Emulsion interfacial viscosity

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 ... 

Insoluble polar molecules (e.g., long chain fatty acids) exhibit an extreme kind of adsorption at liquid surfaces. That is, they can be made to concentrate in one molecular layer at the surface. These interfacial films often provide the stabilizing influence in emulsions since they can both lower interfacial tension and increase the interfacial viscosity. The latter provides a mechanical resistance to coalescence. Such systems also lend themselves to the study of size, shape, and orientation of molecules at an interface. Having an adsorbed layer lowers the surface tension (to Ysolution) by the surface pressure jt= ysoivent - y solution as already noted. 

SAE could be expected to be utilized in Enhanced Oil Recovery, because their low interfacial tension and low interfacial viscosity might be predicted. Finally, emulsion breakers for crude oil may be another interesting application of SAE. 

Surfactants also reduce the coalescence of emulsion droplets. The latter process occurs as a result of thinning and disruption of the liquid film between the droplets on their close approach. The latter causes surface fluctuations, which may increase in amplitude and the film may collapse at the thinnest part. This process is prevented by the presence of surfactants at the O/W interface, which reduce the fluctuations as a result of the Gibbs elasticity and/or interfacial viscosity. In addition, the strong repulsion between the surfactant layers (which could be electrostatic and/or steric) prevents close approach of the droplets, and this reduces any film fluctuations. In addition, surfactants may form multilayers at the O/W interface (lamellar liquid crystalline structures), and this prevents coalescence of the droplets. 

Type and concentration of emulsifier. The viscosity and yield value of emulsions (chemical nature of the emulsifier. Sherman (1955c) proposed two possible reasons for this, namely interfacial viscosity and interfacial adsorption. Interfacial viscosity affects the resistance of droplets to deformation, which is reflected in the resulting emulsion viscosity. A high level of interfacial adsorption enlarges the size of the interfacial layer significantly and increases emulsion viscosity. Adsorption of emulsifier at the interface should also increase with the concentration of emulsifier. The... 

Micro emulsions can be formulated with carbon dioxide in supercritical state instead of a hydrocarbon as nonaqueous solvent. Fluorinated surfactants are commonly used to prepare such microemulsions. Water-in-carbon dioxide microemulsions can be made and the droplet size has been found to be similar to the size of the droplets of water-in-hydrocarbon micro emulsions with similar composition [21]. Such a microemulsion was used for conversion of benzyl chloride to benzyl bromide using KBr as hydrophilic nucleophile. The yield was an order of magnitude higher in the carbon dioxide microemulsion than in a conventional microemulsion at similar conditions, a fact that has been ascribed to low interfacial viscosity [22]. The big advantage with these micro emulsions is the environmental friendliness and the ease of work-up associated with carbon dioxide as solvent. 

Interfacial Viscosity, The foregoing discussion of rheology has dealt with the bulk viscosity properties. A closely related and very important property is the interfacial viscosity, which can be thought of as the two-dimensional equivalent of bulk viscosity, operative in the oil-water interfacial region. As droplets in an emulsion approach each other, the thinning of... 

Low interfacial viscosity is desirable in enhanced oil recovery operations, so that displaced oil globules may readily coalesce into an oil bank. Emulsion stability decreases as interfacial viscosity decreases, and this condition increases the ease with which an oil bank can be formed. Wasan et al. (J5) found a qualitative correlation between coalescence rates and interfacial viscosities for crude oil. 

For the emulsification mechanism to work, however, the interfacial films must be stable. Plus, higher interfacial viscosity increases the stability of emulsions and oil lamellae. Cooke et al. (1974) reported that their qualitative data indicated an increase in interfacial viscosity could increase oil recovery under certain conditions. Regardless, experiences tell us a low interfacial viscosity is needed for a higher recovery. 

Another important use of the PHS-PEO-PHS block copolymer is the formation of a viscoelastic film around water droplets [11, 12] this results from the dense packing of the molecule at the W/O interface, which leads to an appreciable interfacial viscosity. The viscoelastic film prevents transport of water from the internal water droplets in the multiple emulsion drop to the external aqueous medium, and this ensures the long-term physical stability of the multiple emulsion when using polymeric surfactants. The viscoelastic film can also reduce the transport of any a.i. in the internal water droplets to the external phase. This is desirable in many cases when protection of the ingredient in the internal aqueous droplets is required and release is provided on application of the multiple emulsion. 

We have recently reported (6, 7) that those surfactant formulations which yield good oil recovery exhibit both low interfacial tensions and low interfacial viscosities. Our experiments have shown that surfactant formulations which ensure low interfacial viscosity will promote the coalescence of oil droplets and thereby decrease the emulsion stability, thus enhancing the formation of a continuous oil bank. It has been demonstrated that the requirements for emulsion stability are the presence of an interfacial film of high viscosity and a film of considerable thickness. We have observed that the surfactant concentration which minimizes the interfacial tension may not simultaneously minimize the interfacial viscosity. Hence, our results indicate both interfacial tension and interfacial rheology must be considered in selecting surfactant formulations for tertiary oil recovery. 

