PHASE-BEHAVIOR OF AQUEOUS DISPERSIONS OF COLLOIDAL BOEHMITE RODS

We report the phase behavior of aqueous dispersions containing rodlike boehmite particles with two different average lengths, 130 and 280 nm , with length-to-width ratios of approximately 8 and 20, over a range of ionic strengths. At low salt concentrations, where the particle int eraction is predominantly repulsive, a separation into an isotropic up per phase and a birefringent lower phase occurs in the system containi ng the high-aspect-ratio particles. This phase separation was not obse rved in the case of dispersions containing the shorter particles. This seems to indicate that the phase separation is driven by the excluded volume effect. Above a critical (low) particle concentration, the rep ulsive 280 nm long rods form a monophasic birefringent glassy phase. T his concentration is much lower than that at the isotropic-nematic tra nsition as predicted from Onsager's theory for charged rods. Above a c ritical ionic strength a regime is entered where the phase behavior is largely governed by the attractive interaction. Then for both aspect ratios a space-filling amorphous gel is formed.