The role of the cation in the solvation of cellulose by imidazolium-based ionic liquids.

We present a systematic molecular dynamics study examining the roles of the individual ions of different alkylimidazolium-based ionic liquids in the solvation of cellulose. We examine combinations of chloride, acetate, and dimethylphosphate anions paired with cations of increasing tail length to elucidate the precise role of the cation in solvating cellulose. In all cases we find that the cation interacts with the nonpolar domains of cellulose through dispersion interactions, while interacting electrostatically with the anions bound at the polar domains of cellulose. Furthermore, the structure and dimensions of the imidazolium head facilitate the formation of large chains and networks of alternating cations and anions that form a patchwork, satisfying both the polar and nonpolar domains of cellulose. A subtle implication of increasing tail length is the dilution of the anion concentration in the bulk and at the cellulose surface. We show how this decreased concentration of anions in the bulk affects hydrogen bond formation with cellulose and how rearrangements from single hydrogen bonds to multiple shared hydrogen bonds can moderate the loss in overall hydrogen bond numbers. Additionally, for the tail lengths examined in this study we observe only a very minor effect of tail length on the solvation structure and overall interaction energies.