Microemulsions Using Scattering Techniques

Two scattering methods, namely time-average (static) Hght scattering and dynamic (quasi-elastic) light scattering (also referred to as photon correlation spectroscopy PCS), will be discussed in the following sections. 

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Characterisation of Microemulsions Using Scattering Techniques 327 where K is the scattering vector,... 

The behavior of water in oil microemulsions has been studied using different techniques light scattering, electrical conductivity, viscosity, transient electrical birefringence, ultrasonic absorption. All these experiments lead us to propose a picture of the microemulsions structure which assignes an important role to the fluidity of the interfacial region. 

Interactions in microemulsions have been studied using light scattering techniques. In water in oil systems, hard sphere interactions are dominant. The remaining interactions are usually attractive and are of the Van der Waals type. The case of oil in water microemulsion is less known. In those systems, interactions seem to be of a quite different kind. Entropic forces are thought to be important in those media, as will be shown by the study of a simplified system. 

Scattering techniques provide the most definite proof of micellar aggregation. Zielinski et aL (34) employed SANS to study the droplet structures in these systems. Conductivity measurements (35) and SANS (36) were also used to study droplet interactions at high volume fraction in w/c microemulsions formed with a PFPE-COO NH4 surfactant (MW = 672). Scattering data were successfully fitted by Schultz distribution of polydisperse spheres (see footnote 37). A range of PFPE-COO NH/ surfactants were also shown to form w/c emulsions consisting of equal amount of CO2 and brine (38-40). 

Turning back to the lowest interfacial tensions, and in order to fully confirm the critical behavior, the correlation length in the bulk microemulsion was measured by using elastic and inelastic bulk light scattering technique. diverges as... 

After the initial DLS studies were complete, it became apparent that the very strong interdroplet attractive interactions in near-critical and supercritical fluids limited the standard DLS technique to systems of higher dilution or to high fluid densities. Thus, small-angle scattering techniques were later used to better resolve the full dimensional scale range of these microemulsions over a wider range of conditions. 

On the other hand, once an enzyme and its affinity to the interface are well characterized, this enzyme can be a very sensitive probe for tiny changes in the composition of microemnlsion. There are not many other techniques that have such a high sensitivity. Thermodynamic experiments such as calorimetry are also very sensitive, bnt they do not give a detailed insight into structures. By contrast, scattering techniques and NMR yield more detailed pictures, but they are less sensitive. Therefore, enzyme reactions can be a useful complement for the investigation of microemulsion structures. This is the topic of this chapter. 

A beautiful and elegant example of the intricacies of surface science is the formation of transparent, thermodynamically stable microemulsions. Discovered about 50 years ago by Winsor [76] and characterized by Schulman [77, 78], microemulsions display a variety of useful and interesting properties that have generated much interest in the past decade. Early formulations, still under study today, involve the use of a long-chain alcohol as a cosurfactant to stabilize oil droplets 10-50 nm in diameter. Although transparent to the naked eye, microemulsions are readily characterized by a variety of scattering, microscopic, and spectroscopic techniques, described below. 

Small-angle neutron scattering (SANS) can be applied to food systems to obtain information on intra- and inter-particle structure, on a length scale of typically 10-1000 A. The systems studied are usually disordered, and so only a limited number of parameters can be determined. Some model systems (e.g., certain microemulsions) are characterized by only a limited number of parameters, and so SANS can describe them fully without complementary techniques. Food systems, however, are often disordered, polydisperse and complex. For these systems, SANS is rarely used alone. Instead, it is used to study systems that have already been well characterized by other methods, viz., light scattering, electron microscopy, NMR, fluorescence, etc. SANS data can then be used to test alternative models, or to derive quantitative parameters for an existing qualitative model. 

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