Isotropic-nematic phase separation of a dispersion of organophilic boehmite rods

Abstract

An isotropic-nematic phase separation is observed in dispersions of sterically stabilized rodlike boehmite particles. The phase separation process is studied with polarization microscopy. Also, the phase volumes and particle concentrations in the coexisting phases are determined. The dependence of these quantities on the total concentration of the dispersion is analyzed in terms of the Onsager theory for the isotropionematic phase transition extended to bidisperse mixtures of rodlike particles. Qualitative agreement between theory and experiment is obtained. The triphasic isotropic-nematic-nematic equilibrium as predicted by the theory is observed after 6 months.

Liquid crystal phase transitions in suspensions of mineral colloids: new life from old roots

“…A remarkable result is that for sufficiently polydisperse particles two nematic phases may be in equilibrium with an isotropic phase [35]. This has indeed been observed experimentally [36,37], as we will discuss below. Computer simulation…”

“…Colloidal boehmite rods with an average particle length L between approximately 100 and 400 nm (controlled by varying the initial amounts of alkoxide and acid) and aspect ratio L/D varying from 10 to 30 were obtained in this way, showing (some) polydispersity in both the particle length and diameter. Subsequently, these boehmite particles were sterically stabilized by grafting a layer of low-molecular-weight poly(isobutene) (PIB) molecules on their surface [36]. Dispersed in cyclohexane, the sterically stabilized boehmite rods repel each other as a result of steric hindrance of the grafted polymer chains and can be modelled as nearly hard rods.…”

“…Starting from the materials used in the Buining method [42] to produce boehmite rods, Wierenga et al [56] discovered a synthesis route which yields well-formed hexagonal plate-like gibbsite crystals (diameter 100-200 nm and thickness 5-15 nm). Subsequently, van der Kooij & Lekkerkerker [57] succeeded in sterically stabilizing these gibbsite platelets by grafting a layer of low-molecular-weight PIB molecules onto their surface following the method of Buining & Lekkerkerker [36]. This novel model system of hard colloidal platelets was observed to phase-separate into an isotropic and a nematic LC phase [57].…”

Phase behaviour of rod-like colloid+flexible polymer mixtures

“…two nematic phases and an isotropic phase predicted by the present theory will be difficult to realize experimentally. First of all according to the calculations the more concentrated nematic phase will appear at colloid volume fractions above the concentration where the dispersion experimentally is observed to turn into a glass-like state [58]. Another issue that complicates matters here is that at sufficiently high concentrations of rod-like colloidal particles more highly ordered (liquid) crystal phases, such as the colloidal smectic A phase and the colloidal crystal phase are predicted to occur [75,76].…”

“…Recently in our laboratory, in addition to a biphasic isotropic-nematic equilibrium, a three-phase isotropic-isotropic-nematic region was observed in the phase diagram [57] of a colloidal suspension of sterically stabilized rod-like colloidal boehmite particles [58] and polystyrene in orthodichlorobenzene. Such a triphasic equilibrium is reminiscent of the three-phase isotropic-isotropic-solid equilibria recently observed in suspensions of spherical colloidal particles and non-adsorbing polymers [37,43].…”