Microemulsions interface

Macroemulsion and Microemulsion Flooding If a suitable surfactant is injected into the reservoir, it can form macroemulsions and/or microemulsions with the reservoir oil depending on the composition and reservoir conditions. Several articles have been published on the recovery of oil by microemulsion and macroemulsion flooding processes.Among various factors, the most important factor of surfactant flooding in the form of an emulsion is the lowering of the interfacial tension (IFT) at the oil/water interface. Microemulsions are more effective in oil displacement as compared to macroemulsions because microemulsions can provide low IFT systems. 

Compared to macro- and miniemulsions, the water uptake is lower in microemulsions. By virtue of their smaller sizes and, subsequendy, the more efficient packing of the surfactant at the interface, microemulsions are not subject to flocculation effects. In the field of microemulsions, the water uptake is defined as the molar ratio of water to surfactant, W. The corrected Wq (called taking account of the solubility of water in CO2 is defined as ... 

Ex(30) values show a good, often linear, correlation with a large number of other solvent sensitive processes, such as reaction rates and shifts of chemical equilibria. The betaine dye (Scheme 3) and specially designed derivatives are useful molecular probes in the study of micellar interfaces, microemulsions and phospholipid bUayers, of rigid rod-Uke isocyanide polymers, and the retention behaviour in reversed-phase chromatography. In addition to its solvatochromic behaviour, the dye is sensitive to temperature ( thermosolvatochromism ) and pressure changes ( piezosolvatochromism ) and also to the presence of electrol)d es ( halosolvatochromism ). 

Desai S D, Gordon R D, Gronda A M and Cussler E L 1996 Polymerized microemulsions Curr. Opin. Colloid Interface Sc/. 1 519-22... 

In a detersive system containing a dilute surfactant solution and a substrate bearing a soHd polar sod, the first effect is adsorption of surfactant at the sod—bath interface. This adsorption is equivalent to the formation of a thin layer of relatively concentrated surfactant solution at the interface, which is continuously renewable and can penetrate the sod phase. Osmotic flow of water and the extmsion of myelin forms foHows the penetration, with ultimate formation of an equdibrium phase. This equdibrium phase may be microemulsion rather than Hquid crystalline, but in any event it is fluid and flushable... 

For the system studied in [174], it turns out that the oil/water interface is not wetted by the microemulsion, even though the latter is weakly structured. Hence fluctuations do shift the wetting transition beyond the disorder... 

The model has been successfully used to describe wetting behavior of the microemulsion at the oil-water interface [12,18-20], to investigate a few ordered phases such as lamellar, double diamond, simple cubic, hexagonal, or crystals of spherical micelles [21,22], and to study the mixtures containing surfactant in confined geometry [23]. 

In this section we characterize the minima of the functional (1) which are triply periodic structures. The essential features of these minima are described by the surface (r) = 0 and its properties. In 1976 Scriven [37] hypothesized that triply periodic minimal surfaces (Table 1) could be used for the description of physical interfaces appearing in ternary mixtures of water, oil, and surfactants. Twenty years later it has been discovered, on the basis of the simple model of microemulsion, that the interface formed by surfactants in the symmetric system (oil-water symmetry) is preferably the minimal surface [14,38,39]. 

For diffuse and delocahzed interfaces one can still define a mathematical surface which in some way describes the film, for example by 0(r) = 0. A problem arises if one wants to compare the structure of microemulsion and of ordered phases within one formalism. The problem is caused by the topological fluctuations. As was shown, the Euler characteristic averaged over the surfaces, (x(0(r) = 0)), is different from the Euler characteristics of the average surface, x((0(r)) = 0), in the ordered phases. This difference is large in the lamellar phase, especially close to the transition to the microemulsion. x((0(r)) =0) is a natural quantity for the description of the structure of the ordered phases. For microemulsion, however, (0(r)) = 0 everywhere, and the only meaningful quantity is (x(0(r) = 0))-... 

Recently an alternative approach for the description of the structure in systems with self-assembling molecules has been proposed in Ref. 68. In this approach no particular assumption about the nature of the internal interfaces or their bicontinuity is necessary. Therefore, within the same formahsm, localized, well-defined thin films and diffuse interfaces can be described both in the ordered phases and in the microemulsion. This method is based on the vector field describing the orientational ordering of surfactant, u, or rather on its curlless part s defined in Eq. (55). 

Having determined the effect of the diffusive interfaces on the structure parameters, we now turn to the calculation of H and K in microemulsions. In the case of oil-water symmetry three-point correlation functions vanish and = 0. In order to calculate K from (77) and (83) we need the exphcit expressions for the four-point correlation functions. In the Gaussian approximation... 

G. Gompper, M. Schick. Scattering from internal interfaces in microemulsion and sponge phases. Phys Rev E 49 1478-1482, 1994. 

S. Ezrahi, E. Wachtel, A. Aserin, N. Garti. Structural polymorphism in a four component nonionic microemulsion. J Coll Interface Sci 797 277-290, 1997. 

Figure 9.1 illustrates a variety of different stractures. This selection is by no means all-inclusive a host of related stractures such as colloids, microstrands, thin films, microporous solids, microemulsions, and gels could also have been shown. The parts of each of these stractures are distinguished by the zones—interfaces—between them, which often seem to be... 

