The preparation and dispersion of magnetic ferrite particles
Magnetic fluids containing magnetite (Fe(_3)O(_4)) have been prepared using colloidal size particles, obtained by alkaline treatment of mixed iron(II)/iron(III) chloride solutions. The particles have been dispersed in a variety of carrier liquids employing a wide range of surfactant materials. In non-aqueous carrier liquids, a single layer of surfactant is chemically adsorbed at the particle surface to effect steric stabilization. In aqueous media, a primary layer of chemisorbed surfactant interpenetrates a secondary layer which is physically adsorbed. This arrangement places hydrophilic charged groups at the surfactant/water interface, thereby maintaining a favourable interaction between the two phases. A wide variety of surfactants, including a range of straight-chain fatty acids, have been evaluated for use in aqueous magnetic fluids. Fluids incorporating up to 32 wt. % magnetite have been prepared. Stability towards dilution, changes in pH and quantity of dispersing agent have all been studied. Charge-stabilized magnetic fluids, in both acidic and alkaline aqueous media, have been investigated. Control of Fe(_3)O(_4) particle size, between 60Ǻ and 80Ǻ diameter, has-been achieved by altering the conditions under which precipitation occurs, namely temperature, reagent concentrations, viscosity and solvent composition. In addition, water was found to be capable of dispersing larger particles than organic carrier liquids. Particles as small as 30Ǻ diameter have been prepared by in situ magnetite precipitation with naturally occurring polymeric surfactants, such as proteins and polysaccharides. Functional groups along the polymer backbone are thought to coordinate to Fe(^2+) ions, thus maximising the number of particle nucleation sites. Small particles with narrow size distributions result. In this way, magnetic fluids have been prepared, stabilized with low toxicity naturally occurring materials for possible medical applications. Magnetite particles, too large for use in magnetic fluids, have been made by condensing iron(II) hydroxide with a variety of hydrated iron(III) oxide species. Particle size was dependent upon the size and solubility of the iron(III) precursor. Non-colloidal magnetite particles were also obtained by the aerial oxidation of iron(II) hydroxide suspensions at temperatures > 60 C.