Aqueous microemulsions single-phase microemulsion

The Winsor II microemulsion is the configuration that has attracted most attention in solvent extraction from aqueous feeds, as it does not affect the structure of the aqueous phase the organic extracting phase, on the other hand, is now a W/0 microemulsion instead of a single phase. The main reason for the interest in W/0 microemulsions is that the presence of the aqueous microphase in the extracting phase may enhance the extraction of hydrophilic solutes by solubilizing them in the reverse micellar cores. However, this is not always the case and it seems to vary with the characteristics of the system and the type of solute. Furthermore, in many instances the mechanism of extraction enhancement is not simply solubilization into the reverse micellar cores. Four solubilization sites are possible in a reverse micelle, as illustrated in Fig. 15.6 [19]. An important point is that the term solubilization does not apply only to solute transfer into the reverse micelle cores, but also to insertion into the micellar boundary region called the palisade. The problem faced by researchers is that the exact location of the solute in the microemulsion phase is difficult to determine with most of the available analytical tools, and thus it has to be inferred. 

For a given surfactant, the ability to form a single-phase w/o microemulsion is a function of the type of oil, nature of the electrolyte, solution composition, and temperature (54-58). When microemulsions are used as reaction media, the added reactants and the reaction products can also influence the phase stability. Figure 2.2.4 illustrates the effects of temperature and ammonia concentration on the phase behavior of the NP-5/cyclohexane/water system (27). In the absence of ammonia, the central region bounded by the two curves represents the single-phase microemulsion region. Above the upper curve (the solubilization limit), a water-in-oil microemulsion coexists with an aqueous phase, while below the lower curve (the solubility limit), an oil-in-water water microemulsion coexists with an oil phase. It can be seen that introducing ammonia into the system results in a shift of the solubilization... 

The fractional saturation of water in the oil phase is denoted/Wo- For a single-phase microemulsion,/Wo is less than unity. As the amount of water in the phase increases, /Wo progressively increases and reaches the constant value of unity when an excess aqueous phase coexisting with the microemulsion phase appears, creating a two-phase system. 

In addition to single phase microemulsions, several phase equilibria known as Winsor systems [4] are also shown at low surfactant concentrations. A Winsor I (WI) system consists of an 0/W microemulsion that is in equilibrium with an oil phase, while a Winsor II (WII) system is a W/0 microemulsion in equilibrium with an aqueous phase. A Will system has a middle phase (bicontinuous) microemulsion that coexists with both oil and aqueous phases. 

The double inverse microemulsion method was also used to synthesize per-ovskite-type mixed metal oxides [ 155]. One microemulsion solution contained nitrate salts of either Ba(N03)2/Pb(N03)2, La(N03)3/Cu(N03)2 or La(N03)3/ Ni(N03)2, and the other microemulsion contained ammonium oxalate or oxalic acid as the precipitant. These metal oxalate particles of about 20 nm were readily calcined into single phase perovskite-type BaPb03, La2Cu04 and LaNi03. The calcinations required for the microemulsion-derived mixed oxalates were 100-250 °C below the temperatures used for the metal oxalates prepared by a conventional aqueous solution precipitation method. 

Equilibrium with Aqueous Phases. The formation and properties of reverse micelle and microemulsion phases in equilibrium with a second predominantly water continuous phase is of practical interest for extraction processes. Figure 7 compares apparent hydrodynamic diameters observed in the ethane/AOT/water system at 37 C for values of 1, 3 and 16. In single phase systems at W - 1 (a) and 3 (b) the apparent hydrodynamic diameter decreases with increased pressure due to decreased micelle-micelle interactions as the solvent power increases. In contrast for a system with an overall W - 16 (c), where a second aqueous phase exists, hydrodynamic diameter increases continuously with pressure. 

