Experimental and theoretical studies of chitosan dissolution in ionic liquids: Contribution ratio effect of cations and anions

Abstract

Typical hydrogen bonding information (hydrogen bonds, hydrogen bond lengths, and average bond lengths) and weak interaction energy (ΔH) of four ionic liquids (ILs), anion/chitobiose, cation/chitobiose, and IL/chitobiose complexes were determined using density functional theory (DFT) calculations to elucidate the contribution ratio effect of ions (cations or anions of ILs) upon chitosan dissolution.For [Emim]Ac and [Bmim]Ac with same acetate anion, contribution ratio (Rc) values of cations in [Bmim]Ac and [Emim]Ac increased from 37.88% to 38.14%. For [Bmim]Cl and [Etmim]Cl with same chloride anion, contribution ratio (Rc) values of cations increased slightly from 39.39% to 41.65%. As for [Bmim]Cl and [Bmim]Ac with same cation, contribution ratio (Rc) values of Cl- anion in [Bmim]Cl (60.61%) was lower than that of acetate anion in [Bmim]Ac (62.12%).These changes revealed that cations of ILs played more important role in chitosan dissolution.Hydrogen-bond basicity values (β values) of ILs were closely related to the DFT simulation results (weak interaction energy and average hydrogen bond length values) of ILs. Chitosan dissolved faster in acetate based imidazolium ILs ([Emim]Ac and [Bmim]Ac) than the chloride based ILs ([Bmim]Cl and [Etmim]Cl). The chitosan dissolution performance in ILs was consistent with DFT simulation results (weak interaction energy values and average hydrogen bond length values) of IL/chitobiose and hydrogen-bond basicity values of ILs.