Microemulsion is a thermodynamicallystable single-phase system that consists of three components: water, oil and anamphiphilic molecule, called surfactant. The surfactant molecule lowers theinterfacial tension between water and oil resulting in the formation of atransparent solution. The water nanodroplets containing reagents, as ananoreactor, undergo rapid coalescence allowing for a mixing, precipitationreaction and an aggregation processes for the synthesis of Magnetite nanoparticles23 Water-in-oil (W/O) microemulsions (i.e.
reverse micelle solutions) aretransparent, isotropic, thermodynamically stable liquid media. In thesesystems, fine microdroplets of the aqueous phase are trapped within assembliesof surfactant molecules dispersed in a continuous oil phase. Thesurfactant-stabilized microcavities (typically in the range of 10 nm) provide aconfinement effect that limits particle nucleation, growth, and agglomeration.W/Omicroemulsions have been shown to be an adequate, versatile, and simplemethod to prepare nanosized particles and these are the characteristics thatcould make this method useful for both in vivo and in vitro applications.
24Magnetite nanoparticles around 4 nm indiameter have been prepared by the controlled hydrolysis with ammoniumhydroxide of FeCl2 and FeCl3 aqueous solutions within the reverse micellenanocavities generated by using AOT as surfactant and heptane as the continuousoil phase 34. Santra et al. 35 reported a robust methodology for the synthesisof both uncoated and silica-coated MNPs of ultrasmall (<5 nm) and a veryuniform size distribution by water-in-oil microemulsion. They used threedifferent nonionic surfactants (Triton X-100, Igepal CO-520 and Brij-97) forthe preparation of microemulsions, and also used NH4OH and NaOH as base source. By mixingtwo identical water-in-oil microemulsions which one of them containing metal saltsand the other containing the base source, microdroplets will continuouslycollide, coalesce, and break again and finally a precipitate forms in themicelles.
By addition of solvents, such as acetone or ethanol, to the microemulsions,the precipitate can be extracted by filtering or centrifuging the mixture.