Dissolution Mechanism of Cellulose in Ionic Liquids: Understanding the Role of Protic and Aprotic Solvents by Molecular Dynamic Simulations

The present study reveals the dissolution behavior of cellulose in potential Ionic Liquids (ILs) and IL/cosolvent mixtures using both theoretical (Quantum Chemical and MD simulations) and subsequent its experimental... [ view full abstract ]

The present study reveals the dissolution behavior of cellulose in potential Ionic Liquids (ILs) and IL/cosolvent mixtures using both theoretical (Quantum Chemical and MD simulations) and subsequent its experimental validation. For converging upon the recommended IL, 1428 ILs consisting of 42 cations and 34 anions were studied with the Conductor like Screening MOdel for Real Solvents (COSMO-RS) model. Based on the Infinite Dilution Activity Coefficient (IDAC) of the components in IL, the selected anions and cations were then visualized by observing their interactions with cellulose using binding energies and natural bonding orbital (NBO) analysis. The quantum chemical calculations suggests that the anion plays a pronounced role as compared to the cation in the solvation process. Eventually, acetate [OAc]^– anion and 1-ethyl-3-methylimidazolium [EMIM]⁺ cation were found to be good candidates for the dissolution of cellulose. Further, two categories of cosolvents namely protic and aprotic solvents have been selected in order to enhance the cellulose dissolution in IL. The MD simulations were employed to calculate the non-bonded interaction energies and structural properties such as radial distribution functions for better understanding the dissolution mechanism. In the typical IL/cosolvent system, the anion of the IL was found to be strongly solvated by the protic solvents (Formamide and Acetamide) thereby resulting in decreasing cellulose solubility, whereas it gave contrary trend for aprotic solvents (DMSO, DMF, DMAc). On the other hand, the associated ion pairs of [Emim][OAc] was partially dissociated by the addition of cosolvent and aided it to produce more free ions (Figure 1). The dissolution of cellulose was essentially quantified by the cellulose-anion interactions by studying the non-bonded interaction energies (Figure 2). Furthermore the solid-liquid equilibria experiments were conducted for cellulose in IL and IL/cosolvent mixtures to confirm the simulated results (Figure 2). From MD simulations and experimental results, DMSO was found to be a best cosolvent for the enhancement of cellulose dissolution in IL.