A theoretical investigation of the interactions between water molecules and ionic liquids

Quantum chemical calculations have been used to investigate the interaction between water molecules and ionic liquids based on the imidazolium cation with the anions [Cl-], [Br-], [BF4-], and [PF6-]. The predicted geometries and interaction energies implied that the water molecules interact with the Cl-, Br-, and BF4- anions to form X-center dot center dot center dot W (X = Cl or Br, W = H2O), 2X(-)center dot center dot center dot 2W, BF4-center dot center dot center dot W, and W center dot center dot center dot BF4-center dot center dot center dot W complexes. The hydrophobic PF6- anion could not form a stable complex with the water molecules at the density functional theory (DFT) level. Further studies indicate that the cation could also form a strong interaction with the water molecules. The 1-ethyl-3-methylimidazolium cation (Emim(+)) has been used as a model cation to investigate the interaction between a water molecule and a cation. In addition, the interaction between the ion pairs and the water was studied by using 1-ethyl-3-methylimidazolium chloride (Emim center dot Cl) as a model ionic liquid. The strengths of the interactions in these categories follow the trend anion-W > cation-W > ion pair-W.