Emulsions continued interfacial tension

Drop settles and coalesces but is re-entrained] faulty location of exit nozzles for liquid phases/distance between exit nozzle and interface is < 0.2 m/overflow baffle corroded and faUure/interface level at the wrong location/faulty control of interface/liquid exit velocities too high/vortex breaker missing or faulty on underflow line/no syphon break on underflow line/liquid exit velocities too high. [Drop settles but doesn t coalesce [phase inversion] /pH far from zpc/surfactants, particulates or polymers present/electrolyte concentration in the continuous phase < expected/[coalescer pads ineffective] /[drop size decrease] /[secondary haze forms] /[stable emulsion formation] /[interfacial tension too low] /[Maran-goni effect]. ... 

Figure 11.12. Effect of surfactant concentration on the inlerfacial tension (IFT) of TRS 10-410 (a petroleum sulfonate surfactant) -F isobutyl alcohol (IBA) in 1.5% NaCl with dodecane. An ultralow IFT can occur at low or high surfactant concentrations , interfacial tension of o/w micro emulsion A, interfacial tension of w/o micro emulsion. A three-phase system (oil-continuous, water-continuous and middle-phase regions) exists where the ( ) and (A) data overlap...

The model system used by Mabille et al. [149, 150] was a set of monodisperse dilute (2.5 wt% of dispersed oil) emulsions of identical composition, whose mean size ranged from 4 p.m to 11 p.m. A sudden shear of 500 s was applied by means of a strain-controlled rheometer for durations ranging from 1 to 1500 s. All the resulting emulsions were also monodisperse. At such low oil droplet fraction, the emulsion viscosity was mainly determined by that of the continuous phase (it was checked that the droplet size had no effect on the emulsion viscosity). The viscosity ratio p = t]a/t]c = 0.4 and the interfacial tension yi t = 6 mN/m remained constant. 

Another process which leads to HIPE instability is gravitational syneresis, or creaming, where the continuous phase drains from the thin films as a result of density differences between the phases. This produces a separated layer of bulk continuous phase and a more concentrated emulsion phase. The separated liquid can be located either above or below the emulsion, depending on whether the continuous phase is more or less dense, respectively, than the dispersed phase. This process has been studied by Princen [111] who suggests that it can be reduced by a number of parameters, including a high internal phase volume, small droplet sizes, a high interfacial tension and a small density difference between phases. 

Emulsions and foams are two other areas in which dynamic and equilibrium film properties play a considerable role. Emulsions are colloidal dispersions in which two immiscible liquids constitute the dispersed and continuous phases. Water is almost always one of the liquids, and amphipathic molecules are usually present as emulsifying agents, components that impart some degree of durability to the preparation. Although we have focused attention on the air-water surface in this chapter, amphipathic molecules behave similarly at oil-water interfaces as well. By their adsorption, such molecules lower the interfacial tension and increase the interfacial viscosity. Emulsifying agents may also be ionic compounds, in which case they impart a charge to the surface, which in turn establishes an ion atmosphere of counterions in the adjacent aqueous phase. These concepts affect the formation and stability of emulsions in various ways ... 

Microemulsions, like micelles, are considered to be lyophilic, stable, colloidal dispersions. In some systems the addition of a fourth component, a co-surfactant, to an oil/water/surfactant system can cause the interfacial tension to drop to near-zero values, easily on the order of 10-3 - 10-4 mN/m, allowing spontaneous or nearly spontaneous emulsification to very small drop sizes, typically about 10-100 nm, or smaller [223]. The droplets can be so small that they scatter little light, so the emulsions appear to be transparent. Unlike coarse emulsions, microemulsions are thought to be thermodynamically stable they do not break on standing or centrifuging. The thermodynamic stability is frequently attributed to a combination of ultra-low interfacial tensions, interfacial turbulence, and possibly transient negative interfacial tensions, but this remains an area of continued research [224,225],... 

Low internal phase emulsions typically result when high shear conditions are used for emulsification, while low shear mixing can lead to high internal phase, or concentrated, emulsions [435]. There are several conditions needed to form a concentrated emulsion. Low shear mixing is required while the internal phase is slowly added to the continuous phase, and the surfactants used to create the emulsion need to be able to form elastic films [435—438]. The formation of concentrated emulsions has also been linked to surfactant-oil phase interactions [436] and therefore the oil-water interfacial tension and the potential for surfactant-surfactant interactions [439]. 

Rather than using a thermal quench, a more common way to make an emulsion is by mechanical mixing, or agitation, of two or more liquid components, such as occurs in an old fashioned butter chum. Unless surfactants, or emulsifiers, are present, however, when agitation ceases, interfacial tension will drive the two phases back toward separation. This separation occurs by droplet-droplet collision and fusion, if the droplets are Brownian by sedimentation or creaming, if the droplets are non-Brownian or by Ostwald ripening, if the droplet phase is soluble in the continuous phase. 

An emulsion is defined as a dispersion of two immiscible liquids, one of which is finely subdivided and uniformly distributed as droplets (the dispersed phase) throughout the other (the continuous phase). A third component (or multiple additional components), the emulsifying agent(s), is necessary to help stabilize the emulsion. The emulsifying agent(s) coats the droplets and prevents droplet coalescence by either reducing the interfacial tension or by creating a physical repulsion between the droplets. The dispersed phase is occasionally also defined as the internal phase the continuous phase is occasionally also defined as the external phase or dispersion medium. Virtually all emulsions are inherently physically unstable. 

Emulsion stability is affected by temperature, continuous phase viscosity, droplet sizes and their distribution, interfacial tension (IFT), and interfacial film properties. Some of these effects were discussed in the preceding section. This section discusses the effects of viscosity, polymer, IFT, and interfacial film. 

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