Smectic Liquid-Crystalline Order in Suspensions of Highly Polydisperse Goethite Nanorods

摘要

Aqueous dispersions of Goethite rods not only form isotropic, nematic, and columnar phases but also a newly found smectic A phase. Although smectic phases have been found before in colloidal dispersions of rodlike particles (for examples see the literature), the smectic phase of Goethite is first identified in an extremely (over 50%) poly-disperse system, much higher than the terminal polydispersity of 18% determined by computer simulation of spherocylin-ders. It seems very likely that the fractionation and sedimentation-induced macroscopic separation associated with the isotropic-nematic phase transition help to create the circumstances to form a smectic phase within one week. However, it remains remarkable that only a very narrow range of smectic layer spacings is found. Particle lengths that do not fit seem to go to the columnar phase. A second scenario, where individual particles slowly sediment and fractionate and only then phase separate, also seems to be at work, since samples with much lower volume fractions also develop Bragg reflections after about half a year. In both scenarios, the phase development is clearly kinetically helped by the Brownian motion enabled by the presence of an electric double layer around these charged particles.