Emulsions interfacial film effects

Freshly prepared macroemulsions change their properties with time. The time scale can vary from seconds (then it might not even be appropriate to talk about an emulsion) to many years. To understand the evolution of emulsions we have to take different effects into account. First, any reduction of the surface tension reduces the driving force of coalescence and stabilizes emulsions. Second, repulsive interfacial film and interdroplet forces can prevent droplet coalescence and delay demulsification. Here, all those forces discussed in Section 6.5.3 are relevant. Third, dynamic effects such as the diffusion of surfactants into and out of the interface can have a drastic effect. 

Monolayers can be used to retard evaporation (water reservoirs), a related effect is the use of an oil film to dampen waves (surface viscosity), interfacial films are often the stabilizing agent in emulsions (lower y and increase rjs). 

Generally speaking, for a stable emulsion a densely packed surfactant film is necessary at the interfaces of the water and the oil phase in order to reduce the interfacial tension to a minimum. To this end, the solubility of the surfactant must not be too high in both phases since, if it is increased, the interfacial activity is reduced and the stability of an emulsion breaks down. This process either can be undesirable or can be used specifically to separate an emulsion. The removal of surfactant from the interface can, for example, be achieved by raising the temperature. By this measure, the water solubility of ionic surfactants is increased, the water solubility of non-ionic emulsifiers is decreased whereas its solubility in oil increases. Thus, the packing density of the interfacial film is changed and this can result in a destabilisation of the emulsion. The same effect can happen in the presence of electrolyte which decreases the water solubility mainly of ionic surfactants due to the compression of the electric double layer the emulsion is salted out. Also, other processes can remove surfactant from the water-oil interface - for instance a precipitation of anionic surfactant by cationic surfactant or condensing counterions. 

Effect of Demulsifier Mixture. In previous studies (27) Duo-meen C, which was effective in causing flocculation of the water droplets, was not very effective in breaking the interfacial film formed between the water droplets, which inhibits coalescence. (Duomeen C is a mixture of many types of surfactants the general classification is a fatty acid ester nitrogen derivative.) However, Duomeen C in combination with docusate sodium (Aerosol OT), a hydrophilic surfactant, was much more effective in causing water separation compared to the individual chemicals. This effect is shown in Figure 16 for a 6 vol% water-in-oil (Leduc crude) emulsion in which both the UVP signal (20 min after chemical addition) and the volume... 

The interfacial films formed by different crude oils have different characteristics. The physical characteristics of the films are a function of the crude-oil type and gas content, the composition and pH of water, the temperature, the presence of nonionic polar molecules in the water, the extent to which the adsorbed film is compressed, and the contact time allowed for adsorption and concentration of polar molecules in the oil phase 14, 22,23). The rheological properties of the adsorbed emulsifier film have an important effect on the stability of emulsions. 

In extreme cases, material can adsorb at an interface to create a film. Interfacial film formation can occur in crude-oil systems and has been reported by Blair (16), and by Reisberg and Doscher (17), Film formation is relatively common with crude oils and can effectively stabilize emulsions by preventing droplet coalescence even with high values of interfacial tension. 

The gum arabic molecule is, in addition, surface-active, a 4% solution at 30°C has a surface tension of 63.2 mN m b Addition of electrolytes makes the molecule more surface-active either by causing a change in conformation of the molecule at the interface, allowing closer packing, or by increasing the hydro-phobicity of the molecule. It is an effective emulsifier, the stabilisation of the emulsion being dependent mainly on the coherence and elasticity of the interfacial film, which is by no means monomolecular. 

Emulsifiers stabilize emulsions in various ways. They reduce interfacial tension and may form an interfacial film that prevents coalescence of droplets. In addition, ionic emulsifiers provide charged groups on the surface of the emulsion droplets and thus increase repulsive forces between droplets. Emulsifiers can also form liquid crystalline microstructures such as micelles at the interface of emulsion droplets. These are formed only at emulsifier concentrations larger than the critical micelle-forming concentration. These microstructures have a stabilizing effect. 

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. 

For maximum mechanical stability, the interfacial film resulting from the adsorbed surfactants should be condensed, with strong lateral intermolecular forces, and should exhibit high film elasticity. The liquid film between two colliding droplets in an emulsion is similar to the liquid lamella between two adjacent air sacs in a foam (Chapter 7) and shows film elasticity for the same reasons (Gibbs and Marangoni effects). 

Dodd [68] measured the rheological properties of interfacial films in a semiquanti-tative manner by employing an interfacial shear rotational viscometer to study crude-water interfaces with NaCl, acid, and basic additives in the water phase. He concluded that the film must be comprised of naphthenic acids, naphthenic acid soaps, and naphthenic acid anions, in combination with resins, asphaltenes, and waxes. Furthermore, the acidic species must desorb from the interface under basic conditions and partition into the aqueous phase, rendering the interface considerably less rigid. Subsequent researchers have shown that acidic asphaltenes are more effective at emulsion stabilization than their neutral counterparts. 

Monolayers can be used to retard evaporation (water reservoirs), a related effect is the use of an oil film to dampen waves (surface viscosity), and monolayer interfacial films are often the stabilizing agent in emulsions (lower y and increase i/ ). The first scientific studies of spread monolayers appear to have included those by Benjamin Franklin in 1774 and Agnes Pockels in 1891. Experimental methods for determining the retardation of evaporation by monolayers have been reviewed by Barnes [52]. 

Emulsion droplets are normally stabilized by enhancing the mechanical strength of the interfacial film formed around the oil droplets (38). by steric stabilizaticm effects, and/or by the presence of charged surfactants which create an electrostatic barrier. The stabilizing factor of the latter is the electrostatic repulsimi of similarly charged droplets. The emulsion stability can be considerably improved with the use of mixed emulsifying agents (39). 

The stability of cmde oil emulsions determines the effectiveness of enhanced oil recovery and the separation of water and oil in the oil production. Crude oils contains natural interfacially active fractions and particles, for example, resin, asphaltene, and wax particles. The interfacially active fractions and particles tend to present at the water/oil interface and form a tough film surrounding the dispersed droplets. The film can resist the coalescence of the droplets and stabilize the emulsion (8-11). 

JE Strassner. Effect of pH on interfacial films and stability of crude oil—water emulsions. Proceedings of 42nd Annual Spec of AIME Fall Meeting, 1969, Preprint no. SPE-1969. 

The results obtained from flie Langmuir interfacial film studies are important in explaining why certain chemicals are more effective as inhibitors than as demulsifiers. Obviously, the inhibitor/asphaltene interaction is so strong in the bulk oil phase that the interfacial structures being gradually built up will no longer possess properties required to stabilize W/0 emulsions. 

The first step in systematic emulsion breaking would be to characterize the emulsion to be broken in terms of its nature (O/W, W/O, or multiple emulsion), the number and nature of immiscible phases, the presence of a protective interfacial film around the droplets, and the sensitivity of the emulsifiers [32,82,83]. Based on an emulsion characterization, a chemical addition can be prescribed to neutralize the effect of the emulsifier, followed by mechanical means to complete the phase separation. 

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