Breaking, emulsions/dispersions

Demulsification with electrostatic fields appears to be the most effective and economic way for breaking of W/0 emulsion in ELM processes 190, 91]. Electrostatic coalescence is a technique widely used to separate dispersed aqueous droplets from nonconducting oils. Since this type of technique is strictly a physical process, it is most suitable for breaking emulsion liquid membranes to recover the oil membrane phase for reuse. 

Centrifugal separation of gas, oil, water and solid phases, making the dispersion radially inhomogeneous, and possibly breaking emulsions or collapsing foams. 

Viscous, sticky, or waxy materials are easier to dispense in the form of emulsions, as are solids in snspended form. Consequently numerous consumer products are greatly influenced by the knowledge of how to make stable colloidal dispersions. Breaking snch dispersions also has many interesting applications. In secondary oil recovery, for instance, petrolenm is flushed from underground oil fields with water. The material that is extracted is frequently in the form of an emulsion oil-in-water or water-in-od, depending on the relative amounts of the two liquids. As refinery feed streams shonld be free of water, it is necessary to know how to break the emulsion into the two bulk phases. 

Surfactants (surface-active agents) are the most common and the most versatile of all acid additives. There are only a few categories of surfactants, but there are many variations within them. Surfactants are used to reduce interfacial tension, enhance flowback, break emulsions and sludges, and favorably alter wettability. They are also used to disperse and suspend solids in solution and disperse other additives in oil and water. There are excellent sources of information on surfactants available in the literature. For example, as Allen and Roberts describe in the following ... 

Phenomena at Liquid Interfaces. The area of contact between two phases is called the interface three phases can have only aline of contact, and only a point of mutual contact is possible between four or more phases. Combinations of phases encountered in surfactant systems are L—G, L—L—G, L—S—G, L—S—S—G, L—L, L—L—L, L—S—S, L—L—S—S—G, L—S, L—L—S, and L—L—S—G, where G = gas, L = liquid, and S = solid. An example of an L—L—S—G system is an aqueous surfactant solution containing an emulsified oil, suspended soHd, and entrained air (see Emulsions Foams). This embodies several conditions common to practical surfactant systems. First, because the surface area of a phase iacreases as particle size decreases, the emulsion, suspension, and entrained gas each have large areas of contact with the surfactant solution. Next, because iaterfaces can only exist between two phases, analysis of phenomena ia the L—L—S—G system breaks down iato a series of analyses, ie, surfactant solution to the emulsion, soHd, and gas. It is also apparent that the surfactant must be stabilizing the system by preventing contact between the emulsified oil and dispersed soHd. FiaaHy, the dispersed phases are ia equiUbrium with each other through their common equiUbrium with the surfactant solution. 

However, the fundamental theory of simple foams is not as well formulated as the theory for simple emulsions. Because foams consist of gases dispersed in a semisolid film, the properties and behavior immediately become more dramatically subject to external variables, such as temperature and external air pressure. Minute changes in surface tension of the film can make or break the foam. However, a similar approach might be suggested in the foam field. In this case, the variable with which we are most concerned is whether or not a stable foam is produced and the diagrams would be drawn accordingly. 

The interaction between the dispersed-phase elements at high volume fractions has an impact on breakup and aggregation, which is not well understood. For example, Elemans et al. (1997) found that when closely spaced stationary threads break by the growth of capillary instabilities, the disturbances on adjacent threads are half a wavelength out of phase (Fig. 43), and the rate of growth of the instability is smaller. Such interaction effects may have practical applications, for example, in the formation of monodisperse emulsions (Mason and Bibette, 1996). 

An emulsion is a dispersed system of two immiscible phases. Emulsions are present in several food systems. In general, the disperse phase in an emulsion is normally in globules 0.1-10 microns in diameter. Emulsions are commonly classed as either oil in water (O/W) or water in oil (W/O). In sugar confectionery, O/W emulsions are most usually encountered, or perhaps more accurately, oil in sugar syrup. One of the most important properties of an emulsion is its stability, normally referred to as its emulsion stability. Emulsions normally break by one of three processes creaming (or sedimentation), flocculation or droplet coalescence. Creaming and sedimentation originate in density differences between the two phases. Emulsions often break by a mixture of the processes. The time it takes for an emulsion to break can vary from seconds to years. Emulsions are not normally inherently stable since they are not a thermodynamic state of matter. A stable emulsion normally needs some material to make the emulsion stable. Food law complicates this issue since various substances are listed as emulsifiers and stabilisers. Unfortunately, some natural substances that are extremely effective as emulsifiers in practice are not emulsifiers in law. An examination of those materials that do stabilise emulsions allows them to be classified as follows ... 

The previous experiments were all performed on dilute emulsions for which the dispersed phase represents 2.5 wt% of the emulsion. The results obtained for a concentrated emulsion with oil mass fraction equal to 75% sheared at 500 s and 3000 s are reported in Fig. 1.16 [156]. The primary emulsion was polydisperse with d = 57 am. The two previously described regimes still exist. The first regime is particularly efficient in reducing the diameter because one drop breaks into 160 droplets through the Rayleigh instability for an applied shear rate of 500 s (di( = 10.5 am) and into 6200 droplets for an applied shear rate of 3000 s ... 

Ice cream emulsion has a very characteristic degree of stability. The air bubbles should remain dispersed, but as soon it melts in the mouth, the emulsion should break. This leads to the sensation of taste, which is very essential to enjoy its specialness. The sensation of taste on the surface of the tongue is known to be related to molecular shape and physicochemical properties. As soon as these molecules are separated from the emulsion, the taste sensation is recorded in the brain. Therefore, the various components must stay in the same phase after the breakup of the emulsion. Emulsifiers that are generally used have low HLB values (for W/O), and have been found to have considerable effect on the structure of the ice cream. 

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