Nematic liquid crystals were obtained in sterically stabilized suspensions of rodlike particles of sepiolite clay, with an average length up to 900 nm and aspect ratio up to 40. In agreement with computer simulations for hard spherocylinders, the isotropic-nematic transition shifted to lower volume fractions with increasing aspect ratio. However, the coexistence gap was broadened noticeably due to particle polydispersity. The sepiolite crystal structure includes channels filled with zeolitic water, which can be replaced by indigo dye molecules. The indigo molecules are constrained inside the zeolitic channels to be aligned along the long axes of the rods. As a result, the colloidal nematic phase showed a marked dichroism, with an order parameter up to 0.5 for magnetically aligned samples, similar to typical values for dye-doped thermotropic liquid crystals.

Sterically stabilised colloidal rod-sphere mixtures were prepared by mixing sepiolite rods of an average length L ¼ 860 nm and aspect ratio L/D ¼ 40 with silica spheres of average diameter d ¼ 620 nm. Whereas most previous studies have addressed low or high L/d ratios, the present study has an intermediate ratio of 1.4. Samples were studied at rod concentrations from 3 to 9 wt%, in the isotropicnematic coexistence region. No dramatic effects were seen on adding spheres, except for two samples at low rod concentrations where a rapid (local) phase separation resulted. This is ascribed to formation of nematic tactoids, separated by layers of spheres. Samples at higher rod concentrations did not show any rapid phase separation. Microscopy using fluorescent rods however showed a fine network of rods formed in this case. Macroscopic phase separation into sphere-rich and rod-rich phases was not observed in any sample, nor was any rapid clustering of spheres as seen previously in mixtures with L/d ¼ 0.3. The late stage sediment density can be described well by approximating the osmotic pressure of the colloidal rods at second virial level. Whilst the absence of macroscopic phase separation suggests that these mixtures do not reach chemical equilibrium and instead remain stuck in a range of long-lived metastable states, the observations on the sediment density show that nevertheless local mechanical equilibrium is attained.

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