ISOTROPIC AND BIREFRINGENT DISPERSIONS OF SURFACE-MODIFIED SILICA RODS WITH A BOEHMITE-NEEDLE CORE

A method is presented for the preparation of charged and sterically stabilized rodlike silica particles. First, boehmite needles are synthesized by hydrothermal treatment of aqueous aluminum alkoxide solutions. Then controlled silica growth on the needles takes place in an aqueous sodium silicate solution without formation of aggregates or homogeneous silica particles. Finally grafting reactions are performed with (fluorescent) silanes and octadecyl alcohol to obtain stable dispersions in organic solvents. Special attention is given to the silica-covered rods grafted with a dense octadecyl brush, which form isotropic (flow birefringent) dispersions and translucent (permanent birefringent) incompressible low-density sediments in cyclohexane. It is argued that the dominant interparticle force in these systems is a steep hard-rod repulsion, which also accounts for the novel observation of the turbidity decrease at increasing densities.