Microemulsion Technique

Microemulsions are transparent, isotropic dispersions of oil and water (two immiscible liquids) stabilized by the presence of carefully adjusted emulsifier(s) (surfactant molecules that can be cationic, anionic, or neutral), which are located at the interfaces between oil and water domains [339] (Scheme 10).

Microemulsion

Microemulsion

Surfactant molecules

Scheme 10.

Surfactant molecules

Scheme 10.

This systems are clear and thermodynamically stable, and they are prized for their stability and small particle size. The use of microemulsion technique in the synthesis of nano-particles is based upon the use of two nonmiscible phases homogenized with the help of surface-active agents, ligands, and/or polymers, for assisting the growth of nanocrystals in small water droplets homogeneously entrapped in a hydrocarbon solvent. The most common examples are the water-in-oil (W/O) and the oil-in-water (O/W) micro-emulsions, as shown in Scheme 10. The important properties are governed mainly by the water-to-surfactant molar ratio. Typically, the droplets are spherical and have a small polydispersity, in terms of size. The radii of the drops are usually one to four times the thickness of the surfactant film.

The reverse micelle technique is suitable for tailoring small and uniform nanoparticles of discrete nanoscaled oxides or hydroxides with controlled size distribution (<10 nm) [244, 340-351]. The principal advantage of the method is that the concentration of metal-organic in the water droplets is very high, so that the reaction easily forms particles that are gelled throughout. This method has been used to prepare spherical submicrometer particles of titania, zirconia, and alumina [352]. However, a controlled formation of oxide materials is subject to the reactivity of the starting material with water and the reaction parameters, which actually determine the size and agglomeration effects in the final powder. As a consequence, the results can be different for similar precursors. For example, in the microemulsion-based hydrolysis of titanium, alkoxides have yielded uncontrolled aggregation and flocculation [353]; however, TiO2 particles with diameters less than 10 nm were prepared by controlled hydrolysis of titanium iso-propoxide in reverse micelles of sodium bis(2-ethylhexyl)sulfosuccinate-hexane-water [354]. Similarly, transition metal aluminum heterometal alkoxides of the general formula MAl2(OR)8 have been successfully used to perform a size-selective synthesis of nanocrystalline MAl2O4 spinels (Scheme 11).

Scheme 11.

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