(592e) A Molecular Dynamics Investigation of Enhanced Cellulose Solubility in Ionic Liquids with Added Cosolvents

A number of ionic liquids (ILs) have proven to be exceptional solvents of cellulose with significant advantages over today's conventional solvents. High material costs, energy intensive solvent recovery and limited mass transport in the resultant solutions, however, have slowed their adoption in industrial processes. It has been shown that the addition of cosolvents such as DMSO and DMF, to name just a few, can substantially alleviate these mass transport issues while reducing the amount of ionic liquid that is required for dissolution. In some cases, it has been shown that adding cosolvents not only modifies the rheology of solutions, allowing dissolution to take place faster, but that it can actually enhance solubility. Furthermore, for ILs with small amounts of dissolved water, the addition of these same cosolvents can significantly improve solubility. The precise mechanisms by which these cosolvents interact with the other components of the system, leading to this solubility enhancement still remains unclear. In this study explicit, all-atom molecular dynamics simulations are used to study a select number of points on experimentally determined liquid-solid phase diagram. Simulations are performed for both the crystalline and dissolved states of cellulose with different solvent proportions and the thermodynamics of decrystallization calculated. Entropy changes are calculated using the Two-Phase Thermodynamic method allowing us to calculate the Gibb's free energy of dissolution. We find that the calculated changes agree quite well with the experimental phase diagram. Together with an analysis of hydrogen bonding changes and the three dimensional solvent structure, we provide a detailed physical picture for how the addition of moderate amounts of cosolvent enhance cellulose solubility in neat and aqueous ionic liquids.