Emulsions thin-liquid films

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. 

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. 

D. E. Tambe and M. M. Sharma, Hydrodynamics of thin liquid-films bounded by viscoelastic interfaces, J. Colloid Interface Sci. 147, 137-151 (1991) Factors controlling the stability of colloid-stabilized emulsions. 1. An experimental investigation, J. Colloid Interface Sci. 157, 244-253 (1993) Factors controlling the stability of colloid-stabilized emulsions. 2. A model for the rheological properties of colloid-laden interfaces, J. Colloid Interface Sci. 162, 1-10 (1994) Factors controlling the stability of colloid-stabilized emulsions. 3. Measurement of the rheological properties of colloid-laden interfaces, J. Colloid Interface Sci. 171, 456-462 (1995). 

Similarity of Foam Films with Emulsion and Asymmetric Thin Liquid Films... 

The comparison of the results for foam films with those for emulsion films has proved to be very useful, especially with respect to emulsion films of the O/W type. Reason for such a comparison provides the fact that in both cases the thin liquid film is in contact with two hydrophobic phases. It is anticipated that the effects related to adsorption and orientation of surfactant molecules at the film/hydrophobic phase interface are very similar, and there are examples illustrating it. Hence, some regularities established for foam films can be applied to emulsion films and vice versa. 

Effect of Surfactant Type and Concentration. Surfactant concentration and type is of great importance for the stability of thin liquid films and for emulsion stability. Type and concentration of surfactants are responsible for the degree of lowering the interfacial tension and for the viscoelastic properties of droplet surface, as well as for the film thickness between two droplets. 

In the biomedical applications outlined by Ward et al. (7 ), more so than in any other separation application of synthetic polymeric membranes, the goal is to mimic natural membranes. Similarly, the development of liquid membranes and biofunctional membranes represent attempts by man to imitate nature. Liquid membranes were first proposed for liquid separation applications by Li (46-48). These liquid membranes were comprised of a thin liquid film stabilized by a surfactant in an emulsion-type mixture. Wtille these membranes never attained widespread commercial success, the concept did lead to immobilized or supported liquid membranes. In... 

Malhotra AK, Wasan DT. Interfacial rheological properties of adsorbed surfactant films with applications to emulsion and foam stability. In Ivanov IB, ed. Thin Liquid Films, Fundamentals and Applications. Vol. 29. Surfactant Science Series. New York Marcel Dekker, 1988 829-890. 

Thin liquid films in foam and emulsion systems are usually stabilised by soluble surfactants. During the formation of such films the flow-out process of liquid disturbs the surfactant equilibrium state in the bulk and film surfaces. The situation of drainage of a surfactant containing liquid film between two oil droplets is shown in Fig. 3.15. (after Ivanov Dimitrov 1988). Here j" and are the bulk fluxes in the drops and the film, respectively, j and j are the fluxes due to surface diffusion or spreading caused by the Marangoni effect, respectively. 

The coalescence of disperse systems, such as foams and emulsions, and the contact of air bubbles with solid particles, e.g. in the process of flotation, takes place in two steps. The first is characterised by a flocculation of the system, the formation of thin liquid films with an equilibrium thickness. In the second step the film becomes thin enough for the interparticular attractions to overcome the film state so the two separated interfaces form a new interface. The situation where a small bubble attaches a liquid interface is shown in Fig. 2D. 1. 

The important aspect of adsorption processes at a liquid interface is lateral mobility which can lead to lateral excess transport of adsorbed molecules. Lateral transport disturbs the equilibrium state of an adsorption layer. In many important systems, such as emulsions, foams, and bubbly liquids, the properties of a non-equilibrium adsorption layer can be essential. This has been demonstrated in the systematic work of the Russian and Bulgarian schools summarised in monographs like "Thin Liquid Films" by Ivanov, "Coagulation and Dynamics of Thin Films" by Dukhin, Rulyov and Dimitrov, and "Foams and Foam Films" by Krugljakov and Exerowa. These books pay most attention to thick film drainage and stabilisation/destabilisation of thin liquid films. This book is focused on other dynamic processes at liquid interfaces in general or connected with phenomena of emulsions and foams. 

Stable foams may be formed by surfactant solutions. Thin liquid films separate gas bubbles, which can be colloidal but are usually much larger. Once formed, gravity eventually drains the liquid tmtil the films break. Viscous additives can slow drainage and increase bubble hfetime significandy. Solid emulsions and foams are less common, the dispersing phase being sohd while a liquid or gas phase is dispersed. 

Thin, liquid films as such are not only systems of interest, they also occur frequently in nature and in laboratory practice, for example, in foams. In emulsions, to name another case, thin water films between oil (instead of air ) phases are present in a concentrated emulsion of small oil droplets dispersed in water. The properties of the thin water film determine the interaction forces between the oil droplets and will determine, for example, whether the emulsion in stable. Also the reverse case, oil films in water, occurs. Extremely thin ( 5 nm) oil-in-water films are prototypes of lipid bilayers occurring in biological membranes. Thin liquid films on a solid... 

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