Sweet Spot in the Carbon Numbers of Tetraalkylammonium-Based Ionic Liquids for Cellulose Dissolution

Abstract Imidazolium-based ionic liquids/salts have attracted attention as cellulose solvents; however, the structural requirements of ammonium-based ionic liquids/salts for efficient cellulose dissolution remain poorly understood. Here, we established design guidelines for ammonium-based ionic liquids/salts by examining alkyl chain lengths of tetraalkylammonium acetate ([Nm,m,m,n]OAc). The results were categorized by the total carbon number of the cation alkyl chains. The sweet spot of the carbon number ranges from 11 to 16, corresponding to cellulose solubility values of ≥12.5 wt %. Lower carbon numbers (4–10) reduced cellulose solubility (≤5 wt %), likely because of cellulose chain association or strong cation–anion interactions, which decrease the relative effective anion ratio, a concept newly introduced in this study. Excessively large carbon numbers (≥17) also reduced cellulose solubility (≤10 wt %), because lower ion density and cation aggregation similarly reduced the relative effective anion ratio. Based on this guideline, an appropriate [Nm,m,m,n]OAc dissolved more than 10 wt % cellulose.