Quantitative Calorimetric Studies of the Chiral Nematic Mesophase in Aqueous Cellulose Nanocrystal Suspensions

Aqueous suspensions of cellulose nanocrystals (CNCs) can spontaneously form a chiral nematic mesophase at a critical concentration (c*). Unfortunately, no current analytical technique permits rapid detection of c*. Herein, we introduce a facile and accurate approach to assess c* rapidly (<2 h) from a small sample volume and compare our results with those obtained by conventional methods. Our strategy employs isothermal titration calorimetry (ITC) to measure the heat associated with interactions in the suspension, which can identify the onset of mesophase formation as the heat signature is sensitive to the suspension viscosity and thus capable of detecting small changes in the suspension environment. We measure c* for CNC samples differing in surface charge and aspect ratio, and find that both lower aspect ratios and higher surface charges increase c*. Our ITC results reveal the role of CNC interactions prior to the visual observation of mesophase formation and elucidate mesomorphic effects related to nanocrystals and their suspensions.