Phase Separation Behavior in Aqueous Suspensions of Bacterial Cellulose Nanocrystals Prepared by Sulfuric Acid Treatment

Abstract

Phase separation phenomena of aqueous suspensions of cellulose nanocrystals have been studied for bacterial cellulose (BC) prepared by sulfuric acid hydrolysis. Suspensions at concentrations above 0.42 wt % separated into the isotropic and chiral nematic phases with a clear phase boundary. The shape and size distribution of BC nanocrystals in both the phases were determined by transmission electron microscopy (TEM) and atomic force microscopy (AFM). The surface charge density was determined by conductometric titration. The effects of added NaCl (0-5.0 mM) on the phase separation behavior of the aqueous suspensions were investigated for a fixed total cellulose concentration. The volume fraction of the chiral nematic phase had a minimum value at a NaCl concentration of ca. 1.0 mM. At NaCl concentrations ranging from 2.0 to 5.0 mM, the suspensions did not separate into two phases, but became entirely liquid crystalline. The size of the ordered domains in the anisotropic phase decreased with an increase in the NaCl concentration from 0 to 2.75 mM. At 2.75 mM, only tactoids were observed in the entire region. At 5.0 mM, chiral nematic domains were no longer observed. The chiral nematic pitch decreased with increasing concentration of added NaCl, reached a minimum value at approximately 0.75 mM, and then increased sharply with the NaCl concentration up to 2.0 mM.

Cellulose nanocrystal-based materials: from liquid crystal self-assembly and glass formation to multifunctional thin films

“…As CNC samples are highly polydisperse, the influence of the aspect ratio can be recognized even when using a single CNC source. Hirai et al 30 and Dong et al 54 confirmed a fractionation of longer CNC rods into the anisotropic phase and of shorter rods into the isotropic fraction.…”

“…Hirai et al 30 observed an interesting and rather peculiar reversal in phase behavior in their study of bacterial CNC with added NaCl. Although the volume fraction of the cholesteric phase initially decreased with added salt, as expected, it started increasing again at concentrations greater than 0.75 mM NaCl.…”

“…Taking as a starting point the Onsager model 51 (see Supplementary Box 1), which predicts that the required volume fraction for liquid crystal formation is inversely proportional to the rod aspect ratio, the onset concentration of liquid crystallinity is expected to decrease if the rod-like CNC is longer and thinner. Indeed, dispersions of bacterial CNCs, with lengths on the order of 1-2 mm and an aspect ratio in the range of 50-100, 30,52 show nematic ordering well below 1 wt% CNC, cf. Figure 4.…”

“…The effect is weaker for CNCs with a lower surface charge and with a smaller onset concentration for liquid crystalline ordering; hence, the nonlinearity becomes negligible for bacterial CNCs, which are approximately one order of magnitude longer, with B1/20 of the surface charge of wood-derived CNCs. 30,52 Dong et al 55 conducted an elegant experiment to demonstrate the influence of the CNC counter ions on the formation of the liquid crystal phase. By adding HCl at varying concentrations to a series of CNC suspensions within the biphasic regime, such that each suspension had a constant pH of 1.61, they effectively canceled out the effect of the counter ions introduced by the cellulose nanorods.…”

“…For instance, the notion that the reflected color is due to a photonic bandgap resulting from the helical structure has been challenged based on observations of a helix that is too long for Bragg reflection to take place in the visible wavelength range, 29 and a surprising dependence on the ionic strength of the phase sequence and helical pitch has been observed in some systems. 30 One of the aims of this critical review is to emphasize the physics and physical chemistry-related issues involved in CNC self-assembly by comparing it with similar processes in suspensions of other nanorod classes. This approach gives an important additional perspective that is complementary to the current state of understanding, and we believe it can be very beneficial for the future development of the field and the realization of CNC-based functional materials.…”