In an attempt to prepare stable dispersions of cellulose nanocrystals in dipolar aprotic solvents, dilute aqueous suspensions of cellulose nanocrystals were prepared by sulfuric acid hydrolysis of cotton. The aqueous suspensions were freeze-dried, and then sonicated in the solvent of interest. Dispersions of 1 and 3% w/v concentration were prepared in polar organic solvents DMSO and DMF. The dispersions showed flow birefringence. The redispersion was incomplete, and there was some evidence for aggregation in the suspensions. A small amount of water appeared to be critical to suspension stability. Birefringent cellulose films were prepared from the dispersions by drying under vacuum and at ambient conditions.

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Spontaneous entropic phase separation phenomena occur in a wide range of systems containing highly anisotropic colloidal particles. Among these are aqueous suspensions of negatively charged cellulose I nanocrystals produced by sulfuric acid hydrolysis of native cellulose, which phase separate into isotropic and chiral nematic liquid-crystalline phases. Phase separation of an isotropic phase from a completely ordered nanocrystal suspension may be induced by the addition of salts or nonadsorbing macromolecules. In previous work (Edgar, C. D.; Gray, D. G. Macromolecules 2002, 35, 7400-7406), an isotropic phase was found to form over a period of several days when blue dextran (a sulfonated triazine dye, Cibacron blue 3G-A, covalently attached to high-molecular-weight dextran chains) was added to initially ordered suspensions. Here we report work showing that the observed phase separation was associated with the charged dye molecules attached to the dextran. The Cibacron blue 3G-A dye attached to blue dextran was found to induce greater phase separation than free (unbound) dye; at increasing ionic strength, depletion attractions due to the blue dextran increasingly contribute to the phase separation.

The effect of added nonadsorbing macromolecules on the phase behavior of suspensions of electrostatically stabilized cellulose nanocrystals is examined. Triphasic isotropic−isotropic−nematic equilibria develop in cellulose nanocrystal suspensions to which a mixture of dextran and blue dextran is added. Phase diagrams for different combinations of uncharged dextran and anionically charged blue dextran are presented. The range of dextran concentrations at which the triphase equilibrium is produced is strongly influenced by the molecular weight of the blue dextran. Reentrant phenomena as well as regions of biphase coexistence are present in the phase diagrams. A threshold difference in charge density between two blue dextrans of varying degree of dye substitution is necessary for triphase equilibrium to occur at a given number density of each polymer species. Different repulsive electrostatic and attractive entropic forces may dominate at different concentrations of the dextrans, contributing to the rich phase behavior of these suspensions.

Suspensions of negatively charged cellulose nanocrystal are obtained by sulfuric acid hydrolysis of cotton cellulose. Within a specific concentration range, the suspensions spontaneously phase separate to give isotropic and chiral nematic phases. Added anionic dyes of varying charge cause separation of an isotropic phase from the highly concentrated, completely anisotropic suspensions; these dyes have a much greater effect on the suspensions than the equivalent ionic strength of a simple 1:1 electrolyte (sodium chloride). Neutral, cationic and cellulose-binding anionic dyes do not cause phase separation in the anisotropic cellulose suspensions.