Liquid-crystalline behavior in aqueous suspensions of elongated chitin microcrystals

Abstract

Chitin elongated microcrystals are rigid rodlike particles known to form chiral nematic liquid-crystalline phases when suspended in acidic aqueous solutions. These phases remarkably mimic the cholesteric geometry observed in biological tissues such as crustacean exoskeletons where chitin constitutes the fibrous component of a composite material, in association with a protein matrix and a mineral phase. The aim of the present work is to investigate the phase diagram of chitin colloidal dispersions and in particular the chirality of the nematic phase and its dependence on the composition of the suspension. Both the pH and the ionic strength affect the concentrations in the isotropic, c _I , and chiral nematic, c _N , phases at coexistence. It turns out that c _N does not vary monotonically with HCl and goes through a minimum of 5.36±0.29% (in weight) at a HCl concentration of around 5×10⁻⁴ M, with c _I =2.98±0.21%. When the ionic strength of the solution reaches about 10⁻² M the system does not bulk phase-separate anymore; anisotropic droplets form throughout the sample but do not coalesce or sediment. The cholesteric pitch, P ₀, in the biphasic domain of the diagram ranges from 27.5 ± 3.5 to 140 ± 17 μm depending on the composition of the suspension medium. The variations of P ₀ follow a power law as a function of the concentration of crystalline rods in the coexistence domain P ₀∼c ^{− ν}, with 0.3<ν<1.2 as a function of ionic strength. This set of results sheds light on the self-assembly mechanisms that lead to the formation of a complex and ordered biological composite like the cuticle of insects and crustaceans. A better description of the phase diagram of these colloidal liquid crystals is a useful basis for their application in designing, for instance, biomimetic composites and ordered mesoporous materials.