Influence of anionic structure on the dissolution of chitosan in 1-butyl-3-methylimidazolium-based ionic liquids

The high crystallization degree of chitosan and therefore its poor solubility in water and in most of the conventional organic solvents limits its extensive applications in practical processes. In this study, ionic liquids [C(4)mim][HCOO], [C(4)mim][CH3COO], [C(4)mim][CH3CH2COO], [C(4)mim][CH3CH2CH2COO], [C(4)mim][HOCH2COO], [C(4)mim][C6H5COO], [C(4)mim][CH3CHOHCOO] and [C(4)mim][N(CN)(2)] have been used to study the effect of anionic structure on solubility of chitosan and the possible dissolution mechanism in ionic liquids. For this purpose, solubilities of chitosan in these ionic liquids have been determined as a function of temperature, and hydrogen bond accepting ability of the ionic liquids has been estimated by H-1 NMR and solvatochromic UV/vis measurements. At the same time, the interaction between [C(4)mim][CH3COO] and chitosan in the chitosan + [C(4)mim][CH3COO] solution is investigated by C-13 NMR, and the regenerated chitosan from ionic liquids by addition of ethanol is characterized by thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). It is shown that in the ionic liquids investigated, [C(4)mim][CH3COO] is the most efficient ionic liquid for the dissolution of chitosan. Solubilities of chitosan increase almost linearly with increasing hydrogen bond accepting ability of the ILs. Both the anions and the [C(4)mim](+) cation play important roles in the dissolution of chitosan, possibly by the disruption of native hydrogen bonds in chitosan. In addition, good thermal stability has been observed for the regenerated chitosan.