Iron(oxide) ferrofluids: synthesis, structure and catalysis

This thesis describes a study on two connecting research subjects concerning ferrofluids, i.e. the synthesis and use of catalytic magnetic colloids and the microstructural behaviour of ferrofluids in general. An interesting application of ferrofluids (dispersions of magnetic colloids) would be their use in catalysis; catalysts attached to the surface of small magnetic particles could in principle be separated magnetically for recycling after their application as catalyst. A feasibility study of this principle is described in chapter 2. Catalytic magnetic colloids were synthesized and separated by the High Gradient Magnetic Separation technique. Out of fundamental interest, but also for a better understanding of applications of ferrofluids (e.g. as described above), the structural behaviour of ferrofluids in general was studied and is presented in chapters 3-7 of this thesis. Since the 1970s and until today a lot of theoretical work and simulations on the physical behaviour and interparticle structure of dipolar fluids has been performed, showing results on which a full consensus still has not been reached. Here, we describe a systematic and extensive study on the physical and structural properties of metallic iron ferrofluids, in which particle size and surfactant layer can be varied in a broad range, resulting in controllable dipolar and Van der Waals interactions between particles. The preparation and characterization of iron dispersions with different average particle sizes is described in chapter 3. Particle radii could be controlled in the 2-10 nm range by varying the amount of reactants. Scattering measurements (described in chapter 4) were used to study the composition of single particles and revealed that our particles are mono-domain dipolar spherical particles, with a lognormal size distribution of the iron cores, surrounded by a surfactant shell, which is partially penetrated by solvent. Upon oxidation, an oxide layer forms on the particle surface. The structural behaviour of ferrofluids was investigated using Small Angle Scattering techniques, Cryogenic Electron Microscopy and Dynamic Susceptibility measurements, giving a rather complete picture of the physical behaviour of ferrofluids. Our ferrofluids are stable and do not phase separate, as found from susceptibility measurements. The interparticle structure strongly depends on the particle size (related to the dipole moment). SAXS measurements on oleic-acid-coated colloids (chapter 5) indicated that concentrated dispersions of small particles consist of single non-interacting dipolar hard spheres, while dispersions of only slightly larger particles contain a significant contribution of dipolar attractions. Cryo-TEM on dispersions of PIB-coated particles (chapter 6), where Van der Waals attractions are screened by the thick surfactant layer, demonstrated an abrupt transition with increasing particle size (corresponding to increasing dipolar attraction) from separate particles to linear chains or large networks. The behaviour described above was confirmed by dynamic susceptibility measurements on the bulk dispersions (chapter 7); while for small particles the orientation of the magnetic moment changed by thermal rotation inside the particles (Neel rotation) or for slightly larger particles by rotational diffusion of the single particles themselves (Brownian rotation), an enormous increase of relaxation times was found upon increasing the particle radii, demonstrating the presence of large clusters. The rotational diffusion rate of these aggregates was found to increase upon dilution, suggesting that the aggregate size resulted from a dynamic equilibrium. The dipolar nature of the aggregation in zero field was confirmed by alignment of existing chains in the field direction upon vitrification of dispersions in a magnetic field (using cryo-TEM, chapter 6).