A force field for bio-polymers in ionic liquids (BILFF) - part 2: cellulose in [EMIm][OAc]/water mixtures

We present the extension of our force field BILFF (Bio-Polymers in Ionic Liquids Force Field) to the bio-polymer cellulose. We already published BILFF parameters for mixtures of ionic liquid 1-ethyl-3-methylimidazolium acetate ([EMIm][OAc]) with water. Our all-atom force field focuses on a quantitative reproduction of the hydrogen bonds in the complex mixture of cellulose, [EMIm]⁺, [OAc]^- and water when compared to reference ab initio molecular dynamics (AIMD) simulations. To enhance the sampling, 50 individual AIMD simulations starting from different initial configurations were performed for cellulose in solvent instead of one long simulation, and the resulting averages were used for force field optimization. All cellulose force field parameters were iteratively adjusted starting from the literature force field of W. Damm et al. We were able to obtain a very good agreement with respect to both the microstructure of the reference AIMD simulations and experimental results such as the system density (even at higher temperatures) and the crystal structure. Our new force field allows performing very long simulations of large systems containing cellulose solvated in (aqueous) [EMIm][OAc] with almost ab initio accuracy.