Phase behavior of mixtures of magnetic colloids and non-adsorbing polymer

Abstract

In this thesis, we have studied the thermodynamic stability of magnetic fluids, also called ferrofluids. These consist of spherical colloids of typically 10 nm, coated with a monolayer of oleic acid and dispersed in cyclohexane. The core material, Fe3O4, is ferrimagnetic and because of its small size, the core consists of a single magnetic domain with a permanent magnetic moment. An easy preparation method for such colloids is described in chapter 2. The question how interactions between magnetic colloids affect the stability of magnetic fluids is one of practical as well as fundamental interest. Some applications require the magnetic fluid to remain monophasic under working conditions; others need phase separation of the magnetic fluid in order to work. To predict under which conditions a ferrofluid becomes unstable, it is often modeled as a dipolar hard sphere fluid (DHS fluid). Actually, theories for DHS fluids had been developed even before ferrofluids came up. Most of them aimed at describing the dielectric constant of polar liquids in terms of the properties of polar molecules. To assess the applicability of these theories on magnetic fluids, the magnetic susceptibility as a function of concentration and interaction strength of high quality ferrofluids has been studied in chapter 3. The variation of interaction strength was accomplished by separating a polydisperse ferrofluid into fractions with different mean particle sizes, ranging from 8 to 15 nm. None of the prevailing theories describes the measured susceptibilities accurately, although three theories (the Mean Spherical Model, Perturbation Theory and Onsager's theory) are in fair agreement with the experimental data. In chapter 4, two theories are described that are used to predict the stability of ferrofluids against liquid-gas phase separation. The theories show that phase separation in magnetic fluids may be induced by applying a magnetic field, if magnetic interactions between the colloids are sufficiently strong, about 3 kT. In ferrofluids, however, the interaction strength is only 1 kT, though due to polydispersity there will be a small fraction of large particles with much stronger interaction. Even though magnetic interactions alone may be too weak to enable phase separation in magnetic fluids, they will influence the stability of ferrofluids destabilized by other factors, for example by the presence of non-adsorbing polymer. Even without magnetic interactions, non-adsorbing polymer can induce colloidal gas-liquid phase separation in a colloidal dispersion. In chapter 4, a mean field theory for the phase behavior of colloid-polymer mixtures is extended to take magnetic interactions into account. Calculations with this modified theory show that magnetic interactions decrease the stability of colloid-polymer mixtures, and moreover, that the decrease in stability is stronger when a magnetic field is applied. As oleic acid can also be considered as a small polymer, removal of excess oleic acid, which is not always done after the synthesis of magnetic fluids, can improve the stability of magnetic fluids. In chapters 5 and 6, the stability of magnetic fluids containing poly(dimethylsiloxane), a non-adsorbing polymer, is studied experimentally. In chapter 5, it is set out in detail how phase separation is detected and quantified using a susceptibility meter based on a Colpitts oscillator. This instrument can make local measurements of the susceptibility in a sample tube; hence, the colloid concentration in each phase can be measured in a quick and non-destructive way. Moreover, the locations of phase boundaries and therefore the volumes of separate phases can be accurately determined. Chapter 5 also describes a method to obtain the polymer concentration in each separate phase by combining susceptibility measurements of samples with different colloid/polymer ratios. In principle, susceptibility measurements allow for the determination of the full phase diagram of colloid-polymer mixtures, including nodelines. In practice, however, translation of susceptibilities to concentrations was obscured, because phase separation is shown to be accompanied by strong size fractionation, and the susceptibility is very sensitive to changes in particle size. The influence of a magnetic field on the stability of ferrofluid-polymer mixtures is investigated in chapter 6, using the same susceptibility meter as used in chapter 5 and a thermostated electromagnet. Without polymer, the ferrofluid is stable at all attainable field strengths (up to 30 kAm -1 ) and all concentrations. This is inconsistent with many reports in literature, but may be due to the fact that a high quality ferrofluid was used here, i.e. without excess oleic acid and clusters. With polymer, the phase behavior in a magnetic field clearly deviates from the behavior in zero field. The decrease in stability caused by the magnetic field is somewhat stronger that predicted by the theory described in chapter 4, but still of the same order of magnitude. An open question is how the stability is affected by the considerable polydispersity (26%) of the magnetic colloids. That magnetic colloids do not always behave as dipolar hard spheres is demonstrated in chapter 7. Using several experimental techniques, the existence of a strong, anomalous attraction between sterically stabilized magnetite particles and silica spheres was demonstrated. The attraction resulted in irreversible adsorption of magnetite particles, covering up to 30% of the silica surface. The adsorption kinetics appeared to be orders of magnitudes slower than that of a diffusion limited adsorption. Based on this and other observations, proton transfer mechanism was proposed as a possible mechanism to account for the strong attraction.