Anomalous Attraction between Colloidal Magnetite and Silica Spheres in Apolar Solvents

We demonstrate the existence of an anomalous, strong attraction between small (9 nm) oleic acid-grafted magnetite particles and octadecanol-grafted silica spheres (420 nm) in apolar solvents. This attraction manifests itself by irreversible adsorption of magnetite particles onto silica spheres, with surface coverages up to 30%. This “heteroflocculation” is, quite surprisingly, orders of magnitude slower than a diffusion-limited process. The adsorption can be reversed by transferring covered silica particles to solvents with a higher dielectric constant. The findings practically rule out van der Waals forces or Coulomb forces between any pre-existing surface charges as the source of attraction.