(754f) An Investigation of Cellulose Solubility in Ionic Liquids with Added Cosolvents
The development of processes to convert cellulose, the most abundant organic substance on earth, into useful products that are currently made from nonrenewable resources is of the utmost importance for a green and sustainable future. Cellulose is an energy-rich resource but itâs relatively difficult to process since few organic solvents are capable of breaking apart its dense hydrogen bond network. In the past few years, ionic liquids (ILs) have been shown to be efficient at solubilizing cellulose and shown a lot of potential due to the ability to modify the properties of the solvent by varying the constituent cations and anions. Even so, the adoption of ILs in processes has been slowed by their prohibitive costs and mass transport issues. Recently, however, it was shown that the addition of polar aprotic cosolvents can substantially alleviate these mass transport issues while reducing the amount of the costly IL required for dissolution. Furthermore, it was shown that in some cases these cosolvents can significantly improve cellulose solubility and even the water tolerance of the solvents. The precise mechanisms by which this solubility enhancement occurs and however remains unclear. In this study, molecular dynamics simulations are used to study the role that cosolvents play in the enhancement of cellulose solubility. Separate simulations employing either crystalline or dissolved strands of cellulose with ionic liquid, cosolvent and, in some cases, water were performed to understand the underlying molecular driving forces of dissolution and how varying the proportions of these substituents can influence the thermodynamics of the process. Using these two state simulations we quantify the changes in hydrogen bonding, interfacial energy and the free energy of dissolution using the two phase thermodynamic method. We find that the calculated changes agree quite well with the experimentally determined phase diagram and provide a detailed physical picture for how the addition of moderate amounts of cosolvent enhance cellulose solubility in neat and aqueous ionic liquids.