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

Abstract

Aqueous suspensions of cellulose nanocrystals (CNC) 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 to those obtained by conventional methods. Our strategy employs isothermal titration calorimetry (ITC) to measure the heat of interaction, which can identify the onset of mesophase formation since it is sensitive to 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 lower aspect ratios and higher surface charges both increase c*. Our ITC results reveal the role of CNC interactions prior to visual observation of mesophase formation and elucidate effects related to nanocrystals and their suspensions.