One of the main objectives of this study has been to determine the effect of interfacial properties on coalescence, emulsion stability and oil recovery efficiency for various surfactant and caustic systems. We have recently reported (6, 19) that for a petroleum sulfonate system there is no direct correlation between rates of coalescence and interfacial tension or interfacial charge. However, a qualitative correlation has been found between coalescence rates and interfacial viscosities. 

The effects of interfacial tension, interfacial charge and interfacial viscosity on coalescence, and emulsion stability for crude oil emulsions in alkaline solutions have been assessed. It was observed that the NaOH concentration which yields higher interfacial shear viscosity also results in higher emulsion stability. 

Further data on the effect of interfacial viscosity on emulsion stability and its subsequent effect on oil recovery efficiency by alkaline water flooding are needed. 

Interrelationship of Emulsion Stability and Interfacial Viscosity in Improved Oil Recovery," paper presented at the Engineering Foundation Conference on Theory, Practice... 

After adsorption, adsorbed proteins can interact with each other since they have already denatured at an interface (Dickinson and Matsumura, 1991). This is evidenced by an increase in the interfacial viscosity with time after adsorption (Dickinson et al., 1990). Over time, the types and nature of proteins at the interface conld farther alter. This is crucial for storage stability. The ability of P-lg and a-la to displace one another previously adsorbed at the interface is slow and irreversible. In contrast, caseins show rapid and reversible displacement with a-la and P-lg (Dickinson et al., 1989). P-lg is more difficult to be displaced by P-casein than is a-la. Nevertheless, P-lg could not displace P-casein at the interface (Dalgleish et al., 1991). However, some P-lg could be found to adsorb at the interface of the emulsion droplets, which were previously stabilized by P-casein. 

Considering melt flow of BC, it is usually assumed that the test temperature is UCST > T > T, where T stands for glass transition temperature of the continuous phase. However, at Tg < T < T g (T g is Tg of the dispersed phase) the system behaves as a crosslinked rubber with strong viscoelastic character. At UCST > T > T, the viscosity of BC is much greater than would be expected from its composition. The reason for this behavior is the need to deform the domain structure and puU filaments of one polymer through domains of the other. Viscosity increases with increase of the interaction parameter between the BC components in a similar way as an increase of the interfacial tension coefficient in concentrated emulsions causes viscosity to rise [Henderson and Williams, 1979]. 

Some of this mobility control and improvement in sweep could be due to the emulsions formed diich exhibit a higher bulk viscosity. Some work by Wasan et al (32) has shown that the sodium silicates will result in emulsions with lower shear or interfacial viscosities than sodium hydroxide. These lower interfacial viscosities at the micro-level help promote oil coalescence so that oil banking can occur and oil droplets are not retrapped and left behind. The oil banking and rapid emulsification on the macro-level or in the bulk help to give good mobility control. Also some recent work by Wasan has indicated that the silicates tend to keep the surfaces more water-wet, thereby inproving recovery. It has been noted that the thickness of the water film on quartz surfaces is thicker when silicates are present. Further work in these areas is currently being done to determine the limits and effects on the oil recovery mechanisms. 

An emulsion is a mixture of two or more immiscible liquids, whose interface is stabilized by an emulsifier. Essentially, emulsion contains oil and water. Emulsions are inherently thermodynamically unstable system, often recognizable by their cloudy or white appearance since the substances do not mix homogenously. The mixed substances often separate due to reduction in the interfacial energy after a period of time. The possible mechanism of destabilization can be creaming, flocculation, or coalescence. The emulsions possess kinetic stability, which is lost after a period of time. The kinetic stability is associated with the presence of interfacial film around the dispersed droplets at the oil/water interface by the emulsifier. The interfacial film increases the interfacial viscosity, which retards the coalescence of the droplets. ... 

McMahon studied the effect of waxes on emulsion stability as monitored by the separation of water over time (46). The size of the wax crystals showed an effect in some emulsions but not in others. Interfacial viscosity indicated that the wax crystals form a barrier at the water/oil interface which retards the coalescence of colliding water droplets. Studies with octacosane, a model crude oil wax, show that a limited wax/asphaltene/resin interaction occius. A wax layer, even with absorbed asphaltenes and resins, does not by itself stabilize an emulsion. McMahon concludes that the effect of wax on emulsion stability does not appear to be through action at the interface. Instead, the wax may act in the bulk oil phase by inhibiting film thinning between... 

The IFPYV is defined as the shear stress when the shear rate is zero (25). Taubman and Koretskii (26) found that the yield stress of the interfacial film between CCI4 and an aqueous solution of AICI3 was related to the mechanical strength of the emulsifier film and the emulsion stability. Their study concluded that the lifetime of the emulsion, the yield stress, and the interfacial viscosity increase simultaneously. The experiments in our laboratory show that the... 

In addition to bulk viscosity properties, a closely related and very important property is the interfacial viscosity, which can be thought of as the two-dimensional equivalent of bulk viscosity, operative in the oil-water interfacial region. As droplets in an emulsion approach each other the thinning of the films between the drops, and their resistance to rupture, are thought to be of great importance to the ultimate stability of the emulsion. Thus, a high interfacial viscosity can promote emulsion stability by retarding the rate of droplet coalescence, as discussed in later sections. Further details on the principles, measurement, and applications to emulsion stability of interfacial viscosity are reviewed by Malhotra and Wasan [32]. 

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