Theoretical studies on the dissolution of chitosan in 1-butyl-3-methylimidazolium acetate ionic liquid

In this work, the dissolution mechanism of chitosan in imidazolium acetic-based ionic liquid (IL) 1-butyl-3-methylimidazolium acetate was investigated by density functional theory (DFT). Chitobiose is considered to symbolize chitosan during the DFT calculations. [Bmim]OAc is supposed to be the best suitable IL among the investigated ILs for the dissolution of chitosan since the complex formed between [Bmim]OAc and chitobiose has the lowest energy. The hydrogen bonds formed by IL and chitobiose were studied by discussing the geometric parameter variations and the vibration mode analyses. Four strong hydrogen-bond patterns C1-H1 ⋯ O16, C2-H2 ⋯ O16, O38-H39 ⋯ O1 and O40-H41 ⋯ O2 were found, which means the existence of strong interaction between chitosan and [Bmim]OAc. In addition, natural bond orbital (NBO) analysis was used to study the second order perturbation stabilization energies (E(2)) that denotes the intensity of the interactions between chitobiose with H2O and ILs. The E(2) of chitobiose with [Bmim]OAc is larger than that of chitobiose with other ILs and solvents studied, which proves that chitobiose can be dissolved in [Bmim]OAc but cannot in water and other solvents. Atom in molecules (AIM) theory shows that hydrogen bonds between chitobiose and [Bmim]OAc are stronger than that between chitobiose and other solvents. It means that the interactions between [Bmim]OAc and chitobiose interrupt the initial hydrogen bonds in the chitobiose due to the formation of new hydrogen bonds in the complexes. The calculation data provide the interaction mechanism of the dissolution of chitosan in [Bmim]OAc.