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Chiral nematic ordered suspensions of cellulose microcrystallites

Authors
Publisher
McGill University
Publication Date
Keywords
  • Chemistry
  • Physical.

Abstract

Suspensions of rod-like cellulose microcrystallites, 50-250 nm in length and $ sim$7 nm in width, and with negatively charged sulfate groups on the surface have been prepared. When the concentration of cellulose microcrystallites exceeds a critical value, the suspension separates into two phases, forming an upper isotropic phase and a lower chiral nematic anisotropic phase. The critical concentration for phase separation is very sensitive to the ionic strength, particle size, polydispersity of particles, counterion species, and preparation conditions. When the suspension is in pure water and in the biphasic region, the coexisting concentrations in both phases increase with the increase of total cellulose concentration. Suspensions with different counterions have different critical concentrations for phase separation. The critical concentration increases in the order $ rm H sb3{O sp+} < {Na sp+} < {K sp+} < Cs sp+$ for inorganic counterions, and generally increases with increasing counterion size for some organic counterions. The salt-form suspensions demonstrate a good stability with temperature, but the acid-form suspension is unstable at temperatures higher than $50 sp circ$C. The suspensions show induced circular dichroism in the presence of dye, such as Congo red. The anisotropic phase has a very strong negative ICD peak when viewed along the chiral nematic axis, and the peak is much weaker when viewed at right angles to the axis. The isotropic suspension shows a small positive ICD peak. The behavior of the phase separation was compared with the theoretical predictions of Stroobants et al., and a relatively good agreement was obtained between theory and experimental data for suspensions in pure water.

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