Mechanistic insights into cellulose dissolution in deep eutectic solvents from classical MD simulations

There is increasing interest in solubilizing cellulose in deep eutectic solvents (DESs) an emerging class of green solvents. Despite cellulose's general insolubility in DESs according to experimental data, some deep eutectic solvents can dissolve it to a limited extent. However, the molecular mechanisms behind this solubility are unclear, and understanding them is crucial for designing DESs with enhanced cellulose-dissolving capabilities. Here, we employed classical molecular dynamics simulations to explore the molecular mechanism of cellulose solubility in oxaline (choline chloride/oxalic acid) as a prototypical carboxylic acid-based DES, and in reline (choline chloride/urea) the most widely studied DES. The molecular structure of DESs with and without cellulose were studied. Both DESs similarly affect cellulose at room temperature, but at 383 K, reline more effectively separates cellulose chains then oxaline. The main mechanism is the cooperative interaction of cellulose chains with chloride anions and urea molecules, which together promote extension of the cellulose structure; in addition, we propose a molecular mechanism detailing how urea and chloride ions interact with cellulose chains to enable dissolution.