Winsor structures

FIGURE 2.2 Different forms of Winsor structures. (From Moulik, S.P. and Rakshit, A.K., J. Surf. Sci. TechnoL, 22,159, 2006. With permission.)... 

Surfactants employed for w/o-ME formation, listed in Table 1, are more lipophilic than those employed in aqueous systems, e.g., for micelles or oil-in-water emulsions, having a hydrophilic-lipophilic balance (HLB) value of around 8-11 [4-40]. The most commonly employed surfactant for w/o-ME formation is Aerosol-OT, or AOT [sodium bis(2-ethylhexyl) sulfosuccinate], containing an anionic sulfonate headgroup and two hydrocarbon tails. Common cationic surfactants, such as cetyl trimethyl ammonium bromide (CTAB) and trioctylmethyl ammonium bromide (TOMAC), have also fulfilled this purpose however, cosurfactants (e.g., fatty alcohols, such as 1-butanol or 1-octanol) must be added for a monophasic w/o-ME (Winsor IV) system to occur. Nonionic and mixed ionic-nonionic surfactant systems have received a great deal of attention recently because they are more biocompatible and they promote less inactivation of biomolecules compared to ionic surfactants. Surfactants with two or more hydrophobic tail groups of different lengths frequently form w/o-MEs more readily than one-tailed surfactants without the requirement of cosurfactant, perhaps because of their wedge-shaped molecular structure [17,41]. 

Fontell, K., X-ray Diffraetion by Liquid Crystals—Amphiphilie Systems. In Liquid Crystals and Plastic Crystals (G. Gray and P. Winsor, eds.), Vol. 2, Ellis Horwood, Chiehester, 1974. Luzzati, V., Mustaeehi, H., Skoulios, A., and Husson, F., La structure des colloides d associa-tion. I. Les phases liquide-eristalline des systemes amphiphile-eau, Acta Cryst., 13 660-677 (1960). 

The first of these to be discussed will be the Cohesive Energy Ratio, R concept (20), Using the concept of cohesive energy between molecules, Winsor recognized four structures. 

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. 

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 bis(2-ethylhexyl) sodium sulfosuccinate system was initially investigated because its structure of liquid crystalline solution phases and mechanism of solubilization with water had been reported by Rogers and Winsor (10). In our studies, we substituted methanol for water. Table I lists critical micelle concentrations for bis(2-ethylhexyl) sodium sulfosuccinate, triethylammonium linoleate and tetradecyldimethylammonium linoleate in methanol and 2-octanol at 25°C. Literature references for critical micelle concentrations in methanol are sparse, and it has even been suggested that in polar solvents such as ethanol, either micellization does not occur or, if it does, only to a small degree (4). The data of Table I show that micellization occurs in methanol at low concentrations. 

The aggregates discussed above are all anisodimensional, which is the reason for the anisotropic character of the mesophases. In some systems it has been possible to prove the existence of isotropic highly viscous phases of similar structure but which clearly consist of almost isodimensional aggregates. The exact structure of these phases is still the subject of discussion, as is also the case with the complex mesophases. The relation between the isotropic phases and globular proteins and plastic crystals of non-amphiphilic substances has been discussed by Gray and Winsor (5). 

Winsor (17) describes how electrical conductivity varies during addition of an alcohol to an aqueous micellar solution containing some solubilized oil. Conductivity initially decreases as mixed (and probably larger) micelles containing both surfactant and alcohol are formed. When liquid crystal (presumably having a lamellar structure) starts to appear in equilibrium with the micellar solution, conductivity decreases even faster. As more alcohol is added, the aqueous solution disappears, only liquid crystal is present, and the conductivity reaches a minimum. Addition of still more alcohol results in the appearance of an oil-continuous micellar solution and an increase in conductivity. Eventually all liquid crystal disappears, the increase in conductivity ceases, and conductivity... 

A microemulsion that has high oil and water content and is stable while in contact with either bulk oil or bulk water phases. This stability can be caused by a bi-continu-ous structure in which both oil and water phases are simultaneously continuous. In laboratory tube or bottle tests involving samples containing unemulsified oil and water, a middle-phase microemulsion tends to situate between the two phases. See also Winsor Type Emulsions. 

There is as yet no clear-cut relationship between the molecular structure of the mesogenic..unit and the type of mesophase it forms, but several generalizations can be made. Gray and Winsor have divided these factors into how the molecular structure (1) is conducive to liquid crystal formation, (2) affects the thermal stability of the mesophase, and (3) favors the occurrence of smectic versus nematic or cholesteric liquid crystals. 

