Effect of Oil on Cellulose Dissolution in the Ionic Liquid 1-Butyl-3-methyl Imidazolium Acetate

Abstract

While ionic liquids (ILs) are well known to be excellent solvents for cellulose, the exact mechanism of dissolution has been a much disputed topic in recent years and is still not completely clear. In this work, we add to the current understanding and highlight the importance of hydrophobic interactions, through studying cellulose dissolution in mixtures of 1-butyl-3-methyl imidazolium acetate (BmimAc) and medium-chain triglyceride (MCT) oil. We demonstrate that the order in which constituents are mixed together plays a key role, through nuclear magnetic resonance (NMR) spectroscopic analysis. When small quantities of MCT oil (0.25–1 wt %) were introduced to BmimAc before cellulose, the effect on BmimAc chemical shift values was much more significant compared to when the cellulose was dissolved first, followed by oil addition. Rheological analysis also showed small differences in the viscosities of oil–cellulose–BmimAc solutions, depending on the order the constituents were added. On the other hand, no such order effect on the NMR results was observed when cellulose was replaced with cellobiose, suggesting that this observation is unique to the macromolecule. We propose that a cellulose–oil interaction develops but only when the cellulose structure has a sufficient degree of order and not when the cellulose is molecularly dispersed, since the hydrophobic cellulose plane is no longer intact. In all cases, cellulose–BmimAc–oil solutions were stable for at least 4 months. To our knowledge, this is the first work that investigates the effect of oil addition on the dissolving capacity of BmimAc and highlights the need for further re-evaluation of accepted mechanisms for cellulose dissolution in ILs.

Recent Advances in Cellulose-Based Hydrogels Prepared by Ionic Liquid-Based Processes

“…After being combined with various cations possessing alkyl functional groups, Cl − , acetate (Ac − ), dimethyl phosphate (DMP − ), and diethyl phosphate (DEP − ) anions (acidic protons) strongly exhibit the ability to dissolve cellulose [60]. In contrast, weak hydrogen-bonding anions, e.g., BF 4 − , PF 6 − , and Tf 2 N − , are not suitable for cellulose dissolution [61,62]. The Cl − -based ILs are widely adopted as a small-sized hydrophilic hydrogen bond acceptor, for instance BmimCl is frequently used due to its thermoplasticity and thermal processability [63][64][65][66].…”