Process for preparing agglomerate-free nanoscalar iron oxide particles with a hydrolysis resistant coating

A process for producing an-agglomerate-free suspension of stably coated nanosize iron-containing oxide particles, comprising the following steps in the order indicated:

• (1) preparing an aqueous suspension of nanosize iron-containing oxide particles which are partly or completely present in the form of agglomerates;
• (2) adding (i) a trialkoxysilane which has a hydrocarbon group which is directly bound to Si and to which is bound at least one group selected from amino, carboxyl, epoxy, mercapto, cyano, hydroxy, acrylic, and methacrylic, and (ii) a water-miscible polar organic solvent whose boiling point is at least 10° C. above that of water;
• (3) treating the resulting suspension with ultrasound until at least 70% of the particles present have a size within the range from 20% below to 20% above the mean particle diameter;
• (4) removing the water by distillation under the action of ultrasound;
• and
• (5) removing the agglomerates which have not been broken up.