Spontaneous emulsification mechanism

J. C. Lopez-Montilla, P.E. Herrera-Morales, S. Pandey, and D.O. Shah Spontaneous Emulsification Mechanisms, Physicochemical Aspects, Modeling and Applications. J. Dispersion Sci. Technol. 23, 219 (2002). 

In this paper we report first the spontaneous emulsification mechanisms in the petroleum sulfonate and caustic systems. This is followed by the kinetics of coalescence in alkaline systems for both the Thums Long Beach (heavy) crude oil and the Huntington Beach (less viscous) crude oil. Measurements of interfacial viscosity, interfacial tension, interfacial charge and micellar aggregate distributions are presented. Interrelationships between these properties and coalescence rates have been established. 

Spontaneous Emulsification Mechanisms in Sulfonate and Caustic Systems... 

The spontaneous emulsification mechanisms were determined for the 3% (Active) Petrostep 420 and 1.5% NaCl and 0.58% n-hexanol versus non-pre-equilibrated Salem crude oil (Sulfonate System). The mechanisms were also determined for Thums Long Beach Crude Oil (Well 108 B) versus 0.05 M (0.2% by weight)... 

The recovery of naturally acidic oils by alkaline flooding fits into the phase alteration category. The recovery mechanisms of these floods are varied since the surface active salts, which are formed by the in situ acid-base reaction, can adsorb onto the oil-water interface to promote emulsification or can absorb onto the rock surface to alter wettability. The exact recovery mechanism, recently reviewed by Johnson (3) depends on the pH and salinity of the aqueous phase, acidity of the organic phase and wettability of the rock surface (4,5). In this study an additional alkaline recovery mechanism is explored. This mechanism. Emulsification and Coalescence, depends on the valency of the electrolyte as well as the pH and salinity of the aqueous phase. The Emulsification and Coalescence mechanism for the recovery of acidic oils is similar to the Spontaneous Emulsification mechanism suggested by Schechter et al. (6) for the recovery of nonacidic oils with petroleum sulfonate solutions. 

Emulsification and coalescence The possibility of enhancing oil recovery from porous media by a spontaneous emulsification mechanism has been examined by Schechter and coworkers (6). These researchers postulated that residual oil, which is entrapped after a conventional waterflood, can be mobilized by spontaneous emulsification and subsequent coalescence of small droplets with other... 

N. Shahidzadeh, D. Boim, and J. Meunier A New Mechanism of Spontaneous Emulsification Relation to Surfactant Properties. Europhys. Lett 40, 459 (1997). R.W. Greiner and D.F. Evans Spontaneous Formation of a Water-Continuous Emulsion from a W/O Microemulsion. Langmuir 6, 1793 (1990). 

When the minimum of y with respect to T is negative (Fig. 3), the interfacial tension is negative in the range of temperatures in which the curve y = y(T) intersects the abscissa. A negative y makes the water oil interface unstable to thermal and mechanical perturbations and a spontaneous emulsification with the formation of globules of oil in water and water in oil takes place. At low temperatures, because of coalescence, only a fraction of the globules of oil survive... 

In situ formation of oil-in-water emulsions adds the requirement that the emulsification proceed spontaneously or at least with very little energy input due to mixing. Most such systems are associated with the agent-in-oil procedure, and spontaneous emulsification to oil-in-water emulsions sometimes occurs when aqueous caustic is mixed with petroleum oils containing naphthenic acids. Some researchers propose that mass transfer of the naturally occurring surfactants across the interface is the mechanism that causes this phenomena... 

Cash, R. L., et al. Spontaneous Emulsification - a Possible Mechanism for Enhanced Oil Recovery. Proc. 50th SPE Annual Fall Meeting, paper SPE 5562, Dallas, Texas, 1975. 

The caustic method as a means of improved waterflooding for enhanced oil recovery is a complex process. Johnson has outlined four recovery mechanisms (6). Presumably, besides the ultralow tension mode, there are other requirements to ensure efficient and stable recovery of an oil in a given reservoir. To name a few spontaneous emulsification, entrainment, entrappment, wettability reversal in both directions, etc. In order to maintain a particular set of pro-... 

In the caustic system the mechanisms of spontaneous emulsification lead to the formation of both an oil-in-water and a water-in-oil emulsion. A representative photomicrograph has been included in Figure 2. 

Therefore, the mechanism of spontaneous emulsification in the caustic system is interfacial turbulence. The mechanisms for the sulfonate system are diffusion and stranding. 

The mechanisms of a spontaneous emulsification in petroleum sulfonate and caustic systems have been described. 

D. J. Schares, T. Schechter, R. S. and Wade, W. H., "Spontaneous Emulsification—A Possible Mechanism for Enhanced Oil Recovery", SPE 5562. Paper presented at 50th Annual Fall Meeting of the Society of Petroleum Engineers of AIME. Dallas, Texas, September 28 - October 1, 1975. 

The exact mechanism of the spontaneous emulsification is still not known in detail [77]. It most certainly is not a simple swelling of pre-formed droplets. The experimental observation [78] that the presence of small amounts of a highly water-insoluble compound in the monomer phase prior to emulsification prevents formation of a miniemulsion implies that a diffiisional mechanism is involved. 

