Mechanisms of hydrogen bond formation between ionic liquids and cellulose and the influence of water content

We study the dynamics of the formation of multiple hydrogen bonds between ionic liquid anions and cellulose using molecular dynamics simulations. We examine fifteen different ionic liquids composed of 1-alkyl-3-methylimidazolium cations ([C(n)mim], n = 1, 2, 3, 4, 5) paired with either chloride, acetate or dimethylphosphate. We map the transitions of anions hydrogen bonded to cellulose into different bonding states. We find that increased tail length in the ionic liquids has only a very minor effect on these transitions, tending to slow the dynamics of the transitions and increasing the hydrogen bond lifetimes. Each anion can form up to four hydrogen bonds with cellulose. We find that this hydrogen bond "redundancy'' leads to multiply bonded anions having lifetimes three to four times that of singly bound anions. Such redundant hydrogen bonds account for roughly half of all anion-cellulose hydrogen bonds. Additional simulations for [C(2)mim]Cl, [C(2)mim]Ac and [C(2)mim]DMP were performed at different water concentrations between 70 mol% and 90 mol%. It was found that water crowds the hydrogen bond-accepting sites of the anions, preventing interactions with cellulose. The more water that is present in the system, the more crowded these sites become. Thus, if a hydrogen bond between an anion and cellulose breaks, the likelihood that it will be replaced by a nearby water molecule increases as well. We show that the formation of these "redundant'' hydrogen bonding states is greatly affected by the presence of water, leading to steep drops in hydrogen bonding between the anions and cellulose.