(489c) A Molecular Understanding of Cellulose Dissolution in Aqueous Ionic Liquids

A major challenge in the pretreatment of biomass using ionic liquids deals with the removal of water from the solvent. Residual water that is present in biomass releases upon dissolution, commonly inhibiting the ability of the ionic liquid to further dissolve cellulose. Recently however, a class of phosphonium hydroxide-based ionic liquids have been found that can rapidly dissolve cellulose at concentrations between 30% to 60 wt% water. Interestingly, optimal dissolution occurs at intermediate concentrations and any excess water in these systems naturally evaporates, returning the solvent to the optimal dissolution conditions. The unique ability of this ionic liquid to dissolve cellulose in the presence of large amounts of water is not well understood. In this work, molecular dynamics simulations are used to provide a molecular-level understanding of this process. Mixtures of alkylphosphonium hydroxides and water were studied over the entire water concentration range to determine the character and behavior of the pure solvent. Additional simulations of the IL/water mixtures and cellulose were performed and analyzed to provide an in-depth view of ionic liquid structure, hydrogen bond dynamics and thermodynamic changes of state, providing insight into the molecular driving forces of this unique class of aqueous ionic liquids.