Theoretical and experimental studies of ionic liquid-urea mixtures on chitosan dissolution: Effect of cationic structure

Abstract

Two imidazolium chloride ionic liquids (ILs) were mixed with urea for chitosan dissolution. Ionic liquids were [Amim]Cl and [Etmim]Cl (1-allyl-3-methyl-imidazolium chloride and 3-methyl-1-(ethylacetyl)imidazolium chloride, respectively). Physicochemical properties of neat ILs and IL-Urea solutions (including density, viscosity, conductivity, β value, π* value, and thermogravimetric analysis), were thoroughly investigated. Dissolution performance of chitosan in IL-Urea solutions was determined.Chitobiose (KET) as model chitosan structure, density functional theory (DFT) calculations were used to discuss the interactions between IL, urea, and chitobiose, and to clarify the different physicochemical properties of ILs, IL-Urea solutions. Also DFT simulation results were used for elucidating the influence of different cationic substituents and urea addition for chitosan dissolution.[Etmim]Cl showed higher density, viscosity, conductivity, β, and π* value than those of [Amim]Cl. These results revealed strong hydrogen bonds among [Etmim]Cl owing to the carboxylic acid ester group than allyl group of [Amim]Cl. Densities, viscosities, and π* values of IL-Urea solutions increased with increasing urea content. Conductivities and β values of IL-Urea solutions decreased with the increase of urea content at same temperature. The thermal stability of [Amim]Cl-Urea mixture increased when urea content increased from 10% to 30%, which was different from [Bmim]Cl-Urea melt. The thermal stability of [Etmim]Cl-Urea mixture was different to that of [Amim]Cl-Urea mixture.Urea was accommodated in ILs, forming hydrogen bonds with both the anion and cation in ILs, which changed the molecular interactions of ILs. The influence of different substituents in imidazolium ILs and urea addition on chitosan dissolution were analyzed using experimental determination and DFT calculations. Cationic substituents of imidazolium ILs play an extraordinary role in hydrogen bonds formation with chitobiose, so to strengthen chitosan dissolution. The information obtained would provide a guide for designing novel IL solvent systems for chitosan dissolution.