Hydrogen Bonding Behaviors of Binary Systems Containing the Ionic Liquid 1-Butyl-3-methylimidazolium Trifluoroacetate and Water/Methanol

The hydrogen-bonding properties of binary systems consisting of a representative Bronsted acidic hydrophilic ionic liquid (IL) 1-butyl-3-methylimidazolium trifluoracetate ([Bmim] [CF3CO2]) and a cosolvent, water or methanol, over the entire concentration range have been investigated by methods of attenuated total reflectance infrared spectroscopy, H-1 NMR spectroscopy, and quantum chemical calculations. It has been found that the hydrogen-bonding interactions between the anion [CF3CO2](-), rather than the cation, and the cosolvent molecules are dominant at low concentration of cosolvent. The H-bond interaction site between the IL anion and water/methanol is the O atom in the -COO group, while the -CF3 group makes a positive contribution by donating electron to the carboxylic group, forming a cooperative hydrogen-bonding system. For the cation [Bmim](+), although the C2-H is the favorable proton donor in H-bonding interactions, the water/methanol molecules form H-bonds with the alkyl C-H at low water/methanol concentration due to the stronger interaction between C2-H and [CF3CO2](-). Interestingly, we found that the interaction between methanol and the IL is stronger than that between water and the IL because the methyl group in methanol has a positive contribution to the formation of H-bonds. The following sequential order of interaction strength is established: [Bmim](+)-methanol-[CF3CO2](-) > [Bmim](+)-water-[CF3CO2](-) > [Bmim](+)-[CF3CO2](-) > [CF3CO2](-)-methanol > [CF3CO2](-)-water > [Bmim](+)-methanol > [Bmim](+)-water.