Magnetic Nanorods Confined in a Lamellar Lyotropic Phase

Abstract

The dilute lamellar phase of the nonionic surfactant C 12EO 5 was doped with goethite (iron oxide) nanorods up to a fraction of 5 vol %. The interaction between the inclusions and the host phase was studied by polarized optical microscopy (with or without an applied magnetic field) and by small-angle X-ray scattering. We find that, when the orientation of the nanorods is modified using the magnetic field, the texture of the lamellar phase changes accordingly; one can thus induce a homeotropic-planar reorientation transition. On the other hand, the lamellar phase induces an attractive interaction between the nanorods. In more concentrated lamellar phases (under stronger confinement) the particles form aggregates. This behavior is not encountered for a similar system doped with spherical particles, emphasizing the role of particle shape in the interaction between doping particles and the host phase.

Effect of shape biaxiality on the phase behavior of colloidal liquid-crystal monolayers

“…We hope that our study will serve as a guide for future experimental studies of confined board-shaped colloidal systems, such as goethite nanorods [30][31][32] and the recently synthesized lead carbonate nanoplatelets [36]. geometry of a single particle:…”

“…By inserting goethite nanorods into a soft lamellar matrix of non-ionic surfactant, it is also possible to examine the effect of dimensional reduction on the stability of mesophases [30][31][32]. The confined nanorods between the bilayers of a lamellar phase have been shown to undergo a first-order in-plane (two-dimensional) isotropic-nematic phase transition, where the isotropic and nematic phases correspond to planar and biaxial nematic phases, respectively.…”

Mutual Alignment of Block Copolymer−Magnetic Nanoparticle Composites in a Magnetic Field

“…Using this kind of scheme to induce reorientation in lyotropic liquid crystal/nanomagnet composites has also been demonstrated. 18,19 The composite under study consisted of a polystyrene-blockpoly(2-vinylpyridine) (PS-P2VP) diblock copolymer, which forms cylindrical microdomains of P2VP in a PS matrix, 20 and spindletype hematite (R-Fe 2 O 3 ) particles, which were synthesized by the forced hydrolysis of Fe(ClO 4 ) 3 , according to procedures previously described by Ocaña et al 21 The particles, an example of which is shown in the transmission electron microscopy (TEM) image in Figure 1a, had an average diameter of 55 ( 6 nm and an average long axis length of 330 ( 38 nm (see the Supporting Information, Figure S1). Such hematite particles are weakly ferromagnetic and have recently been shown to orient with their major axes perpendicular to an applied magnetic field, B, 22 as drawn in Figure 1b (B applied in the x-direction).…”

Infrared dichroism of gold nanorods controlled using a magnetically addressable mesophase

“…This coupling occurs because both the goethite nanorods 18 and the gold nanorods (see Figure 1b and the discussion below) are confined within the water layers of the lamellar phase. Furthermore, it was shown that the orientation of the lamellar phase could be controlled by the application of a magnetic field, since the magnetic rods impose their orientation to the surfactant mesophase 20 . Consequently, the magnetic field indirectly controls the orientation of the gold nanorods.…”

“…To this end, we further add to the lamellar phase goethite nanorods that confer magnetic sensitivity to the system, at a concentration φ g = 1.5 vol% (Note that φ g is much higher than φ Au !). The goethite nanorods were thoroughly characterized in previous work 14,19 , and the lamellar phase doped with these nanoparticles (but without gold nanorods) was also investigated 20,21 . .…”