Microemulsion micelle nanocavities generated by using AOT as surfactant

Microemulsion is a thermodynamically
stable single-phase system that consists of three components: water, oil and an
amphiphilic molecule, called surfactant. The surfactant molecule lowers the
interfacial tension between water and oil resulting in the formation of a
transparent solution. The water nanodroplets containing reagents, as a
nanoreactor, undergo rapid coalescence allowing for a mixing, precipitation
reaction and an aggregation processes for the synthesis of Magnetite nanoparticles
23 Water-in-oil (W/O) microemulsions (i.e. reverse micelle solutions) are
transparent, isotropic, thermodynamically stable liquid media. In these
systems, fine microdroplets of the aqueous phase are trapped within assemblies
of surfactant molecules dispersed in a continuous oil phase. The
surfactant-stabilized microcavities (typically in the range of 10 nm) provide a
confinement effect that limits particle nucleation, growth, and agglomeration.
W/Omicroemulsions have been shown to be an adequate, versatile, and simple
method to prepare nanosized particles and these are the characteristics that
could make this method useful for both in vivo and in vitro applications.24
Magnetite nanoparticles around 4 nm in
diameter have been prepared by the controlled hydrolysis with ammonium
hydroxide of FeCl2 and FeCl3 aqueous solutions within the reverse micelle
nanocavities generated by using AOT as surfactant and heptane as the continuous
oil phase 34. Santra et al. 35 reported a robust methodology for the synthesis
of both uncoated and silica-coated MNPs of ultrasmall (<5 nm) and a very uniform size distribution by water-in-oil microemulsion. They used three different nonionic surfactants (Triton X-100, Igepal CO-520 and Brij-97) for the preparation of microemulsions, and also used NH4OH and NaOH as base source. By mixing two identical water-in-oil microemulsions which one of them containing metal salts and the other containing the base source, microdroplets will continuously collide, coalesce, and break again and finally a precipitate forms in the micelles. By addition of solvents, such as acetone or ethanol, to the microemulsions, the precipitate can be extracted by filtering or centrifuging the mixture.