Probing molecular interactions of cellulose fibers with isomeric deep eutectic solvents using NMR spectroscopy

Deep eutectic solvents (DESs) are promising media for cellulose fiber (CF) modifications, such as swelling, dissolution, functionalization, and disintegration, owing to their low toxicity, biodegradability, and versatility. These modifications are governed by CF–DES interactions, regulated by the chemical structures of DES constituents. Here, nuclear magnetic resonance (NMR) spectroscopy was used to investigate the molecular interactions and dynamics between CFs and two isomeric non-derivatizing DESs comprising triethylmethylammonium chloride (TEMACl) with imidazole (Imi), TEMACl–Imi, or TEMACl with pyrazole (Pyra), TEMACl–Pyra. The NMR approach encompassed variable-temperature 1 H diffusion, T 1 -and- T 2 -relaxation, and 13 C NMR experiments. Significant CF swelling occurred in TEMACl–Imi, highlighted by reduced Imi and almost unchanged TEMACl relaxation times. This indicated that Imi primarily accounted for the interaction with cellulose. Correspondingly, the diffusion coefficients ( D ) of both DES components reduced, probably because of the increased viscosity due to CF swelling, as well as restricted diffusion inside the swollen CFs. Further, the 13 C NMR spectra displayed characteristic cellulose-backbone signals, indicating a swelling-induced increase in CF mobility. Conversely, TEMACl–Pyra exhibited significantly suppressed changes in 1 H-relaxation times and D , and no cellulose signal appeared in its 13 C spectra. This indicated suppressed CF swelling and CF–DES interaction. The more significant CF swelling in TEMACl–Imi might stem from the formation of stronger hydrogen bonds by Imi with the hydroxyl groups in cellulose compared with those formed by Pyra. Overall, these findings highlight how the molecular configuration of DES facilitates cellulose interactions and the profound impact of these interactions on CF modification.