Addition of oil yields an oil-in-water microemulsion with nearly spherical drops. Within limits, the higher the molecular weight of the oil added to produce an oil-in-water microemulsion, the less oil is needed to formulate single phases with polymer for mobility control (Hirasaki et al., 2008). During screening tests, a clear surfactant-polymer solution with oil added does not mean the corresponding aqueous solution without oil will be clear. Therefore, aqueous stability tests with polymer added in the surfactant solution are necessary and important. 

Systems whose composition is located inside the three-phase triangle split into an amphiphile-rich phase (m). which is in the center of the diagram at the boundary of the single-phase region, and two excess phases, which are essentially pure aqueous phase and pure oil. The amphiphile-rich phase, which is found to be a bicontinuous microemulsion, has been called the middle phase because it appears in-between the oil and water phase in a lest tube. Since the middle phase is at equilibrium with both excess phases, it cannot be diluted cither by water or oil. and it is thus neither water- nor oil-continuous. 

Microemulsions are ternary systems containing oil, water, and surfactant. The terms oil and water in a microemulsion system normally refer to oil phase (oil and oil soluble components such as cyclosporine) and aqueous phase (water and water soluble components such as sodium chloride), respectively. The phase behavior of water-oil-surfactant mixtures was extensively studied by Winsor (1948). Based on his experimental observations, Winsor classified equilibrium mixtures of water-oil-surfactant into four systems (1) type I (Winsor I) system where water continuous or oil-in-water (0/W) type microemulsion coexists with the oil phase. In these systems, the aqueous phase is surfactant-rich (2) type II (Winsor II) system where oil continuous or water-in-oil (W/0) type microemulsion coexists with the aqueous phase. In these systems, the oil phase is surfactant-rich (3) type III (Winsor III) system where bicontinuous type microemulsion (also referred to as surfactant-rich middle-phase) coexists with excess oil at the top and excess water at the bottom and (4) type IV (Winsor IV) system where only a single-phase (microemulsion) exists. The surfactant concentration in type IV microemulsion is generally greater than 30 wt%. Type IV microemulsion could be water continuous, bicontinuous, or oil continuous depending on the chemical composition. The phase behavior of microemulsions is often described as a fish diagram shown in Figure lO.I (Komesvarakul et al. 2006). 

The addition of alcohol, as cosurfactant, to the [Cgmim][TfjN]/AOT/water system leads to stable w/IL microemulsions. DLS and protein solubilization experiments confirm the existence of an aqueous nanoenvironment in the IL phase of [C mirnTf N]/ AOT/l-hexanol/water microemulsions [67]. The kinetics of the enzymatic reactions were performed in this quaternary system. Specifically, lipase-catalyzed hydrolysis of p-nitrophenyl butyrate (p-NPB) was used as a model reaction [68]. In a similar way, the hpase-catalyzed hydrolysis of p-NPB was investigated to evaluate the catalytic efficiency in water/AOT/Triton X-100/[C mim][PFJ [69]. A large single-phase microemulsion region can be obtained from the combination of two surfactants in IL. 

Graciaa et al assumed that microemulsion phases (O, W , surfactant phase microemulsion (M5), type 3 oleic microemulsion (M,), type 3 aqueous microemulsion (M )), despite being single thermodynamic phases, are composed of submi-croscopic regions of oil and water separated by an interfacial layer of surfactant. Consequently, it is possible for each of the different microemulsion phases to be constructed from the three constituents parts described earlier. 

Another important phase classification that can often be found in literature has been introduced by Winsor (Winsor 1948), who found four general types of phase equilibria. A Winsor type IV phase corresponds to classical single phase microemulsions consistent with Hoar s Schulman s definition (Hoar Schulman 1943). A Winsor type I system denotes two phases in equilibrium, an o/w structure and an almost pure oil upper phase. On the contrary, in a Winsor type II system an aqueous phase containing surfactant is in equilibrium with an w/ o microemulsion in the upper phase. Finally, Winsor type III structures equal a three-phase system consisting of a surfactant poor water phase, a bicontinuous middle phase, and an upper almost pure oil phase. 

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