Chitosan dissolution with sulfopropyl imidazolium Brönsted acidic ionic liquids

Two Brönsted acidic ionic liquids (1-methy-3-(3-sulfopropyl) imidazolium acetate, as mPSAc, and 1-butyl-3-(3-sulfopropyl) imidazolium acetate, as bPSAc) were synthesized for chitosan dissolution. Physical properties, including density, pH value, viscosity, Hammett acidity (H0), and Kamlet-Taft parameters (β and π*) of Brönsted acidic ionic liquids (BAILs), were studied and analyzed. The dissolution performance and regeneration properties of chitosan in BAILs were studied and compared with other ordinary ionic liquids ([Emim]Cl, [Bmim]Cl, [Amim]Cl, [Emim]Ac, [Bmim]Ac, [Amim]Ac). 1% chitosan was dissolved in two BAIL aqueous solutions (5% w/w) at 30 °C (35 min and 37 min, respectively). The regenerated chitosan was characterized by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and X-ray diffraction (XRD). Molecular weight and deacetylation degree results showed that mPSAc endowed slight hydrolysis upon chitosan, only dissolution of chitosan at lower temperature. Density functional theory (DFT) simulations were performed to study the interactions between sulfopropyl imidazolium acidic ionic liquids and chitobiose. Four types of hydrogen bonds (C-H…O, O-H…O, N-H…S, N-H…O) and many strong hydrogen bonds were found in BAIL-chitobiose, suggesting strong interactions between BAILs and chitobiose. DFT simulation results indicated that the active hydrogen atom of imidazole ring, sulfonate ion and acetate ion played important roles in the chitosan dissolution process by the disruption of native hydrogen bonds of chitobiose.