Microemulsions quaternary systems

The phase behavior observed in the quaternary systems A and B is also evidenced in ternary systems. Figure 4 shows the phase diagrams for systems made of AOT-water and two different oils. The phase diagram with decane was established by Assih (14) and that with isooctane has been established in our laboratory. At 25°C the isooctane system does not present a critical point and the inverse micellar phase is bounded by a two-phase domain where the inverse micellar phase is in equilibrium with a liquid crystalline phase, as for system B or system A when the W/S ratio is below 1.1. In the case of decane, a critical point has been evidenced by light scattering (15). Assih and al. have observed around the critical point a two-phase region where two microemulsions are in equilibrium. A three-phase equilibrium connects the liquid crystalline phase and this last region. 

Another polar solvent that has been used in SDS-stabilized microemulsions is glycerol. Hexanol or decanol have been used as cosurfactants and systems both with and without oil have been studied. The ternary system with hexanol as cosurfactant was examined with SANS and NMR self-diffusion measurements by two different groups and both found the microemulsions to be structureless solutions [130,131], Similar behavior was found from a self-diffusion study of the quaternary systems with p-xylcnc or decane as the oil component [131,132],... 

As a brief conclusion it can be noted that many nonaqueous microemulsions reported do not seem to contain an organized structure, being simply molecular solutions. Since the degree of organization already in many aqueous microemulsion is low, in particular for quaternary systems containing ionic surfactant and cosurfactant, this is not really surprising. 

In the preceding sections, the phase behaviour of rather simple ternary and quaternary non-ionic microemulsions have been discussed. However, the first microemulsion found by Schulman more than 50 years ago was made of water, benzene, hexanol and the ionic-surfactant potassium oleate [1, 3]. Winsor also used the ionic-surfactant sodium decylsulphate and the co-surfactant octanol to micro-emulsify water/sodium sulphate and petrol ether [2], In the last 30 years, in-depth studies on ionic microemulsions have been carried out [7, 8, 65, 66]. It toned out that nearly all ionic surfactants which contain one single hydrocarbon chain are too hydrophilic to build up microemulsions. Such systems can only be driven through the phase inversion if an electrolyte and a co-surfactant is added to the mixture (see below and Fig. 1.11). 

Figure 1.11 Section through the phase tetrahedron of the pseudo-quaternary system E O/NaCI-n-decane-sodium dodecyl sulphate (SDS)-1-butanol (C4E0) at tf> = 0.58, 8 = 0.10 and T = 20°C [26]. Note that the pseudo-quaternary ionic system can be driven through phase inversion by adding C4E0 as was the case for the quaternary alkylpolyglucoside microemulsions. (From Ref. [26], reprinted with...

Figure 1.15 Water/oil interfacial tension crab (plotted on log-scale) as function of the relevant tuning parameter, (a) Variation of crab with temperature T, exemplarily shown for the water-n-octane-C- oE4 system [17]. (b) Variation of crab with the composition of the amphiphilic film 8yi in the quaternary system hbO-n-octane-fS-CsG-i-CsEo at T = 25°C [90]. Both systems show that the water/oil interfacial tension runs through a distinct minimum in the middle of the three-phase region. The full line is calculated considering the bending energy difference between a curved amphiphilic film in the microemulsion and the flat film of the macroscopic interface [96].

Penders, M.H.G.M. and Strey, R. (1995) Phase behavior of the quaternary system H20/ -octane/C8E5/ -octanol Role ofthe alcohol in microemulsions./. Phys. Chem.,99,10313-10318. 

Very early, the Swedish school attempted to determine the extent and shape of the region of existence of microemulsions in quaternary systems [76-78]. By examination of sections of the phase diagram at several levels of oil, Friberg and coworkers established a direct connection between the microemulsion areas and the inverse micellar solutions described by Ekwall [1]. Thus, prior to describing the phase diagrams of the quaternary systems, those of ternary systems made of water, sodium dodecylsulfate (SDS), and an alcohol are first presented here. 

Also, for W/O microemulsions, the conductance usually increases with the water content of the system, which is customarily expressed as the molar ratio of water to surfactant, symbolized by u o or R. Naturally the situation is even worse for quaternary systems of any type (W/O or O/W) if the fourth component is an electrolyte. In this case, the birefringence experiment ( jump) is associated with a simultaneous temperature jump that complicates the interpretation of the data. Nevertheless, usefiil information has been obtained from such experiments. 

The phase behavior of ternary and quaternary systems of the type water-oil-surfactant-cosurfactant is affected strongly by the addition of other components. Therefore, it is questioned how the solubilization of soil during the use of microemulsions as cleaning media in the washing process influences their existence region in the phase diagram and their solubilization power. To test this effect, the temperature dependence of the phase behavior of samples 10-28 (see Table 2) after their use in model... 

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. 

For this reason, the solubilization capacity of the microemulsions formulated with IPM is lower than that observed with R (+)-LlM. The oil behavior can also be governed by the chain compatibility between oil and surfactants. The change in the solubilization capacity behavior of the two oils when the mixing ratio (w/w) of ethoxylated mono-di-glyceride increases to 3/1 or in the quaternary systems water/ethoxylated mono-di-glyceride/oil could be attributed to the better chain compatibility between the mixed surfactants chains and the IPM chain length. 

The power of the method will be illustrated by two applications. First, we investigate the influence of the alcohol content on the quaternary system for which the formation of multilamellar vesicles has been discovered by Hofftnaim et al. [10]. Second, diffusion in the water-in-oil microemulsion and in the bicontinuous cubic gel phase of the AOT/octanol/ water system is investigated. 

Microemulsions, unlike classical emulsions, are usually transparent and thermodynamically stable systems requiring a significant amount of surfactant. The addition of a cosurfactant, although not always required, often appears to be necessary so that microemulsions in their simplest compositions are ternary or quaternary systems. Microemulsions are thus some sort of swollen micellar systems in which the core of the droplets accommodates a more or less important amount of dispersed oil (or water in the case of reverse micelles). Research concerning such systems, which were first mentioned by Hoar and Schulman, literally exploded in the late... 

In the recent years, more and more important theoretical and practical importance gains pol5mierizations carried out in mini-and in microemulsions. The latter are defined as the most complex quaternary systems composed of a monomer, a phase diffuser, a surface-active compound (surfactant) and a cosurfactant. Depending on the degree of hydrophobic-ity or hydrophilicity of monomer used water or an appropriate hydrocarbon can be used as a phase diffuser. 

We have discussed [2,8] a similar (but rather unusual in the case of microemulsions) behavior of some quaternary systems based on sugar esters,... 

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. 

Use of a microemulsion to overcome reagent incompatibility can be seen as an alternative to the more conventional approach of carrying out the reaction in a two-phase system with the use of a phase transfer catalyst. The latter is usually either a quaternary ammonium salt or a crown ether. There are several examples in the literature of comparisons between the microemulsion concept and phase transfer catalysis. The topic has also recently been reviewed [46]. 

Schomacker compared the use of nonionic microemulsions with phase transfer catalysis for several different types of organic reactions and concluded that the former was more laborious since the pseudo-ternary phase diagram of the system had to be determined and the reaction temperature needed to be carefully monitored [13,29]. The main advantage of the microemulsion route for industrial use is related to the ecotoxicity of the effluent. Whereas nonionic surfactants are considered relatively harmless, quaternary ammonium compounds exhibit considerable fish toxicity. 

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