Winsor 1 systems

The main advantage to the Winsor system is its heuristic feature of treating all cohesive interactions in a two-phase surfactant system. However, to date only the simple form of Equation 22 has been exploited quantitatively (21, 23) as... 

Organic reactions in micro emulsions need not be performed in one-phase systems. It has been found that most reactions work well also in two-phase Winsor I or Winsor II systems, i.e. an oil-in-water microemulsion coexisting with excess oil or a water-in-oil microemulsion coexisting with excess water, respectively [7, 8]. A Winsor III system, i.e. a three-phase system in which a middle phase microemulsion coexists with both oil and water, has also been successfully used as reaction medium [9]. The transport of reactants from the excess oil or water phase to the microemulsion phase, where the reaction takes place, is evidently fast compared to the rate of the reaction. This is a practically important aspect on the use of micro emulsions as media for chemical reactions because it simplifies the formulation work. Formulating a Winsor I or Winsor II system is usually much easier than formulating a one-phase microemulsion of the whole reaction mixture. Winsor systems can also be of value to simplify the work-up process, in particular to separate the product from the surfactant, as will be discussed below in Sect. 2.4 (see also [6]). 

The use of Winsor systems as reaction media is not unique to organic synthesis. Winsor I and Winsor II systems, as well as three-phase systems of Winsor III type, have been successfully employed as media for a variety of enzymatic -reactions [7,10-12]. 

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 observation that the reaction runs approximately as fast in a Winsor system as in a one-phase microemulsion has later been seen also for another bimolecular substitution reaction [16]. The fact that a Winsor system can be used instead of a one-phase microemulsion is practically important. Formulation of a one-phase microemulsion is often a problem, particularly when one wants a high loading of reactants into the oil and water domains, and one may end up with various types of two-phase or three-phase systems. Evidently, such systems may be just as useful as reaction media, as long as one of the phases is a microemulsion. The excess phase (or phases) can be regarded as reservoirs for the reactant (or reactants) while the reaction occurs at the oil-water interface of the microemulsion phase. 

This brief survey begins in Sec. II with studies of the aggregation behavior of the anionic surfactant AOT (sodium bis-2-ethylhexyI sulfosuccinate) and of nonionic pol-y(ethylene oxide) alkyl ethers in supercritical fluid ethane and compressed liquid propane. One- and two-phase reverse micelle systems are formed in which the volume of the oil component greatly exceeds the volume of water. In Sec. Ill we continue with investigations into three-component systems of AOT, compressed liquid propane, and water. These microemulsion systems are of the classical Winsor type that contain water and oil in relatively equal amounts. We next examine the effect of the alkane carbon number of the oil on surfactant phase behavior in Sec. IV. Unusual reversals of phase behavior occur in alkanes lighter than hexane in both reverse micelle and Winsor systems. Unusual phase behavior, together with pressure-driven phase transitions, can be explained and modeled by a modest extension of existing theories of surfactant phase behavior. Finally, Sec. V describes efforts to create surfactants suitable for use in supercritical CO2, and applications of surfactants in supercritical fluids are covered in Sec. VI. 

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. 

Previous studies have shown that as the alkane carbon number (ACN) of the oil phase increases, it generally becomes a poorer solvent for the surfactant. In Winsor systems this effect leads to a 2-3-2 transition [36]. In one-phase AOT reverse micelle systems, water solubilization at the phase boundary, decreases as ACN increases [48]. None of these studies, however, used a solvent lighter than hexane. 

The trends in phase behavior as a function of oil-phase ACN indicate an optimum point in the neighborhood of pentane or hexane. This behavior is a consequence of the enthalpic and entropic components of the solvent-surfactant tail interaction [25,43]. Alkanes heavier than hexane are hindered in their ability to penetrate between the surfactant tails, which makes the combinatorial (chain length compatibility) effect the dominant one for these solvents. Alkanes lighter than hexane penetrate between surfactant tails very easily, but their enthalpic interaction with the tails is weak. At pentane and hexane, the combinabon of enthalpic and entropic effects is balanced. This balance favors the formation of the 2 configuration in Winsor systems. In reverse micelle systems, the optimum solvent-surfactant tail interaction stabilizes the reverse micelles against phase separation driven by micelle-micelle interactions. Also, there is a minimum in the attractive intermicellar dispersion interaction [13]. Therefore W reaches a maximum. 

Figure 6 Interfacial tension between oil and water in the presence of a saturated surfactant monolayer versus temperature. The surfactants are alkyl polyoxyethylene glycol ethers C12E5 with hexane, C10E4 with octane, and CsEi with decane. The vertical bars indicate the transition temperatures between the different Winsor systems. (Data from Ref. 50.)...

The lowest interfacial tensions in Winsor systems have a different origin than that of the largest ones they do not depend on the surfactant film properties and are due to the nearness of a critical point. Close to the boundaries Winsor I Winsor III, S = S and Winsor III -> Winsor II, S = Si, the corresponding excess phase becomes slightly turbid. This is reminiscent of the vicinity of a critical point. This particular type of critical point, where... 

In the SDS system, does not become large enough to determine the ratio p/v accurately. This ratio was determined by Kim et al. [58,59] in another Winsor system where y falls below 10 mN/m and increases above 100 nm they found v//ti =0.52 0.04. Criti-... 

Winsor systems are extremely useful for studying transfer processes from one phase to another. Due to the existence of well-defined liquid-liquid interfaces, they have been used to examine how solubilizates can be transferred in and transported by microemulsion droplets. ° ... 

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