Microemulsion bicontinuous structure

When oil is recovered from sedimentary formations by conventional means, more than one-half of it can be left behind in the rock (48). This oil is very difficult to remove because it is coating the rock surfaces and not free-flowing. Surfactant-based systems have been developed to enhance the recovery of the trapped oil. When these surfactant solutions are pumped underground, they appear to form microemulsions, bicontinuous structures, and possibly very fine macroemulsions, with the oil. The flow properties of these emulsions through porous media are quite important, therefore much elfort has been invested in rheological studies (49). Once the emulsified oil is removed from the ground, the emulsion needs to be broken in order for the oil to be recovered from the process stream. Another application of emulsions in the petroleum industry is to produce relatively low viscosity emulsions of viscous crude oil to make pipeline transport much easier. 

On a microscopic scale, a microemulsion is a heterogeneous system and, depending on the relative amounts of the constituents, three main types of structures can be distinguished [69] oil in water (OAV, direct micellar structure), water in oil (W/O, reverse micellar structure) and a bicontinuous structure (B) (Figure 6.1). By adding oil in water, OAV dispersion evolves smoothly to a W/O dispersion via bicontinuous phases. 

FIG. 2 Example media (a) Surfactant-water phase diagram. (Reprinted from Ref. 206, Copyright 1991, with permission from Elsevier Science.) (b) Ordered periodic and bicontinuous structures. (Reprinted from Ref. 178 with permission from Academic Press, Ltd.) (c) Nonordered membrane structures from ternary microemulsions. (Reprinted with permission from Ref. 177, Copyright 1989, American Chemical Society.)... 

Microemulsions are thermodynamically stable mixtures. The interfacial tension is almost zero. The size of drops is very small, and this makes the microemulsions look clear. It has been suggested that microemulsion may consists of bicontinuous structures, which sounds more plausible in these four-component microemulsion systems. It has also been suggested that microemulsion may be compared to swollen micelles (i.e., if one solubilizes oil in micelles). In such isotropic mixtures, short-range order exists between droplets. As found from extensive experiments, not all mixtures of water-oil-surfactant-cosurfactant produce a microemulsion. This has led to studies that have attempted to predict the molecular relationship. 

Many reports are available where the cationic surfactant CTAB has been used to prepare gold nanoparticles [127-129]. Giustini et al. [130] have characterized the quaternary w/o micro emulsion of CTAB/n-pentanol/ n-hexane/water. Some salient features of CTAB/co-surfactant/alkane/water system are (1) formation of nearly spherical droplets in the L2 region (a liquid isotropic phase formed by disconnected aqueous domains dispersed in a continuous organic bulk) stabilized by a surfactant/co-surfactant interfacial film. (2) With an increase in water content, L2 is followed up to the water solubilization failure, without any transition to bicontinuous structure, and (3) at low Wo, the droplet radius is smaller than R° (spontaneous radius of curvature of the interfacial film) but when the droplet radius tends to become larger than R° (i.e., increasing Wo), the microemulsion phase separates into a Winsor II system. 

The increase in conductivity is due to increase in dissolved surfactant, and this increase continues until all the crystallites dissolve. The peak in specific conductance is attained when the microemulsion is formed and the specific conductance levels off. The plateau of Figure 6 is often referred to as a "percolation threshold" (] ) and is reached when there is a disordered interspersion capable of bicontinuous structures ( ). Further addition of methanol results in a lowering of conductivity explained by the solution eventually approaching the conductivity of methanol. This is the region of molecular dispersion. These conductivity curves are similar to those observed by Lagues and Santerey (13) on a system of water, cyclohexane, sodium dodecylsulfate and 1-pentanol. 

Although NMR results provide perhaps the most convincing evidence of the bicontinuous structure of some microemulsions, many other techniques support their existence. These techniques include electrical conductimetry, x-ray and neutron scattering, quasielastic light scattering, and electron... 

Oh et al. [16] have demonstrated that a microemulsion based on a nonionic surfactant is an efficient reaction system for the synthesis of decyl sulfonate from decyl bromide and sodium sulfite (Scheme 1 of Fig. 2). Whereas at room temperature almost no reaction occurred in a two-phase system without surfactant added, the reaction proceeded smoothly in a micro emulsion. A range of microemulsions was tested with the oil-to-water ratio varying between 9 1 and 1 1 and with approximately constant surfactant concentration. NMR self-diffusion measurements showed that the 9 1 ratio gave a water-in-oil microemulsion and the 1 1 ratio a bicontinuous structure. No substantial difference in reaction rate could be seen between the different types of micro emulsions, indicating that the curvature of the oil-water interface was not decisive for the reaction kinetics. More recent studies on the kinetics of hydrolysis reactions in different types of microemulsions showed a considerable dependence of the reaction rate on the oil-water curvature of the micro emulsion, however [17]. This was interpreted as being due to differences in hydrolysis mechanisms for different types of microemulsions. 