Hessel [79] has proposed a mechanism of spontaneous emulsification which involves a two-step process (1) the monomer phase swells the bilayers of the unilamellar vesicl which results in microemulsion droplets budding off the bilayer, and (2) the microemulsion droplets swell to miniemulsion droplet dimensions (50-400 nm). Diffusion of the monomer through the aqueous phase is... 

Several mechanisms may be proposed to explain the process of spontaneous emulsification, all of which are related to the properties of the interfacial film. The first mechanism is due to interfacial turbulence that may occur as a result of mass transfer or by non-uniform adsorption of the surfactant molecules at the OAV interface. The interface shows unsteady motions - streams of one phase are ejected and penetrate into the second phase. This is illustrated in Figure 4.1(a) which shows the localized reduction in interfacial tension caused by non-uniform adsorption of surfactants or mass transfer of surfactants across the interface (5-7). When the two phases are not in chemical equilibrium, convection currents may be formed which transfers the liquid rich in surfactants towards the areas deficient in surfactants. These convection currents may give rise to... 

The second mechanism of spontaneous emulsification is based on diffusion and stranding, as represented in Figure 4.1(b). This is best illustrated by carefully placing an ethanol-toluene mixture (containing, say, 10% alcohol) on to water. The aqueous layer eventually becomes turbid as a result of the presence of toluene droplets (8). The alcohol molecules diffuse into the aqueous phase, carrying some toluene in a saturated three-component sub-phase (9). At some distance from the interface, the alcohol becomes sufficiently diluted in water to cause the toluene to precipitate as droplets in the aqueous phase. This process occurs when the third component (the alcohol) increases the mutual solubility of the two previously immiscible phases (oil and water). 

Several other mechanisms have been proposed to explain the dynamics of spontaneous emulsification. Direct observation by using phase contrast and polarizing microscopy showed that in some cases vesicles (closed bilayers) are produced in the oil phase near the interface with the water. These vesicles tend to explode , thereby pulverizing oil droplets into the aqueous phase. The above structures can be produced, for example, by using a mixture of nonionic surfactant (alcohol ethoxylate) and a long-chain alcohol such... 

Art and magic aside, there are three principal methods of emulsion preparation which are most often employed. A fairly comprehensive coverage of those methods is presented in the work by Becher et al., cited in the Bibliography. The three methods most often employed include (1) physical emulsification by drop rupture, (2) emulsification by phase inversion, and (3) spontaneous emulsification. The latter two methods may be described as chemically based processes in that the nature of the final emulsion will be controlled primarily by the chemical makeup of the system (the chemical nature of additives, the ratios of the two phases, temperature, etc.), while in the first it will depend more on the mechanical nature of the process (e.g., amount and form of energy input.), as well as the rheological and chemical properties of the components. Other possibilities exist (see Table 11.1) however, most are of limited practical importance. 

It is, of course, not necessary to form an emulsion by mechanical dispersion of oil and water phases. One method that has been nsed to form oil-in-water emnlsions with small and uniform drops in a nonionic snrfactant system is to start with the surfactant phase near the PIT and cool it rapidly by perhaps 20°C to 30°C (Friberg and Solans, 1968 Forster et al., 1995 Sagitani, 1992). The capacity for solubilization of oil decreases dramatically npon cooling, and the excess oil nucleates as small drops from the supersaturated microemulsion. Provided that it solubihzes substantial oil, the lamellar liqnid crystalline can also be cooled in this manner to form oil-in-water emulsions (Forster et al., 1995). Spontaneous emulsification can also be prodnced by diffusion, as discussed in Chapter 6. 

FIGURE 6.25 Schematic diagram showing mechanism of spontaneous emulsification for a drop of n-hexadecane containing suitable amounts of pure C12E6 and n-octanol. 

At first sight the process of generation of small particles by solvent displacement is deceptively simple after mixing of the two solvents, the organic substance (polymer or active principle) suddenly finds itself surrounded by a water-rich environment and, because of its low solubiUty in water, it precipitates, generating solid particles. An everyday occurrence of this process is the spontaneous emulsification of alcoholic drinks when diluted with water, such as Ouzo in Greece and Fastis in France. However, the detailed description of the mechanism is more complex and is still controversial. 

The second mechanism that may account for spontaneous emulsification is based on diffusion and stranding. This is best illustrated by carefully placing an ethanol-toluene mixture (containing say 10% alcohol) onto water. The aqueous layer eventually becomes turbid due to the presence of toluene droplets [20]. In... 

The third mechanism of spontaneous emulsification may be due to the production of an ultralow (or transiently negative) interfacial tension. This mechanism is thought to be the cause of formation of microemulsions when two surfactants, one essentially water soluble and one essentially oil soluble, are used [22, 23]. This mechanism is described in detail in Chapter 10 on microemulsions. 

The above interfadal tension results may throw some light on the mechanism of spontaneous emulsification in the present model EC. As mentioned before, there are basically two main mechanisms of spontaneous emulsification, namely creation of local supersaturation (i.e. diffusion and stranding) or by mechanical breakup of the droplets as a result of interfadal turbulence and/or the creation of an ultralow (or transiently negative) interfacial tension. Diffusion and stranding is not the likely mechanism in the present system since no water-soluble co-solvent was added. To check whether the low interfadal tension produced is sufficient to cause spontaneous emulsification, a rough estimate may be made from consideration of the balance between the entropy of dispersion and the interfacial energy, i.e. 

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