Light scattering, microemulsions stabilized

By dynamic light scattering it was found that, in surfactant stabilized dispersions of nonaqueous polar solvents (glycerol, ethylene glycol, formamide) in iso-octane, the interactions between reversed micelles are more attractive than the ones observed in w/o microemulsions, Evidence of intermicellar clusters was obtained in all of these systems [262], Attractive intermicellar interactions become larger by increasing the urea concentration in water/AOT/ -hexane microemulsions at/ = 10 [263],... 

Light scattering measurements and theoretical treatment strongly support the idea that attractive interactions between inverse micelles play an important role in the stability of oil rich microemulsions. In the system containing pentanol, attractions between (i)/o micelles can be sufficient to give rise to a phase separation between two microemulsion phases. 

The problem of particle nucleation was first addressed in the 1980s by Candau and coworkers [14,104] for the case of water-in-oil microemulsions. Their studies concerned the polymerization of acrylamide inside water-swollen micelles stabilized by AOT and dispersed in toluene. A thorough investigation of the structures prior to and after polymerization by elastic and quasi-elastic light scattering (QELS), viscomelry, and ultracentrifugation experiments yielded two key experimental results ... 

FIGURE 5. Light scattering curves show micellization first at 15% water per weight in a W/0 microemulsion with 50% decane, stabilized by potassium oleate and pentanol (Sjoblom, 1978), (30% counted on water, surfactant and cosurfactant). 

AN EXPERIMENTAL INVESTIGATION OF STABILITY OF MICROEMULSIONS AND POLYDISPERSITY EFFECTS USING LIGHT-SCATTERING SPECTROSCOPY AND SMALL ANGLE X-RAY SCATTERING... 

In most cases, microemulsions are easily differentiated from emulsions by their ease of formation, stability and translucent nature. Differentiation is most difficult in the case of optimized microemulsion formulations with very efficient surfactants (10, 11). Such microemulsions contain only 1 to 3 wt% surfactant, and qualitatively look like opaque emulsions due to strong light scattering. However, careful study of stability and reproducibility by using short-optical-path vials, and an understanding of the phase behaviour (see below), allows such concentrated microemulsions to be distinguished from standard emulsions. 

Microemulsions are thermodynamically stable systems. Oil-in-water (0/W) microemulsions are mixtures of monomer(s), water, surfactant, and, in some cases, cosurfactant. The cosurfactant is a surface-active compound that, in combination with the surfactant, reduces the interfacial tension between the monomer and the aqueous phase to very low values, ensuring the thermodynamic stability of the microemulsion. Alcohols are often used as cosurfactants. The low interfacial tension results in a frequent fluctuation in size and shape of the microemulsion droplets. In water-in-oil (W/0) microemulsions, a mixture of water-soluble monomers and water are dispersed in an organic solvent with the help of a surfactant. The use of a cosurfactant is not needed often because the monomers are surface active. The amount of surfactant required in microemulsion polymerization (>10wt%) is substantially higher than that used in emulsion polymerization. The droplet (swollen micelle) size of the both 0/W and W/0 microemulsions is in the range of 5-20 nm in diameter. Since these small droplets only weakly scatter light, the microemulsions are transparent. Bicontinuous microemulsions are sometimes formed using blends of nonionic surfactants [100]. Microemulsion polymerization has been reviewed [101]. 

Microemulsions, like micelles, are considered to be lyophilic, stable, colloidal dispersions. In some systems the addition of a fourth component, a co-surfactant, to an oil/water/surfactant system can cause the interfacial tension to drop to near-zero values, easily on the order of 10-3 - 10-4 mN/m, allowing spontaneous or nearly spontaneous emulsification to very small drop sizes, typically about 10-100 nm, or smaller [223]. The droplets can be so small that they scatter little light, so the emulsions appear to be transparent. Unlike coarse emulsions, microemulsions are thought to be thermodynamically stable they do not break on standing or centrifuging. The thermodynamic stability is frequently attributed to a combination of ultra-low interfacial tensions, interfacial turbulence, and possibly transient negative interfacial tensions, but this remains an area of continued research [224,225],... 

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