Microemulsions are thermodynamically stable, clear fluids, composed of oil, water, surfactant, and sometimes co-surfactant that have been widely investigated during recent years because of their numerous practical applications. The chemical structure of surfactants may have a low molecular weight as well as being polymeric, with nonionic or ionic components [138-141]. For a water/oil-continuous (W/O) microemulsion, at low concentration of the dispersed phase, the structure consists of spherical water droplets surrounded by a monomolecular layer of surfactant molecules whose hydrophobic tails are oriented toward the continuous oil phase (see Fig. 6). When the volume fractions of oil and water are high and comparable, random bicontinuous structures are expected to form. 

Thevenin, M. A., Grossiord, J. L., and Poelman, M. C. (1996), Sucrose esters cosurfactant microemulsion systems for transdermal delivery—assessment of bicontinuous structures,/. I. Pharm., 137(2), 177-186. 

Addition of salting-out type electrolytes to oil-water-surfactant (s) systems has also a strong influence on their phase equilibria and interfacial properties. This addition produces a dehydration of the surfactant and its progressive transfer to the oil phase (2). At low salinity, a water-continuous microemulsion is observed in equilibrium with an organic phase. At high salinity an oil-continuous microemulsion is in equilibrium with an aqueous phase. At intermediate salinity, a middle phase microemulsion with a bicontinuous structure coexists with pure aqueous and organic phases. These equilibria were referred by Vinsor as Types I,II and III (33). 

Microemulsions. The structure of microemulsion systems has been reviewed (22). Both bicontinuous and droplet-type structures, among others, can occur in microemulsions. The droplet-type structure is conceptually more simple and is an extension of the emulsion structure that occurs at relatively high values of IFT. In this case, very small thermodynamically stable droplets occur, typically smaller than 10 nm (7). Each droplet is separated from the continuous phase by a monolayer of surfactant. Bicontinuous microemulsions are those in which oil and water layers in the microemulsion may be only a few molecules thick, separated by a monolayer of surfactant. Each layer may extend over a macroscopic distance, with many layers making up the microemulsion. 

Middle-Phase Microemulsion A microemulsion, with high oil and water contents, that is stable while in contact with either bulk-oil or bulk-water phases. This stability may be due to a bicontinuous structure in which both oil and water phases are continuous at the same time. In laboratory tube or bottle tests involving samples containing unemulsified oil and water, a middle-phase microemulsion will tend to be situated between the two former phases. See also Bicontinuous Microemulsion. 

Fig. 5 shows a hypothetical phase diagram with representation of microemulsion structures. At high water concentrations, microemulsions consist of small oil droplets dispersed in water (o/w microemulsion), while at lower water concentrations the situation is reversed and the system consists of water droplets dispersed in oil (w/o microemulsions). In each phase, the oil and water droplets are separated by a surfactant-rich film. In systems containing comparable amounts of oil and water, equilibrium bicontinuous structures in which the oil and the water domains interpenetrate in a more complicated manner are formed. In this region, infinite curved channels of both the oil and the water domains extend over macroscopic distances and the surfactant forms an interface of rapidly... 

Figure 7.18 Diagrammatic representation of microemulsion structures (a) a water-in-oil microemulsion droplet (b) an oil-in-water microemulsion droplet and (c) an irregular bicontinuous structure.

Based on data such as those showing a gradual decrease in electrical conductivity of microemulsions near optimal conditions as they became more lipophilic, Scriverf suggested that both oil and water were continuous under these conditions, in contrast to microemulsions far from optimal conditions, which were either oil continuous or water continuous. Several years later the bicontinuous structure was observed using a special electron microscopy technique in which the microemulsion is vitrified, i.e., cooled so fast that it freezes without crystallization of ice. ... 

Scriven, L.E., Equilibrium bicontinuous structures, in Micelles, Solubilization, and Microemulsions, Vol. 2, Mittal, K.L., Ed., Plenum Press, New York, 1977, 877. 

Figure 1.20 Micrographs of a bicontinuous microemulsion of the system H20-n-octane-C 2Es prepared near the X-point at 4> = 0.50, -y = 0.06 and T = 32.4°C. (a) Freeze-fracture direct imaging (FFDI) picture showing particularly in the middle of the image a sponge-like bicontinuous structure consisting of white and black domains. Note that the colours are inverted, (b) The freeze-fracture electron microscopy (FFEM) picture supports the FFDI result. (From Ref. [105], reprinted with permission of the American Chemical Society.)...

Knowing the shape (topology) of the microstructure one can obtain an estimate of the length scales from the composition of the microemulsion. Thus, the diameter of the domains (d = ,) in a bicontinuously structured microemulsion can be calculated from... 

As was described in Chapter 1, microemulsions are structured on a nanometre scale with domain sizes of 5-50 nm, which corresponds to surface areas of 300-30 m2 g 1. Moreover, a microemulsion can have various microstructures, ranging from discrete particles of one phase dispersed in the second (oil droplets in water or vice versa) to a bicontinuous structure consisting of two equal subphases (oil and water). Thus, using micro emulsions as templates should lead to a material with a high surface area and a structure equal to the structure of the template. This material would be suitable for a range of apphcations that... 

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