Synthesis and Analytical Centrifugation of Magnetic Model Colloids

Abstract

This thesis is a study of the preparation and thermodynamic properties of magnetic colloids. First, two types of magnetic model colloids are investigated: composite colloids and single-domain nanoparticles. Thermodynamics of magnetic colloids is studied using analytical centrifugation, including a specially adapted centrifuge for measuring heavy and strongly light absorbing colloids. Magnetic composite colloids can be prepared from thermodynamically stable Pickering emulsions of 3-methacryloxypropyl trimethoxysilane (TPM) oil in water stabilized by magnetite or cobalt ferrite nanoparticles. The emulsion droplet size can be increased by the relative volume of oil, the amount of salt, and the evolution of the droplets over time. This evolution over time is due to a gradual transfer of the interfacial nanoparticles to the oil phase, which can be utilized to controllably transfer aqueous nanoparticles to the TPM oil phase by the grafting of TPM onto the surface of the nanoparticles. Magnetic nanoparticles can be prepared by various methods, which yield particles that either have good crystallinity, contain twinning defects, or have a high density of dislocations. These crystal lattice defects can have a detrimental influence on the magnetic properties of the particles. It is shown that a low geometric size polydispersity of magnetic nanoparticles does not guarantee low polydispersity of the magnetic dipole moments. To study the thermodynamics of magnetic colloids by analytical centrifugation, a LUMiFuge stability analyzer is used for the first time to measure the osmotic equation of state of concentrated dispersions of magnetic nanoparticles. The LUMiFuge is equipped with homebuilt measurement cells with glass capillaries with an internal thickness of only 50 micrometers that allow measurement of concentrated, strongly light absorbing colloidal dispersions. For sufficiently small colloids also an analytical ultracentrifuge (AUC) can be used to determine the osmotic equation of state. Homebuilt AUC centerpieces with optical path lengths as low as 50 micrometers have been realized and the results are compared with the equations of state from the LUMiFuge. Sterically stabilized magnetic iron oxide nanoparticles with a diameter of 13 nm dispersed in an apolar solvent are used as an experimental realization of the dipolar hard spheres known from theory and computer simulations. The experimental osmotic pressures of the magnetite fluids are significantly below Van 't Hoff's law due to magnetic interparticle attractions. The osmotic second virial coefficient obtained from the experimental osmotic pressures corresponds to a dipolar coupling constant in the range of 2.0-2.4. An isotropic, Van der Waals-like phase separation, if any, is expected at a value of the dipolar coupling constant of 2.5, close to our experimentally obtained value, yet no indications for such a phase transition are found. The dipolar coupling parameter obtained by analytical centrifugation is in reasonable agreement with the coupling parameter calculated from independent magnetic measurements of the nanoparticles. This agreement clearly confirms that analytical centrifugation is an accurate method to quantify interactions between colloidal particles.