Microemulsion phase

Nevertheless, possibiUties for confusion abound. From the definitions of microemulsions and macroemulsions and from Figure 1, it immediately follows that in many macroemulsions one of the two or three phases is a microemulsion. Until recentiy (49), it was thought that all nonmultiple emulsions were either oil-in-water (O/W) or water-in-oil (W/O). However, the phase diagram of Figure 1 makes clear that there are six nonmultiple, two-phase morphologies, of which four contain a microemulsion phase. These six two-phase morphologies are oleic-in-aqueous (OL/AQ, or O/W) and aqueous-in-oleic (AQ/OL, or W/O), but also, oleic-in-microemulsion (OL/MI), microemulsion-in-oleic (MI/OL), aqueous-in-microemulsion (AQ/MI), and microemulsion-in-aqueous (MI/AQ) (49). 

FIG. 12 The behavior of the internal energy U (per site), heat capacity Cy (per site), the average Euler characteristic (x) and its variance (x") — (x) close to the transition line and at the transition to the lamellar phase for/o = 0. The changes are small at the transition and the transition is very weakly first-order. The weakness of the transition is related to the proliferation of the wormhole passages, which make the lamellar phase locally very similar to the microemulsion phase (Fig. 13). Note also that the values of the energy and heat capacity are not very much different from their values (i.e., 0.5 per site) in the Gaussian approximation of the model [47]. (After Ref. 49.)... 

MEEKC is a CE mode similar to MEKC, based on the partitioning of compounds between an aqueous and a microemulsion phase. The buffer solution consists of an aqueous solution containing nanometer-sized oil droplets as a pseudo-stationary phase. The most widely used microemulsion is made up of heptane as a water-immiscible solvent, SDS as a surfactant and 1-butanol as a cosurfactant. Surfactants and cosurfactants act as stabilizers at the surface of the droplet. 

J Sjoblom, R Lindberg, SE Friberg. Microemulsions-phase equilibria characterization, structures, applications and chemical reactions, Adv Colloid Interf Sci 65 125-287, 1996. 

Wadle, A., Forster, Th. and von Rybinski, W. (1993) Influence of the microemulsion phase structure on the phase inversion temperature emulsification of polar oils. Colloids and Surfaces A Physicochemical and Engineering Aspects, 76, 51-57. 

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. 

Figure 7. Topological fluctuations of the lamellar phase at different points of the phase diagram, (a) Single fusion between the lamellae by a passage (this configuration is close to the topological disorder line), (b) Configuration close to the transition to the disordered microemulsion phase the Euler characteristic is large and negative.

The Landau-Ginzburg model of a ternary mixture of oil, water, and surfactant studied here was proposed by Teubner and Strey [115] on the basis of the scattering peak in the microemulsion phase. Later it was refined by Gompper and Schick [116]. Its application to various bulk and surface phenomena is described in detail in Ref. 117. 

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. 

A. Wadle, T. Forster, and W. Von Rybinski Influence of the Microemulsion Phase Structure on the Phase Inversion Temperature Emulsiflcation of Polar Oils. Colloid and Surfaces A Physicochem. Eng. Aspects 76, 51 (1993). 

Graciaa A, Lachaise J, Sayous JG, Grenier P, Yiv S, Schechter RS, Wade WH (1983) The partitioning of complex surfactant mixtiu es between oil-water-microemulsion phases at high surfactant concentration. J Colloid Interface Sci 93 474-486... 

Upadhyaya A, Acosta EJ, Scamehorn JF, Sabatini DA (2006) Microemulsion phase behavior of anionic-cationic surfactant mixtures Effet of tail branching. J Surfact Deterg 9 169-179... 

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. 

Two main microemulsion microstructures have been identified droplet and biconti-nuous microemulsions (54-58). In the droplet type, the microemulsion phase consists of solubilized micelles reverse micelles for w/o systems and normal micelles for the o/w counterparts. In w/o microemulsions, spherical water drops are coated by a monomolecular film of surfactant, while in w/o microemulsions, the dispersed phase is oil. In contrast, bicontinuous microemulsions occur as a continuous network of aqueous domains enmeshed in a continuous network of oil, with the surfactant molecules occupying the oil/water boundaries. Microemulsion-based materials synthesis relies on the availability of surfactant/oil/aqueous phase formulations that give stable microemulsions (54-58). As can be seen from Table 2.2.1, a variety of surfactants have been used, as further detailed in Table 2.2.2 (16). Also, various oils have been utilized, including straight-chain alkanes (e.g., n-decane, /(-hexane),... 

Microemulsions are dynamic systems in which droplets continually collide, coalesce, and reform in the nanosecond to millisecond time scale. These droplet interactions result in a continuous exchange of solubilizates. The composition of the microemulsion phase determines the exchange rate through its effect on the elasticity of the surfactant film surrounding the aqueous microdomains. Compared with nonionic surfactant-based microemulsions, AOT reverse micelles have a more rigid... 

FIGURE 7.3 Hypothetical three-phase diagram of surfactant/oil/water composition illustrating the rapid change in the constitution as a microemulsion phase is diluted in water. 

The review by Attwood and Florence (1998) is valuable because it provides a comprehensive survey of the earlier pharmaceutical literature. From this we can learn that microemulsions are formed spontaneously from fixed compositions of surfactant(s), oil, and water but these compositions change with temperature. As a stable system is diluted progressively with, for example, water, the microemulsion phase will spontaneously revert to other phases on the diagram, including unstable emulsions or a solution. This becomes relevant when considering what happens to the system when it is administered. No matter how it is administered the microemulsion phase will be diluted, most likely with water in some form or another, and revert to some other composition in the body. 

Activity and stability are often comparable to values in aqueous media. Many substrates which cannot be made to react in water or in pure organic solvents such as hexane owing to lack of solubility can be brought to reaction in microemulsions. Whereas enzyme structure and mechanism do not seem to change upon transition from water to the microemulsion phase (Bommarius, 1995), partitioning effects often are very important. Besides an enhanced or diminished concentration of substrates in the vidnity of microemulsion droplets and thus of enzyme molecules, the effective pH values in the water pool of the droplets can be shifted in the presence of charged surfactants. Frequently, observed acceleration or deceleration effects on enzyme reactions can be explained with such partitioning effects (Jobe, 1989). 

A correlation of the detergency performance and the equilibrium phase behavior of such ternary systems is expected, based on the results presented by Miller et al. (3,6). The phase behavior of surfactant - oil - water (brine) systems, particularly with regard to the formation of so-called "middle" or "microemulsion" phases, has been shown by Kahlweit et al. (7,8) to be understandable in teims of the... 

Figure 3.28 Illustrative section from the phase prism of a mixture of oil, water, and surfactant. This section is for constant surfactant concentration (T is temperature). The section shows a middle-phase microemulsion phase existing together with oil (upper) and water (lower) phases. The surfactant is partitioned among all of the phases. The cross-hatching shows how the microemulsion can be O/W (to the left), or W/O (to the right), or bicontinuous (centre). From Schwuger et al. [226]. Copyright 1995, American Chemical Society.

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