(263c) Interface transport of nanoparticles - synthesis of oil-based magnetic fluids

The wet chemical synthesis of magnetite nanoparticle dispersions can be seen as state of the art for the first step in the production of magnetic fluids. Two iron salts contaning Fe2+ and Fe3+ ions are mixed in an aqueous solution in stoichiometric ratio. The precipitation uses a fast pH-shift from about 2 to above 9. The precipitation agent is a caustic soda or ammonia solution. The during the precipitation particle synthesis particle agglomeration and particle growth are superimposed. The magnetite nano-particles have to be stabilized to maintain their size and with it the superparamagnetic properties. For most applications the stabilisation takes place in a non-aqueous solvent. Therefore a separation step is necessary to transfer the particles from the aqueous colloid into the non-aqueous oil based liquid. In conventional processes the nano-magnetite is colleted by oil based detergent droplets from the aqueous dispersion, leading to a high consumption of detergent. The process study presents the continuous transfer of nano-particles from the aqueous to the second non-miscible, non-aqueous liquid phase. The transfer bases on the sedimentation of the dispersed particles. In a fist step the aqueous dispersion is destabilised to promote reversible agglomeration of the magnetite particles. The agglomerates are large enough to sediment in a centrifugal field. In the centrifuge below the aqueous dispersion the second phase is placed. During sedimentation the particles pass the liquid-liquid interface, where the particle surface is covered with the stabilising detergent. The detergent further causes the disintegration of the agglomerates. This process allows very low detergent concentration within the magnetic fluid, which is important for further application, i.e. synthesis for polymer-nanocomposite. Furthermore the phase transfer of the nano-particles shows a high efficiency up to 100%. The particle size within the oil-based magnetic fluid corresponds to the primary particle size of the precipitated magnetite. The content of impurities like water within the oil-based magnetic fluid is very low. The process itself is not limited to the synthesis of magnetic fluids it can be operated for several different phase transfer problems involving nano-particles.

Keywords: sedimentation, colloid, dispersion, stabilized suspension, precipitation, detergent