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

## TL;DR

This study uses NMR spectroscopy to explore how different deep eutectic solvents interact with cellulose fibers, revealing how their molecular structures affect fiber swelling and modification.

## Contribution

The paper introduces a novel NMR-based approach to probe molecular interactions between cellulose fibers and isomeric deep eutectic solvents.

## Key findings

- TEMACl–Imi caused significant cellulose fiber swelling, indicated by changes in NMR relaxation times and diffusion coefficients.
- 13C NMR showed increased cellulose mobility in TEMACl–Imi due to swelling, while TEMACl–Pyra showed minimal interaction and no cellulose signal.
- Imidazole in TEMACl–Imi likely forms stronger hydrogen bonds with cellulose hydroxyl groups compared to pyrazole in TEMACl–Pyra.

## Abstract

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 1H diffusion, T1-and-T2-relaxation, and 13C 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 13C NMR spectra displayed characteristic cellulose-backbone signals, indicating a swelling-induced increase in CF mobility. Conversely, TEMACl–Pyra exhibited significantly suppressed changes in 1H-relaxation times and D, and no cellulose signal appeared in its 13C 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.

The online version contains supplementary material available at 10.1007/s10570-025-06913-0.