The preparation and stability of homodisperse colloidal haematite (alpha-Fe2-O3)

N.H.G. Penners

    Research output: Thesisinternal PhD, WU

    Abstract

    <p/>Since the foundation of colloid chemistry as a branch of science, much attention has been paid to the subject of colloid stability, i.e. the stability of colloid systems against aggregation. Gradually, our knowledge of the mechanisms involved has improved and models were developed, comprised in the DUO theory, which form the basis of a quantitative description of the stability of a colloidal system. There is plenty of experimental evidence which substantiate the correctness of the principles of the DUO theory, and hence, this theory is regarded as one of the fundaments of colloid chemistry. However, in one respect the theory is not confirmed by experiments: calculations predict pronounced size effects, but in practice stability seems to be little affected by particle size.<p/>It was the purpose of this study to gain insight in this contradictory matter. <strong>Chapter 1</strong> offers a more extended introduction to the problem as well as the outline of this study.<p/><strong>Chapter 2</strong> focusses attention on a model system which meets the outlined requirements regarding surface charge, homodispersity, sphericity and particle size range: a method is described for the synthesis of homodisperse haematite (α-Fe <sub>2</sub> O <sub>3</sub> ) sols containing particles whose sizes vary from 35 nm up to 700 nm. This method is principally based on the gradual growth of haematite seeds in supersaturated FeCl <sub>3</sub> solutions (heterogeneous nucleation) up to the desired paticle size. As slight deviations in the composition of the growth medium have drastic effects on the shape of the final colloid, emphasize is given to the description of optimal synthesis conditions for spherically shaped particles. Kinetic experiments, performed to unravel the principles of particle growth revealed that the precipitation process is governed by diffusion.<p/>The coagulation experiments described in chapter 5 are monitored by turbidity measurements. Therefore the characterisation of the optical properties of the sols is a prerequisite for further studies. The sols under investigation are homodiperse and contain spherically shaped particles. This makes them particularly suited for such an evaluation, as their scattering behaviour can be interpreted in terms of the Mie theory. Such a comparsion, leading to values for the refractive index (n) and the absorption coefficient (K) in the wavelength range from 400 to 800 nm is made in <strong>chapter 3</strong> . The agreement between calculations and experiments is good for any wave length in the visible range, and for any particle size studied, although the particles are monocrystalline and no perfect spheres.<p/>Any study dealing with electrostatic stabilisation demands some knowledge of the electrochemical behaviour of the system under study. <strong>Chapter 4</strong> pays attention to the electrochemical characterisation of the haematite surface by comparing haematite samples from different origines. In these studies, potentiometric titrations, streaming potential measurements on haematite-coated capillaries and micro electrophoresis were used as the experimental tools to get access to the surface properties. Instead of providing unambiguous data, being valid for all iron oxides occuring in the (α-Fe <sub>2</sub> O <sub>3</sub> ) modification, the reported experiments emphasize that the crystal structure of the bulk phase is not the exclusive parameter in determining the electrochemical behaviour of an oxide. The purification procedure, or a heat treatment of the sol (aqueous or dried) plays an important part as well. There is some evidence that the crystal habit of the haematite surface is pH-dependent and that in some cases precipitated amorphous oxide may share in determining the surface properties. Though not going too much into details, the study gives evidence of the fact that the charging mechanisms of haematite are more complicated than expected on the grounds of purely crystallographic considerations.<p/>Inevitably this finding has its impact on the question of colloid stability, which is the dominating item of <strong>chapter 5</strong> . Considering the outcome of the electrochemical study, is it still justified to assume that the haematite particles meet the demands of the stability theory regarding the sharpness of the boundary between bulk material and surrounding liquid? How could a diffuse surface layer be accounted for in existing stability models? Within the restrictions set by such questions, chapter 5 deals with the item of colloid stability with special reference to the influence of particle size on stability: though the value of the critical coagulation concentration depends on particle size and shows a minimum (!), the slopes of the log W - log C plots are virtually size independent. Such trends can be accounted for by the concepts of the DUO theory, if coagulation reversibility and shear effects are incorporated in the analysis. Deviations from sphericity, which are definitely observed for the systems under investigation. might explain some of the observed effects as is shown by some simple double layer calculations dealing with orientational effects in the interaction of a cubic particle with a half space.Finally, <strong>chapter 6</strong> reflects on the preceding Items and pays attention to its limitations. Furthermore, it points to subjects which deserve further elaboration and mentions the means to make them experimentally accessible.
    Original languageEnglish
    QualificationDoctor of Philosophy
    Awarding Institution
    Supervisors/Advisors
    • Lyklema, J., Promotor, External person
    • Koopal, L.K., Co-promotor, External person
    Award date15 Mar 1985
    Place of PublicationWageningen
    Publisher
    Publication statusPublished - 1985

    Keywords

    • colloids
    • dispersion
    • emulsions
    • haematite
    • iron hydroxides
    • iron oxides
    • isolation
    • processing
    • purification
    • suspensions

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