Microemulsions optical transparency

In 1959, J. H. Schulman introduced the term microemulsion for transparent-solutions of a model four-component system [126]. Basically, microemulsions consist of water, an oily component, surfactant, and co-surfactant. A three phase diagram illustrating the area of existence of microemulsions is presented in Fig. 6 [24]. The phase equilibria, structures, applications, and chemical reactions of microemulsion have been reviewed by Sjoblom et al. [127]. In contrast to macroemulsions, microemulsions are optically transparent, isotropic, and thermodynamically stable [128, 129]. Microemulsions have been subject of various... 

The reason for this is that if a stable, optically transparent emulsion is to be obtained, the relationship between the amount of the oil phase and the surfactants has to be within a relatively narrow range. Microemulsion elec-trokinetic chromatography has been shown to be a highly applicable technique for the analysis of complex mixtures such as multicomponent formulations and drug-related impurities. This technique opens a new way to determine water-insoluble neutral species such as steroids, which are difficult to analyze by CE. It is therefore likely that the MEEKC method will be increasingly applied for pharmaceutical and biopharmaceutical analyses in coming years. 

Emulsions are two-phase systems formed from oil and water by the dispersion of one liquid (the internal phase) into the other (the external phase) and stabilized by at least one surfactant. Microemulsion, contrary to submicron emulsion (SME) or nanoemulsion, is a term used for a thermodynamically stable system characterized by a droplet size in the low nanorange (generally less than 30 nm). Microemulsions are also two-phase systems prepared from water, oil, and surfactant, but a cosurfactant is usually needed. These systems are prepared by a spontaneous process of self-emulsification with no input of external energy. Microemulsions are better described by the bicontinuous model consisting of a system in which water and oil are separated by an interfacial layer with significantly increased interface area. Consequently, more surfactant is needed for the preparation of microemulsion (around 10% compared with 0.1% for emulsions). Therefore, the nonionic-surfactants are preferred over the more toxic ionic surfactants. Cosurfactants in microemulsions are required to achieve very low interfacial tensions that allow self-emulsification and thermodynamic stability. Moreover, cosurfactants are essential for lowering the rigidity and the viscosity of the interfacial film and are responsible for the optical transparency of microemulsions [136]. 

A microemulsion, Fig. 1, has a similar organization to that characteristic of a micelle but employs, rather than one, multiple surfactant components, allowing for introduction of other additives into the hydrophobic core [11], As with micelles, microemulsions are optically transparent and can be easily studied by standard spectroscopic methods. One important use of such microemulsions is in the photoinduced initiation of polymerization of monomers with low water solubility many such reactions involve a mechanism occurring through photoinduced interfacial electron transfer. 

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. 

As common emulsions described above, micro-emulsions also exist as oil/water and water/oil emulsions. Unlike macro-emulsions, micro-emulsions are optically transparent and usually thermodynamically stable they do not tend to separate into phases. The diameter of microemulsion droplets is of the order of 10-100 nm, less than the wave length of visible light. 

Recently it was proposed that PEMLC electrocatalysts may also be prepared by water-in-oil microemulsions. These are optically transparent, isotropic, and thermodynamically stable dispersions of two nonmiscible liquids. The method of particle preparation consists of mixing two microemulsions carrying appropriate reactants (metal salt + reducing agent), to obtain the desired particles. The reaction takes place during the collision of water droplets, and the size of the particles is controlled by the size of the droplets. Readers are referred to the early work of Boutonnet et al. [149], the review paper of Capek [150] and refs. [128,151], and 152 for fuel cell apphcations. The carbonyl route has the ability to control the stoichiometry between bimetallic nanoparticles, but also the particle size. The reader is referred to review papers for more details [106,107]. Other methods, including sonochemical and radiation-chemical, have been used successfully for the preparation of fuel cell catalysts (see, e.g., review articles 100 and 153). 

As compressed carbon dioxide is a nonpolar molecule with weak van der Waals forces (low polarizability per volume), it is a relatively weak solvent [1], Thus, many interesting separations and chemical reactions involving insoluble substances in CO2 can be expected to take place in heterogeneous systems, for example, microemulsions, emulsions, latexes and suspensions. Microemulsion droplets 2-10 nm in diameter are optically transparent and thermodynamically stable, whereas kinetically stable emulsions and latexes in the range from 200 nm to 10 pm are opaque and thermodynamically unstable. 

These AOT microemulsions are characterized by a high surfactant to monomer ratio, (2.5-3). If the amount of AOT is too low, the optically transparent microemulsions evolve towards turbid and unstable latexes during polymerization, due to a shift in the emulsion region of the phase diagram [55,56]. However, it should be noted that AM plays the role of a cosurfactant in these systems, owing to its surface-active properties, thus leading to an increase in the micellar stabilization capacities [48]. 

A water-in-oil microemulsion is a thermodynamically stable, optically transparent dispersion of two immiscible liquids stabilized by a surfactant. The important properties are... 

Figure 3 represents the percentage of surfactant(s) required for the formation of an AM-NaAMPS microemulsion as a function of the HLB number for different compositions of the monomer feed [30]. The curves delineate the transition between a turbid emulsion and an optically transparent microemulsion. The transition is sharp and can be easily detected by turbidimetry or visually. It can be seen that microemulsions are found in an HLB domain ranging between 8 and 11. The curves exhibit a minimum for an optimum HLB value, which increases as the content of ionic monomer in the feed increases. Note also the low surfactant concentration needed for the formation of clear systems (5.5% < min < 7.5%) in spite of the large proportions of monomers incorporated ( 22%). 

The main difficulty encountered by most of the authors and one that precludes the use of higher monomer concentrations lies in retaining the optical transparency and stability of the microemulsions upon polymerization. In addition to entropic factors contributing to the destabilization of microemulsions during polymerization, the compatibility between polymer and cosurfactant also influences the system [64]. This is especially true when styrene is polymerized within O/W microemulsions that contain an alcohol because the latter is not... 

The term microemulsion indicates an optically transparent, thermodynamically stable, isotropic dispersion of nanometric-sized droplets of one liquid in another (immiscible) liquid, stabilized by interfacial layers of surfactant molecules. Note that there is no generally agreed extent of solubilization when a micellar solution can be said to have transformed into a microemulsion [99]. However, in case of water solubilization, Pileni [100] indicates that when the [water]/[surfactant] molar ratio (= w) exceeds a value of 15, we have a microemulsion, below which the term reverse micelle is preferred. In this text, this differentiation has not been strictly adhered to instead, a general term reverse microemulsion has often been used as a matter of convenience. 

To fix the labile structures of microemulsions with a view to useful applications, notably in photochemistry (microemulsions are optically transparent). 

In contrast to emulsions, which are unstable macrodisperse systems (1-10 pm in droplet diameter), microemulsions are homogeneous, optically transparent, thermodynamically stable systems that can be formed only in specific ranges of temperature, pressure, and composition. They consist of droplets of water tens of nanometers in size dispersed within an immiscible organic (oil) phase [inverse micelles, or water-in-oil (W/0) microemulsions] or vice versa, oil pools dispersed within an aqueous phase [direct micelles, or oil-in water (OAV) microemulsions]. The droplets are encased in a surfactant shell as in emulsions or, more frequently, in a shell consisting of a suitable surfactant and a cosurfactant (usually an alcohol) and are thus stabilized. 

Besides biocatalytic applications, it is interesting to note that nanostructures within a microemulsion are recognized as models of biological structures that facilitate the investigation of protein/enzyme structure—activity relationship under conditions that mimic biological environments in optically transparent solutions. 

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