Synthesis of magnetic nanoparticles and nanocomposites via water-in-oil microemulsions

The effects of surfactants, co-surfactants, aqueous phase and temperature on the solubilisation capability of microemulsion systems were investigated. Appropriate water-in-oil (w/o) microemulsion systems for the synthesis of nanomaterials were selected in consideration of the higher solubilisation and the better thermo-stability. Mono-dispersed iron oxide nanoparticles with the size of 1-5 nm were synthesised via Igepal CO-520/cyclohexane w/o microemulsion at 25°C. The size of particles increased from 1 nm to 10 nm with the increase of the size of water pools. The original particles as prepared were identified as magnetite, which transformed into maghemite after 2-hour hydrothermal at 120°C and into hematite after 2-hour hydrothermal at 140°C, accompanied with the increased crystallite size. Precipitation was employed for basic studies of starting materials, reaction time and temperature. Compared with precipitation-derived particles, microemulsion-derived nanoparticles show smaller particle size, are less aggregated and exhibit higher activities and a lower saturation magnetisation and coercivity both at 5K and 300K. Poly-(methacrylic acid) (PMAA), Polyacrylamide (PAM) nano-spheres were synthesised via Triton X-114/cyclohexane and Brij 97/cyclohexane w/o microemulsions at 60°C, respectively. The size of PMMA spheres is 30-100nm while the size of PAM spheres increased from 50 nm to 200 nm with the increase of surfactant concentration from 19.3% to 28.9%. The increased size of water pools from 2.43 to 4.32 also resulted in the increased size of PAM from 50 nm to 500 nm. The effects of reaction time and temperature, and reagent concentration on PAM polymerisation in microemulsion were investigated in terms of the conversion, molecular weight and morphology of polymers produced. Core-shell structured silica coated iron oxide nanoparticles were synthesised via Igepal CO-520/cyclohexane systems at 25°C, with 5 nm core and 5 nm shell. Nanocomposites of PAM embedded with iron oxide were synthesised in Brij 97/cyclohexane at 60°C, with the size of 120 nm. The crystallinity of magnetic nanoparticles was affected by the coating process.