Simulations Reveal Conformational Changes of Methylhydroxyl Groups during Dissolution of Cellulose Iβ in Ionic Liquid 1-Ethyl-3-methylimidazolium Acetate

In this work, we use molecular dynamics (MD) simulations to study the dissolution of microcrystalline cellulose in the ionic liquid 1-ethyl-3-methylimidazolium acetate (abbreviated as [C2mim][OAc]) at 20 wt % loading. The interactions of [C(2)mim][OAc] with the I-beta cellulose structure at 120 degrees C were studied. The results show that both the cation and the anion of [C(2)mim][OAc] penetrate into the cellulose I-beta crystal structure but that the anion in particular forms strong hydrogen bonds with cellulose. Our results also show that the methylhydroxyl groups of cellulose solvated in [C(2)mim][OAc] are predominantly in the gauche-trans (gt) conformation, in contrast to the dominant trans-gauche (tg) conformation of cellulose I-beta in air or the gauche gauche (gg) conformation for cellulose chains in water or after pretreatment with ammonia. Because the gt conformation is found mainly in cellulose II, these simulations suggest that regenerated cellulose under similar pretreatment conditions is composed mainly of cellulose II, and this result was confirmed by X-ray diffraction of samples processed under similar pretreatment conditions. These simulations provide new insight into the efficacy of [C(2)mim][OAc] pretreatment, suggesting that [C(2)mim][OAc] interacts with and biases the methylhydroxyl groups of cellulose toward orientations that are consistent with the experimentally observed more easily hydrolyzed cellulose II.