Effect of sodium oleate as a buffer on the synthesis of superparamagnetic magnetite colloids

In this study, superparamagnetic monodisperse magnetite colloids, around 5 nm in size, were prepared by dissolving iron chlorides, sodium hydroxide (NaOH) and sodium oleate (NaOL), in toluene/ethanol/water mixtures and refluxing for 4 h. The concentrations of NaOH and NaOL were varied to systematically investigate the effect on the surface properties, size, dispersion, and magnetic properties of magnetite nanoparticles (MNPs). The samples were characterized via XRD, FTIR, TGA, TEM, SAED, DLS, and VSM. The results indicated that the surface coatings of MNPs could be manipulated from oleate to hydroxyl groups via increasing the molar ratio of NaOH/Fe(II) more than 8. The amount of NaOH had no obvious influence on the size and the saturation magnetization of MNPs. Therefore NaOH was not a necessary reactant for forming magnetite crystals. On the contrary, NaOL was shown to be the most important component for synthesizing stable magnetite colloids. The NaOL acted as both a key reactant to buffer the pH environment and a surfactant to keep the MNPs stable in nonpolar solvent media.