Stability of magnetizable colloidal suspensions by addition of oleic acid and silica nanoparticles

An experimental investigation is described on the stability of suspensions of micron-sized iron particles by (1) using oleic acid-mineral oil mixtures as a continuous phase and (2) adding silica nanoparticles to the continuous phase (silicone oil). The time evolution of the optical absorbance of the suspensions as a function of oleic acid (OA) concentration, silica nanoparticles concentration and magnetic field strength and direction has been analyzed. Adsorption isotherm demonstrated that OA adsorbs onto iron particles. However, only after the addition of very large amounts of OA (i.e. much more than the amount needed to form a monolayer coating) it is possible to design stable suspensions. The application of an external magnetic field provokes a faster sedimentation due to the formation of field-induced aggregates. When silica nanoparticles are used as stabilizer, a threshold value has been found in the silica concentration. Above this value iron suspensions are stable. The application of a magnetic field decreases this threshold concentration value due to the combined effect of the induced iron chains aggregation and the silica gel network. (c) 2005 Elsevier B.V. All rights reserved.