Abstract
We have found that the dissolution of cellulose in the binary mixed solvent tetrabutylammonium acetate/dimethyl sulfoxide follows a previously overlooked near-stoichiometric relationship such that one dissolved acetate ion is able to dissolve an amount of cellulose corresponding to about one glucose residue. The structure and dynamics of the resulting cellulose solutions were investigated using small-angle X-ray scattering (SAXS) and nuclear magnetic resonance techniques as well as molecular dynamics simulation. This yielded a detailed picture of the dissolution mechanism in which acetate ions form hydrogen bonds to cellulose and causes a diffuse solvation sheath of bulky tetrabutylammonium counterions to form. In turn, this leads to a steric repulsion that helps to keep the cellulose chains apart. Structural similarities to previously investigated cellulose solutions in aqueous tetrabutylammonium hydroxide were revealed by SAXS measurement. To what extent this corresponds to similarities in dissolution mechanism is discussed.
Highlights
Cellulose is the most abundant natural polymer on Earth and an important raw material in a bio-based economy
Cellulose solubility corresponding to a molar ratio 0.48:1 anhydroglucose units (AGU):ionic liquids (IL) have been reported for several neat acetate IL (Wang et al 2012)
The results above are all consistent with a mechanistic picture where approximately one acetate ion binds to each AGU by strong, long-lived hydrogen bonds
Summary
Cellulose is the most abundant natural polymer on Earth and an important raw material in a bio-based economy. Tetrabutylammonium acetate (TBAAc) solutions in DMSO dissolve cellulose of high degree of polymerization (DP) in concentrations suitable for fibre spinning close to room temperature (Miao et al 2014; Sun et al 2015). As TBAAc has a melting point at the very upper edge of the IL range, the use of co-solvent is necessary to allow cellulose dissolution near room temperature. This is not necessarily a disadvantage; addition of dimethyl sulfoxide (DMSO) to 1-ethyl-3-methylimidazolium acetate (EmimAc) has previously been shown to promote the dissolution of cellulose (Li et al 2016). Measured absorbance is due to scattering and can be used to calculate the turbidity as
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