FIG. 1 Geometries of electrolyte interfaces, (a) A planar electrode immersed in a solution with ions, and with the ion distrihution in the double layer, (b) Particles with permanent charges or adsorbed surface charges, (c) A porous electrode or membrane with internal structures, (d) A polyelectrolyte with flexible and dynamic structure in solution, (e) Organized amphophilic molecules, e.g., Langmuir-Blodgett film and microemulsion, (f) Organized polyelectrolytes with internal structures, e.g., membranes and vesicles. 

The rates of multiphase reactions are often controlled by mass tran.sfer across the interface. An enlargement of the interfacial surface area can then speed up reactions and also affect selectivity. Formation of micelles (these are aggregates of surfactants, typically 400-800 nm in size, which can solubilize large quantities of hydrophobic substance) can lead to an enormous increase of the interfacial area, even at low concentrations. A qualitatively similar effect can be reached if microemulsions or hydrotropes are created. Microemulsions are colloidal dispersions that consist of monodisperse droplets of water-in-oil or oil-in-water, which are thermodynamically stable. Typically, droplets are 10 to 100 pm in diameter. Hydrotropes are substances like toluene/xylene/cumene sulphonic acids or their Na/K salts, glycol.s, urea, etc. These. substances are highly soluble in water and enormously increase the solubility of sparingly. soluble solutes. 

Tjandra et al. (1998) have proposed an interfacial reaction model for the kinetics of the reaction between 1-bromo octane and sodium phenoxide to give 1-phenoxyoctane in a nonionic microemulsion. In this model the microemulsion is assumed to consist of the aqueous phase and the interface is covered by a monolayer of surfactant molecules. It is thus possible to assess the interfacial area from the concentration of the surfactant in the microemulsion medium. 

Liquid liquid interfaces occur as macro- and also as micro- and nanoheterogenous systems (termed small systems), described in colloidal chemistry as, e.g., miscelles, vesicles, and microemulsions [14,19] (see also Section V). Up to now, fast progress concerns mainly the macrosystems (> 100/rm), including all types of natural and artificial membranes. 

The potential x as the difference of electrical potential across the interface between the phase and gas, is not measurable. But its relative changes caused by the change of solution composition can be determined using the proper voltaic cells (see Section IV). The name surface potential is unfortunately also often used for the description the ionic double layer potential (i.e., the ionic part of the Galvani potential) at the interfaces of membranes, microemulsion droplets and micelles, measured usually by the acid-base indicator technique (Section V). 

Another example of chemical-potential-driven percolation is in the recent report on the use of simple poly(oxyethylene)alkyl ethers, C, ), as cosurfactants in reverse water, alkane, and AOT microemulsions [27]. While studying temperature-driven percolation, Nazario et al. also examined the effects of added C, ) as cosurfactants, and found that these cosurfactants decreased the temperature threshold for percolation. Based on these collective observations one can conclude that linear alcohols as cosurfactants tend to stiffen the surfactant interface, and that amides and poly(oxyethylene) alkyl ethers as cosurfactants tend to make this interface more flexible and enhance clustering, leading to more facile percolation. 

The ITIES with an adsorbed monolayer of surfactant has been studied as a model system of the interface between microphases in a bicontinuous microemulsion [39]. This latter system has important applications in electrochemical synthesis and catalysis [88-92]. Quantitative measurements of the kinetics of electrochemical processes in microemulsions are difficult to perform directly, due to uncertainties in the area over which the organic and aqueous reactants contact. The SECM feedback mode allowed the rate of catalytic reduction of tra 5-l,2-dibromocyclohexane in benzonitrile by the Co(I) form of vitamin B12, generated electrochemically in an aqueous phase to be measured as a function of interfacial potential drop and adsorbed surfactants [39]. It was found that the reaction at the ITIES could not be interpreted as a simple second-order process. In the absence of surfactant at the ITIES the overall rate of the interfacial reaction was virtually independent of the potential drop across the interface and a similar rate constant was obtained when a cationic surfactant (didodecyldimethylammonium bromide) was adsorbed at the ITIES. In contrast a threefold decrease in the rate constant was observed when an anionic surfactant (dihexadecyl phosphate) was used. 

Forster, Th., von Rybinski, W. and Wadle, A. (1995) Influence of microemulsion phases on the preparation of fine-disperse emulsions. Advances in Colloid and Interface Science, 58, 119-149. 

Arriagada, F.J., and Osseo-Asare, K. (1995) Synthesis of nanosize silica in aerosol OT reverse microemulsions./. Colloid Interface Sci. 170, 8-17. 

Here scalar order parameter, has the interpretation of a normalized difference between the oil and water concentrations go is the strength of surfactant and /o is the parameter describing the stability of the microemulsion and is proportional to the chemical potential of the surfactant. The constant go is solely responsible for the creation of internal surfaces in the model. The microemulsion or the lamellar phase forms only when go is negative. The function/(<))) is the bulk free energy and describes the coexistence of the pure water phase (4> = —1), pure oil phase (4> = 1), and microemulsion (< ) = 0), provided that/o = 0 (in the mean-held approximation). One can easily calculate the correlation function (4>(r)(0)) — (4>(r) (4>(0)) in various bulk homogeneous phases. In the microemulsion this function oscillates, indicating local correlations between water-rich and oil-rich domains. In the pure water or oil phases it should decay monotonically to zero. This does occur, provided that g2 > 4 /TT/o — go- Because of the < ), —<(> (oil-water) symmetry of the model, the interface between the oil-rich and water-rich domains is given by... 

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