A. Xu et al. / Carbohydrate Polymers 92 (2013) 540–544
543
Table 1
The solubility (%) of microcrystalline cellulose in ILs/DMSO (RDMSO = 2.54:1) solvents and the ˇ parameters of the ILs at 25 ◦C.
Solventa
Cellulose solubility (%)
IL
ˇb
[C4mim][CH3COO]/DMSO (RDMSO = 2.54:1)
[C4mim][(C6H5)COO]/DMSO (RDMSO = 2.54:1)
[C4mim][HOCH2COO]/DMSO (RDMSO = 2.54:1)
[C4mim][CH3CHOHCOO]/DMSO (RDMSO = 2.54:1)
[C4mim]Cl/DMSO (RDMSO = 2.54:1)
15.0
9.0
Insoluble
Insoluble
Insoluble
Insoluble
[C4mim][CH3COO]
[C4mim][(C6H5)COO]
[C4mim][HOCH2COO]
[C4mim][CH3CHOHCOO]
[C4mim]Cl
1.161
0.987
0.967
0.964
0.840
0.621
[C4mim][N(CN)2]/DMSO (RDMSO = 2.54:1)
[C4mim][N(CN)2]
a
RDMSO is the mole ratio of DMSO to IL.
Ref. Xu et al.
b
3.4. Influence of co-solvent nature on the solubility of cellulose
1-allyl-3-methylimidazolium acetate [Amim][CH3COO], addition
of 10% N-methylimidazole into [Amim][CH3COO] gives 14% solu-
bility.
Encouraged by the above results, we have used other
aprotic polar solvents to see whether their addition in the
[C4mim][CH3COO] can enhance the solubility of cellulose.
DMF and DMA are selected for this purpose. It is found
that at 25 ◦C, solubility of cellulose in [C4mim][CH3COO]/DMF
(RDMSO = 2.54:1) and [C4mim][CH3COO]/DMA (RDMSO = 2.54:1) sol-
vents also increases markedly compared with that of cellulose in
order: [C4mim][CH3COO]/DMSO (15.0%) > [C4mim][CH3COO]/DMF
(12.5%) > [C4mim][CH3COO]/DMA (5.5%) at the same molar ratio of
through ion–dipole interaction (Hanke et al., 2002), and the dif-
ference in cellulose solubility would partially be ascribed to the
difference in the dipole moment of these aprotic polar solvents.
According to the literature (Covington & Dickinson, 1973), dipole
moment of the aprotic polar solvents we used follows the order:
DMSO (3.96D) > DMF (3.86D) > DMA (3.81D). Thus, the solvated
strength of the [C4mim]+ cations by the aprotic polar solvents and
the corresponding concentration of “free” [CH3COO]− anions in the
solvent systems should follow the same order. Consequently, under
the same conditions, [C4mim][CH3COO]/DMSO gives the highest
cellulose solubility, and the lowest cellulose solubility is observed
in [C4mim][CH3COO]/DMA solvent.
4. Conclusions
The novel solvents designed in this work can directly dissolve
cellulose in high concentration without heating. The conductivity
results confirm that the enhanced dissolution of cellulose is mainly
ascribed to the increased concentration of the “free” [CH3COO]−
anions from the dissociated IL owing to the preferential solvation
of [C4mim]+ by the aprotic polar solvents. This supports the con-
clusion that cellulose solubility in ILs is mainly determined by the
hydrogen bond accepting ability of their anions, and explains why
protic polar solvents such as water and methanol can be used to
precipitate cellulose from ILs solutions. This finding opens up new
horizons in the design of cellulose solvents, and provides a new
platform for the practical use of the cellulose processing at ambient
temperature.
Acknowledgements
This work was supported financially by the National Basic
Research Program of China (973 Program, No. 2011CB211702) and
the National Natural Science Foundation of China (No. 21133009).
Furthermore, based on our concept mentioned above, if a protic
polar solvent is added into an ionic liquid, solubility of cellulose
will decrease in such a solvent because anions rather than cations
of the ILs will be solvated preferentially by formation of hydro-
gen bonding. In order to examine this speculation, we tried to
measure solubilities of cellulose in [C4mim][CH3COO]/water and
[C4mim][CH3COO]/methanol solvents at 25 ◦C. It was found that in
lose in these solvents were too small to be determined. This is why
the protic polar solvents, like water and methanol, are often used
as precipitants to regenerate cellulose from ILs solutions (Swatloski
et al., 2002; Xu et al., 2010). Alternatively, we determined cellu-
lose solubility at 25 ◦C in [C4mim][CH3COO]/DMSO (1:2.54) with
addition of 1.0% methanol, then 20.0% decrease in the cellulose
solubility was observed. This confirms the role of preferential solva-
tion of anions of ionic liquids by protic polar solvents in the decrease
of cellulose solubility.
Appendix A. Supplementary data
Supplementary data associated with this article can be
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