Synthesis of goethite as a model colloid for mineral liquid crystals

We describe the synthesis and modification of lathlike goethite (alpha-FeOOH) model colloids, where we focused on the reduction of the size polydispersity (determined from transmission electron microscopy pictures.) The length polydispersity could be strongly reduced from 55% down to 17% using a combination of a forced hydrolysis method and an improved centrifugation procedure. Preliminary small-angle X-ray scattering experiments demonstrated the potential of these model particles to study the influence of the length polydispersity on the occurrence of different liquid crystalline phases. A coating of polyisobutene was applied on the goethite surface to obtain a stable dispersion of hard-core laths in toluene, which retained the peculiar magnetic alignment behavior of aqueous goethite particles (changing from parallel to perpendicular orientation at increasing field strength.) The goethite laths could further be coated with a silica layer of tunable thickness which also allowed the incorporation of a fluorescent dye. Finally, the goethite core of silica-coated goethite was dissolved with acid to obtain hollow silica laths. These modifications give additional possibilities for the use of goethite as model colloids.