls within the colloidal size range. Thus a foam typically contains either very small bubble sizes or, more commonly, quite large gas bubbles separated by thin liquid films. The thin liquid films are called lamellae (or laminae ). Sometimes distinctions are drawn as follows. Concentrated foams, in which liquid films are thinner than the bubble sizes and the gas bubbles are polyhedral, are termed polyederschaum . Low-concentration foams, in which the liquid films have thicknesses on the same scale or larger than the bubble sizes and the bubbles are approximately spherical, are termed gas emulsions , gas dispersions , or kugelschaum . See also Evanescent Foam, Froth, Aerated Emulsion. 

Malhotra, A.K. Wasan, D.T. Interfacial Rheological Properties of Adsorbed Surfactant Films with Applications to Emulsion and Foam Stability in Thin Liquid Films, Ivanov, I.B. (Ed.), Dekker New York, 1988, pp. 829-890. 

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