Winsor III system

Fatty alcohol- (or alkyl-)ethoxylates, CoE, are considered to be better candidates for LLE based on their ability to induce rapid phase separation for Winsor II and III systems. (Winsor III systems consist of excess aqueous and organic phases, and a middle phase containing bicontinuous microemulsions.) However, C,E,-type surfactants alone cannot extract biomolecules, presumably because they have no net negative charge, in contrast to sorbitan esters [24,26,30,31]. But, when combined with an additional anionic surfactant such as AOT or sodium benzene dodecyl sulfonate (SDBS), or affinity surfactant, extraction readily occurs [30,31]. The second surfactant must be present beyond a minimum threshold value so that its interfacial concentration is sufficiently large to be seen by... 

Previous work has shown that binary surfactant systems containing Dowfax 8390 and the branched hydrophobic surfactant AOT can form Winsor III systems with both PCE and decane whereas DOWFAX 8390 by itself cannot (Wu et. al. 1999). This binary surfactant system was used in conjunction with hydrophobic octanoic acid to help with phase behavior and lessen the required concentration of CaCl2. Since this formulation is rather complicated, questions about field robustness arise. Thus, for the phase behavior studies presented here, we used the simple binary system of the nonionic TWEEN 80 and the branched hydrophobic AOT, and we optimized the NaCl concentration to give the Winsor Type III system. The lesser electrolyte concentration requirement for the binary TWEEN 80/ AOT system helps to decrease the potential for undesirable phase behavior such as surfactant precipitation, thereby increasing surfactant system robustness. 

At low temperatures an O/W microemulsion (0/Wm) is formed which is in equilibrium with an excess oil phase. This condition is termed a Winsor I system. At high temperatures the headgroup requires less space on the interface and, thus, a negative curvature can result. A phase inversion o ccurs and a W/O microemulsion (W/Om) is formed which is in equilibrium with an excess water phase. This situation is termed a Winsor II system. At intermediate temperatures three phases - a water phase, a microemulsion D and an oil phase - are in equilibrium. This is called a Winsor III system. Here the curvature of the interfaces is more or less zero. Hence, the interfacial tension is minimum as depicted in Figure 3.24 (right) for the system C12E5, tetradecane and water. 

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]). 

An attempt was also made to accelerate the same reaction performed in a microemulsion based on water, nonionic surfactant and hydrocarbon oil [9]. The reaction was performed in a Winsor III system and the same Q salt, tetra-butyl ammonium hydrogen sulfate, was added to the formulation. In this case the addition of the phase transfer catalyst gave only a marginal increase in reaction rate. Similar results have been reported for an alkylation reaction performed in different types of micellar media [52]. The addition of a Q salt gave no effect for a system based on cationic surfactant, a marginal increase in rate for a system based on nonionic surfactant and a substantial effect when an anionic... 

Whereas the surfactant will always reside in the microemulsion phase, the product is likely to partition into an excess oil phase if it is an apolar substance and into an excess water phase if it is a polar compound. The principle is illustrated in Fig. 5.14 for hydrolysis of a lipophilic ester in a Winsor I system (an oil-in-water microemulsion in equilibrium with excess oil) followed by transition into a Winsor III system [60]. The ester partitions between the excess oil phase and the oil droplets and the hydroxyl ions reside in the continuous water domain of the microemulsion. The reaction takes place at the interface. After completed reaction, acid is added to protonate the alkanoate formed and the temperature is raised so that a Winsor I to Winsor III transition occurs. The lipophilic... 

Figure 5.14 Ester hydrolysis in a Winsor I system followed by a heat-induced transition to a Winsor III system.

All types of micro emulsions were obtained in salinity scans with mixtures of Aerosol MA (sodium dihexyl sulphosuccinate) and twin-tailed (Guerbet and Exxon type) alcohol ethoxy and propoxy sulphates for perchloroethylene (PCE), carbon tetrachloride, 1,2-dichlorobenzene and trichloroethylene [59] at 25°C. At lower temperatures, however, stable macro emulsions are formed. Chloroform, 1,2-dichloroethane and other chlorinated hydrocarbons were found to be too polar for those anionic surfactants. Extremely hydrophilic and temperature-insensitive surfactants are necessary for effective solubilisation of chlorinated hydrocarbons yielding Winsor III systems. N-methyl-N-D-glucalkaneamide surfactants showed good performance for DNAPL solubilisation even at 16°C [56]. 

Whereas Winsor III systems exhibit ultra-low interfacial tensions between the three phases and also very high solubilisation capacity, Winsor I systems have higher interfacial tensions and much lower solubilising power. At the transition between the two types of microemulsion systems, an intermediate behaviour can be found which is called supersolubilisation [47,70]. The uptake of oils into surfactant aggregates is usually enhanced by one to two orders of magnitude compared to effective micellar systems, but interfacial tension reduction is still moderate. The transition point can be adjusted by varying the salinity or organic components. 

WII Winsor II system (W/0 pern in equilibrium with excess water) WIN Winsor III system (pern in equilibrium with excess oil and water) and WIV Winsor IV (single-phase pern). 

Under well-chosen conditions (the so-called optimal formulation), the interfacial tension T ater/oU between the aqueous and the oil phases is ultralow and, as a consequence, a Winsor III system appears spontaneously. In that case, it becomes a very effective reaction medium with the following features ... 

Microemulsion research has since its inception been stimulated by the great potential for practical applications. In particular, considerable research interest has been invested in the possibility of using microemulsions for enhanced oil recovery (EOR). It was observed that surfactant formulations forming three-phase microemulsion systems, often termed Winsor III systems [29], in the oil well could increase the oil yield considerably. Important contributions to the understanding of the mechanisms involved were made by Shah and Hamlin [30] and the Austin group led by Schechter and Wade (see Bourrel et al. [31]). 

If more and more surfactant is added to a Winsor III system, the surfactant-rich phase swells at the expense of the excess oil and water phases. From a certain point, a single surfactant-rich phase is found. Upon increasing the amount of surfactant even further, a first-order transition to a lamellar phase may be observed. In a special case, it has been shown that the coexistence region between the (bicontinuous) microemulsion phase and a lamellar phase was extended into the region where the surfactant-rich phase coexists with excess oil and water, leading to a four-phase equilibrium water-lamellar phase-microemulsion phase-oil [58]. In Ref. 46, even a three-phase equilibrium, water-lamellar phase-oil, was observed, the bicontinuous microemulsion phase apparently being absent. 

The largest interfacial tension in the Winsor III systems (bicontinuous microemulsions in equilibrium with both excess oil and water) is also equal to the interfacial tension between oil and water in the presence of a saturated surfactant monolayer, i.e.. 

Another workup approach has been to use the inherent phase behavior of oil-water-surfactant systems to separate product from remaining reactants and from surfactant. A Winsor III system made with a branched-tail phosphonate surfactant was used as reaction medium for lipase-catalyzed hydrolysis of trimyristin. The enzyme resided almost exclusively in the middle-phase microemulsion together with the surfactant. The products formed, 2-myristoylglycerol and sodium myristate, partitioned into the excess hydrocarbon and water phases, respectively, and could easily be recovered [129]. A similar procedure was used for cholesterol oxidation using cholesterol oxidase as catalyst [130]. 

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). 

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