Magnetic-field-induced orientational order in the isotropic phase of hard colloidal platelets

The magnetic-field-induced orientational order in the isotropic phase of colloidal gibbsite [Al(OH)3] platelets is studied by means of optical birefringence and small-angle x-ray scattering (SAXS) techniques. The suspensions display field-induced ordering at moderate field strengths (a few Tesla), which increases with increasing particle concentration. The gibbsite particles align their normals perpendicular to the magnetic field and hence possess a negative anisotropy of their diamagnetic susceptibility Δχ. The results can be described following a simple, Onsager-like approach. A simplified model is derived that allows one to obtain the orientational distribution function directly from the scattering data. However, it leads to an underestimate of the diamagnetic susceptibility anisotropy Δχ. This accounts for the difference between the Δχ values provided by the two experimental techniques (SAXS and magneto-optics). The order of magnitude Δχ∼10-22 J/T2 lies in between that of goethite suspensions and that of suspensions of organic particles