The complex phase behaviour of suspensions of goethite (α-FeOOH) nanorods in a magnetic field

Abstract

In 1902, Majorana reported the magneto-optical properties of aqueous colloidal suspensions of mixed iron oxides. Oddly enough, the magnetic-field induced birefringence displayed a non-monotonic dependence upon field intensity. This behaviour was later interpreted as due to the existence in these sols of at least two different chemical species. During the course of our studies of mineral liquid crystals, we have revisited this problem by examining aqueous suspensions of pure goethite (alpha-FeOOH) nanorods. Although they are comprised of a single chemical species, these suspensions show the same odd behaviour reported by Majorana. Moreover, we show that, as the volume fraction increases, the suspensions have an isotropic liquid/nematic/rectangular columnar phase sequence, with first-order transitions between these phases. The non-monotonic dependence of the field-induced birefringence can be explained by the existence of a remanent magnetic moment of the nanorods and the negative anisotropy of their magnetic susceptibility. Therefore, the nanorods align parallel to a weak field but realign perpendicular to the field beyond Bc approximately 375 mT. In addition, other interesting phenomena appear upon application of a magnetic field: the disordered (i.e. isotropic in zero-field) phase becomes highly anisotropic and difficult to distinguish from the nematic phase. Both phases then acquire not only quadrupolar order but also dipolar order. The rectangular columnar phase is strongly stabilised versus the nematic one. Our experimental observations raise new theoretical questions about the phase diagram of these suspensions with respect to volume fraction and magnetic field intensity.

Liquid crystal phase transitions in suspensions of mineral colloids: new life from old roots

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Advances in the Study of Inorganic Lyotropic Liquid Crystals

“…Подробные исследования мезофаз дисперсий гетита были проведены группой Лемери [33,34]. Было найдено, что изотропная фаза существует до объемной доли 5,5 %.…”

NIR transmittance tuneability under a magnetic field of colloidal suspensions of goethite (α-FeOOH) nanorods

“…3,22 In particular, aqueous colloidal goethite suspensions show a magneticeld induced birefringence, called "Majorana phenomenon", 23 that is related to the orientation of anisometric particles by the eld. It is possible to observe a clear difference in absorption along the particle axes, i.e.…”