1D and 2D NMR of nanocellulose in aqueous colloidal suspensions

This is the first report on surface structural elucidation of individual nanocellulose as colloidal suspensions by 1D H-1, 2D heteronuclear single quantum coherence (HSQC) as well as C-13 nuclear magnetic resonance (NMR). H-1 NMR of rice straw CNCs (4.7 nm thick, 143 nm long, 0.04 sulfate per AG or 19.0% surface hydroxyl to sulfate conversion) resembled that of homogeneous cellulose solution. Conventional 2D HSQC NMR of CNC, CNF 1.5 (2-14 nm thick, several micrometers long, 0.10 -COOH per AG) and CNF10 (2.0 nm thick, up to 1 mu m long, 0.28 -COOH per AG) gave H1:H2 ratios of 1.08:1,0.97:1 and 0.94:1, respectively, all close to the theoretical 1:1 value for cellulose. The H1:H6 ratios determined from 2D HSQC NMR for CNCs, CNF1.5 and CNF10 were 1:1.47, 1:0.88 and 1:0.14, respectively, and corresponded to 26%, 56% and 93% C6 primary hydroxyl conversion to sulfate and carboxyl groups, consistent with, but more sensitive than those by conductometric titration and X-ray diffraction. Both H-1 and 2D HSQC NMR data confirm that solution-state NMR detects nanocellulose surface carbons and protons primarily, validating this technique for direct surface characterization of nanocellulose in aqueous colloidal suspensions, presenting a sensitive and meaningful NMR tool for direct characterizing individual nanocellulose surfaces in never-dried state. (C) 2014 Elsevier Ltd. All rights reserved.