Physical properties of aqueous suspensions of goethite ( $\alpha$ -FeOOH) nanorods

Abstract

Depending on volume fraction, aqueous suspensions of goethite (alpha-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.

Magnetic field control of the ordering of two-component suspension of hard rods

“…The main physical properties of goethite were described in earlier studies [12,13]. Goethite magnetic nanorods have a lath-like shape of a typical size of 330 × 40 × 18 nm 3 and a small permanent magnetic moment of 1.1 × 10 −17 emu.…”

“…In a strong magnetic field, the amplitude of interaction between rods 2 and the magnetic field is much larger than the steric interaction between the rods, i.e. a cb 22 and a cb 12 . In this case, the angular distribution of rods 2, f 2∞ , takes the Boltzmann form:…”

Influence of polydispersity on the phase behavior of colloidal goethite

“…Goethite can easily form liquid crystals that are susceptible to external fields. Lemaire et al [7][8][9][10][11] further revealed that the boardlike goethite particles have a permanent magnetic moment along their long axis, presumably due to uncompensated spins within their antiferromagnetic crystal structure, combined with an induced magnetic moment oriented predominantly along the shortest particle dimension. The interplay between them leads to peculiar field-strength-dependent reorientation phenomena.…”

Direction Dependent Diffusion of Aligned Magnetic Rods by Means of X-Ray Photon Correlation Spectroscopy

“…However, because of the limited access to well defined anisotropic particles with a tunable aspect ratio only a few systems consisting of such particles have been investigated. Anisotropic particles, however, should give rise to a phase behavior with enhanced complexity compared to spherical particles due to the orientational degree of freedom [4][5][6][7][8][9][10]. Because of particle interactions or interactions with an external field such systems are expected to form analogs to liquid crystalline, i.e., nematic or smectic phases.…”