Concentration Dependence of the Structure of Cellulose/Ionic Liquid: A Molecular Dynamics Simulation Study

In this study, we conducted molecular dynamics simulations of cellulose oligomers (10-mers) as model cellulose molecules in the ionic liquid 1-ethyl-3-methylimidazolium acetate, varying their concentration from 5 to 50 mol %. As the cellulose concentration increased, the hydrogen bonding ratio between the hydroxyl groups of cellulose and the anions changed from 1:1 to 2:1. This transition, known as anion bridging, occurs when one anion interacts with multiple OH groups across different cellulose chains simultaneously. This phenomenon explains the ability of cellulose to dissolve in the ionic liquid at the remarkably high concentration of 40 mol %, as observed experimentally. We also simulated X-ray scattering factors as a function of cellulose concentration. While the simulations did not reveal a liquid crystalline phase, which contrasts with the experimental data, a scattering peak corresponding to periodic structures appeared at high concentrations, primarily due to a cellulose-cellulose contribution. The observed anion bridging and scattering peak are in good agreement with a previously proposed experimental model and enhance our understanding of highly concentrated cellulose/ionic liquid systems at the molecular level.