Impact of Cellulose Dissolution on 1‑Butyl-3-Methylimidazolium Chloride Crystallization Studied by Raman Spectroscopy, Wide-Angle X‑ray Scattering, and Solid-State NMR

• Published in: Crystal growth & design Vol. 20; no. 3; pp. 1706 - 1715
• Main Authors: Kotov, Nikolay, Raus, Vladimír, Urbanová, Martina, Zhigunov, Alexander, Dybal, Jiří, Brus, Jiří
• Format: Journal Article
• Language: English
• Published: American Chemical Society 04-03-2020
• ISSN: 1528-7483; 1528-7505
• DOI: 10.1021/acs.cgd.9b01458

Ionic liquid 1-butyl-3-methylimidazolium chloride (bmimCl) is a widely applied solvent for cellulose. In this work, we studied the crystallization behavior of bmimCl in the presence of cellulose, aiming at obtaining further insight into bmimCl–cellulose interactions. To induce crystallization of bmimCl, the mixtures with 3 and 5 wt % of cellulose were subjected to −25 °C for various periods of time and investigated by Fourier transform-Raman spectroscopy, wide-angle X-ray scattering, and solid-state NMR. Even though the presence of cellulose noticeably delayed the crystallization process, bmimCl still adopted the monoclinic form M0, characterized by gauche–anti conformation of the butyl chain, which was reported previously for neat bmimCl. This lack of changes in the crystal forms adopted by bmimCl at different conditions highlights its resistance toward the perturbation of its ordering behavior. Unexpectedly, ssNMR analysis revealed the presence of a 15% crystalline fraction of cellulose in the crystallized mixture, showing that bmimCl crystallization induces partial self-organization of cellulose. Furthermore, the ssNMR spectra indicated that cellulose interacts with bmimCl ring carbon atoms, implying the existence of carbohydrate–aromatic stacking interactions in this system. Finally, the impact of water contamination (0.6 and 2.9 wt %) on the bmimCl/cellulose mixtures was studied. The higher water content effectively suppressed the crystallization of bmimCl. On the other hand, the mixture containing less water eventually crystallized, which was accompanied by the macroscopic phase separation of water and recrystallization of bmimCl into the more energetically favorable orthorhombic crystal form. This highlights the important role of cellulose in the recrystallization process as without cellulose this effect was reported only for bmimCl with considerably higher water content.