Cellulose is a kind of biomaterials with properties of biodegradability, biocompatibility and stability and has been widely used for biorenewable resource research. However, cellulose is insoluble in water and common organic solvents. Previous research showed that ionic liquids (IL) have the advantages of negligible vapor pressure, high thermal stability and nonflammable and are considered as green solvents for cellulose.
It was also reported several highly effective cellulose solvents consisting of 1-butyl-3-methylimidazolium acetate [C4mim][CH3COO] and aprotic solvents and the enhanced dissolution of cellulose mainly because of the preferential salvation of cations of the IL by the cosolvents. Little is known about the molecular mechanism of the cosolvents in the dissolution of cellulose in IL-based system.
Researchers from Institute of Process Engineering, Chinese Academy of Sciences chose four kinds of cosolvents including two aprotic solvents and two protic solvents to study the role of cosolvents on cellulose dissolution in [C4mim][CH3COO] on molecular level. Molecular dynamics (MD) simulations and quantum chemistry calculations are common methods to investigate the interactions of cellulose with ILs at the molecular level.
MD simulations were performed on the mixtures consisting of cellulose, [C4mim][CH3COO] and one of the cosolvents at 298K. Quantum chemistry calculations were used to study the interactions of cations and anions with the cosolvents.
It was found that the hydrogen bond interactions between [CH3COO]- anions and the hydroxyl protons of cellulose played an important role in the dissolution of cellulose in IL/cosolvent systems. And the effect of cosolvents on the solubility of cellulose also mainly determined by the interactions.
The aprotic solvents form weak C-H…O hydrogen bonds with [C4mim]+ and [CH3COO]-. They can partially break down the ionic association of [C4mim][CH3COO] by salvation of the cation and anion but no preferential solvation was observed. And the protic solvents form strong O-H…O hydrogen bonds with [CH3COO] and weak C-H…O hydrogen bonds with [C4mim]+. The strong preferential salvation of [CH3COO]- by protic solvents can compete with the cellulose-[CH3COO]- interaction in the dissolution process, resulting in decreased cellulose solubility.
Furthermore, the influence of the molar ratio of the aprotic solvent to IL on the dissolution of cellulose was also studied. The research proposed a possible mechanism of cosolvent on solubility of cellulose in ionic liquids on the molecular level. The work was published on the Journal of Physical Chemistry B.