Effect of small amount of water on the dynamics properties and microstructures of ionic liquids

A series of systems of 1-butyl-3-methylimidazolium acetate ([Bmim][Ac]), 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]), and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Bmim][Tf2N]) with a small amount of water were simulated. Viscosities of systems were obtained by nonequilibrium molecule dynamics simulation and the results show that the viscosities change in different ways: for [Bmim][BF4] and [Bmim][Tf2N], viscosities decrease rapidly in the first stage, and then decrease slowly with the increase of water content. But for [Bmim][Ac], the viscosities increase first and then decrease. The unique phenomenon of [Bmim][Ac] can be attributed to the formation of chain-like structure of anion center dot center dot center dot water center dot center dot center dot anion center dot center dot center dot. Hydrogen bond (HB) interaction between ion pairs is weakened, but the number of HB between water and anions increases with increase of water content. Besides, the microstructures of water in ionic liquids-water systems were compared and found that the distribution of water is more concentrated in [Bmim][Tf2N]-H2O system, while it is isotropy in [Bmim][Ac]-H2O system. (c) 2016 American Institute of Chemical Engineers AIChE J, 63: 2248-2256, 2017