Microemulsions scattering techniques

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. 

Hellweg, Th. and Langevin, D. (1999) The dynamics in dodecane/C10E5/water microemulsions determined by time resolved scattering techniques. Physica A, 264, 370-387. 

Hellweg, T., Brfilet, A. and Sottmann, T. (2000) Dynamics in an oil-continuous droplet microemulsions as seen by quasielastic scattering techniques. Phys. Chem. Chem. Phys., 2, 5168-5174. 

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). 

As mentioned above, the strength of the NMR relaxation technique in comparison to other (say, scattering) techniques for studying micelles and microemulsions is related to the fact that rotational dynamics are monitored. This has the advantage that the dynamics are (1) sensitive to small size variations and (2) essentially independent of interactions. In an extensive study of a three-component microemulsion system composed of the nonionic surfactant pentaethylene oxide dodecyl ether, water, and decane oil-swollen micelles dispersed in water were investigated over a large concentration range [58]. 

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... 

Table 2 Structure of Microemulsions in Compressible Fluids from Scattering Techniques Including DLS, SAXS, and SANS...

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. 

Compared to widely used dynamic light scattering, the electrochemical approach provides a faster and less expensive tool for characterizing microemulsions. Electrochemical techniques do not require any prior knowledge of physical properties except viscosity and are also applicable to opaque systems. However, caution is recommended in interpreting electrochemical diffusion coefficients, as discussed in the following section. 

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