The first reports on nonaqueous microemulsions, isotropic solutions containing a hydrophilic and a lipophilic component, stabilized by a surfactant, were made by Palit and McBain in 1946 [116] and by Winsor in 1948 [117]. They both used glycols as polar solvents. The microemulsion regions were only observed visually so no structural information could be obtained. 

Changes in temperature, concentration of surfactant, additives in the liquid phase, and structural groups in the surfactant may all cause change in the size, shape, and aggregation number of the micelle, with the structure varying from spherical through rod- or disklike to lamellar in shape (Winsor, 1968). 

Microemulsions are macroscopically isotropic mixtures of at least a hydrophilic, a hydrophobic and an amphiphilic component. Their thermodynamic stability and their nanostructure are two important characteristics that distinguish them from ordinary emulsions which are thermodynamically unstable. Microemulsions were first observed by Schulman [ 1 ] and Winsor [2] in the 1950s. While the former observed an optically transparent and thermodynamically stable mixture by adding alcohol, the latter induced a transition from a stable oil-rich to a stable water-rich mixture by varying the salinity. In 1959, Schulman et al. [3] introduced the term micro-emulsions for these mixtures which were later found to be nano-structured. 

Winsor, P.A. (1968) Binary and multicomponent solutions of amphiphilic compounds. Solubilization and the formation, structure and theoretical significance of liquid crystalline solutions. Chem. Rev., 68, 1. 

Winsor, P. A., Chem. Rev. (1968) 68, 1-40, "Binary and Multicomponent Solutions of Amphiphilic Compounds. Solubilization and the Formation, Structure, and Theoretical Significance of Liquid Crystalline Solutions."... 

The thermodynamic modeling of microemulsions has taken various lines and gave conflicting results in the period before the thermodynamic stability and microstructure were established. It was early realized that a maximal solubilization of oil and water simultaneously could be discussed in terms of a balance between hydrophilic and lipophilic interactions the surfactant (surfactant mixture) must be balanced. This can be expressed in terms of the HLB balance of Shinoda,Winsor s R value, and a critical packing parameter (or surfactant number), as introduced to microemulsions by Israelachvili et al. [37], Mitchell and Ninham [38], and others. The last has become very popular and useful for an understanding of surfactant aggregate structures in general. 

This chapter is intended to deal with the elucidation and characterization of structures occurring in monophasic Winsor IV equilibria. However, in many cases we use examples of binary amphiphilic systems whose isotropic solutions cannot, obviously, be considered microemulsions. This is because many of the microstructures found in three-component systems are simple extensions of their binary counterparts [16,27]. Furthermore, we included important results concerning liquid crystalline phases, as this information provides, we believe, the fundamental basis for more profound understanding of surfactant organization in multicomponent systems. 

Figure 5 indicates the different cases of phase behavior and self-assembly structures according to Winsor notation, which has been followed by many other researchers who rediscovered Winsor s work during the enhanced oil recovery research drive of the 1970s [20-22]. 

Figure 5 Phase behavior, R ratio, and types of structures according to Winsor notation. (From Ref. 11.)...

In some cases Winsor [11] reported that the single-phase region (which he called type IV phase behavior) could also contain liquid crystalline phases with lamellar or other structure. 

It is worth noting that this description assumes implicitly that the = 1 case is associated with a zero-curvature C layer, which could be provided either by a lamellar liquid crystal structure with alternating O and W flat layers or a zero curvature surface of the Schwartz type or as a transient and fluctuating combination of Si and S2 structures. It is now well recognized that middle-phase microemulsions, which are in equilibrium with both oil and water excess phases, exhibit bicontinuous structures as shown in Fig. 7 [27] that are not far from the transient mixture of Si and S2 swollen micelles predicted by Winsor. 

A ternary system composed of oil, water, and surfactant can form a wide variety of aggregated structures. Two characteristic compositions are frequently studied reverse micelle systems in which the amount of oil greatly exceeds the amount of water, and systems in which oil and water are present in relatively equal amounts (Winsor systems). Reverse micelle systems were discussed in the previous section this section is devoted to Winsor systems having an oil phase composed of a supercritical fluid or compressed liquid alkane. It should be noted, however, that these two types of systems merely represent two specific regions in the space of ternary oil-water-surfactant compositions, and both are subject to the same thermodynamic considerations. 

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