The Effects of Ionic Liquid Structure on the Thermodynamics of Cellulose Dissolution

A big challenge in the production of lignocellulosic biofuels lies in the selection of an optimal pretreatment solvent. A number of ionic liquids (ILs) have been identified that can dissolve cellulose effectively and many more are theoretically possible. Tailoring these solvents has the potential to increase performance and reduce costs, however without a much deeper understanding of the structure-property relationships within ILs this remains a difficult task. Using molecular dynamics we systematically examine the effect of chemical structure on the free energy of dissolution of crystalline bundles of cellulose. We focus on 15 different ionic liquid combinations and calculate the thermodynamic differences between the crystalline and dissolved states. Anions of 3 different sizes are paired with 5 different imidazolium-based cations with differing tail length. Entropies are calculated using the two-phase thermodynamic (2PT) method, allowing us to identify each of the contributions arising from the anion, the cation and cellulose as well as their translational, rotational and vibrational contributions. In doing so we show how changes to the chemical structure affects the overall energy and enthalpy as well as quantify the role that solvent entropy plays in the dissolution process. The free energy differences obtained from the simulations agree well with experimental solubility measurements and we show how these methods can be used to accurately predict solubilities a priori.