Microemulsions soft surfactant systems

It is generally accepted that the soft-core RMs contain amounts of water equal to or less than hydration of water of the polar part of the surfactant molecules, whereas in microemulsions the water properties are close to those of the bulk water (Fendler, 1984). At relatively small water to surfactant ratios (Wo < 5), all water molecules are tightly bound to the surfactant headgroups at the soft-core reverse micelles. These water molecules have high viscosities, low mobilities, polarities which are similar to hydrocarbons, and altered pHs. The solubilization properties of these two systems should clearly be different (El Seoud, 1984). The advantage of the RMs is their thermodynamic stability and the very small scale of the microstructure 1 to 20 nm. The radii of the emulsion droplets are typically 100 nm (Fendler, 1984 El Seoud, 1984). 

A related field of research where NMR investigations have yielded a large variety of information are soft colloidal volume systems, so-called association colloids, composed of amphiphilics (surfactants, lipids) or copolymers, e.g. block-copolymers. Structures such as micellar solutions, microemulsions, or liquid crystalline phases possess internal interfaces where the arrangement of molecules can be governed by similar principles as at a solid/liquid interface. NMR methods have very successfully been employed in this area, as has been reviewed [8, 9], and some ideas and approaches can be transferred to liquid interfacial layers in colloidal particle dispersions. 

A well-studied model system for the case of soft charged interfaces is that of the AOT surfactant, having two branched short chains, in the presence of oil and water. Solubilization in water-in-oil microemulsions has been studied by Zemb and co-workers (45) in the case of the water-oil-AOT system. The topology of these microemulsions is dominated by geometrical constraints as the interfacial area, cr = 0.6 nm /molecule, of the surfactant is invariant with composition within experimental uncertainty. If partial molar volume of water variations are neglected, the radius of the droplets is imposed by the composition of the microemulsions. 

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