Eur. Phys. J. E (2004) 13: 291-308
https://doi.org/10.1140/epje/i2003-10078-6

Physical properties of aqueous suspensions of goethite (-FeOOH) nanorods

Part I: In the isotropic phase

¹ Laboratoire de Physique des Solides, UMR CNRS 8502, Bâtiment 510, Université Paris-Sud, 91405, Orsay, France
² European Synchrotron Radiation Facility, B.P. 220, 38043, Grenoble, France
³ Nemoptic, 1 rue Guynemer, 78114, Magny-les-Hameaux, France
⁴ Laboratoire de Chimie de la Matiére Condensée, UMR CNRS 7574, Université Paris 6, 4 Place Jussieu, 75252, Paris, France

^* e-mail: davidson@lps.u-psud.fr

Abstract

Depending on volume fraction, aqueous suspensions of goethite (-FeOOH) nanorods form a liquid-crystalline nematic phase (above 8.5%) or an isotropic liquid phase (below 5.5%). In this article, we investigate by small-angle X-ray scattering, magneto-optics, and magnetometry the influence of a magnetic field on the isotropic phase. After a brief description of the synthesis and characterisation of the goethite nanorod suspensions, we show that the disordered phase becomes very anisotropic under a magnetic field that aligns the particles. Moreover, we observe that the nanorods align parallel to a small field (< 350 mT), but realign perpendicular to a large enough field (> 350 mT). This phenomenon is interpreted as due to the competition between the influence of the nanorod permanent magnetic moment and a negative anisotropy of magnetic susceptibility. Our interpretation is supported by the behaviour of the suspensions in an alternating magnetic field. Finally, we propose a model that explains all experimental observations in a consistent way.