MAGNETIC SILICA DISPERSIONS - PREPARATION AND STABILITY OF SURFACE-MODIFIED SILICA PARTICLES WITH A MAGNETIC CORE

Preparation and properties are discussed of a novel magnetic dispersio n, containing surface-modified silica colloids with a core of single-d omain magnetite particles. The underlying idea is to tailor the silica shell thickness and surface properties such that the colloids are sta ble spheres with isotropic interactions, whereas an external magnetic field produces weak dipolar attractions and consequently reversible an isotropic structures. The influence of the shell thickness is analyzed in terms of a dipolar-sphere potential. Preparation of magnetite core s, silica growth, and surface modifications with an alcohol and a sila ne are described in detail. Particle properties are investigated, amon g other things, with SAXS, light scattering, and electron microscopy. The major conclusion is that the preparation route yields stable, nona ggregated magnetic silica particles with a shape and internal structur e which is mainly determined by small magnetite clusters in the starti ng ferrofluid.