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 Brønsted acidic hydrophilic ionic liquid (IL) 1-butyl-3-methylimidazolium trifluoroacetate ([Bmim][CF3CO2]) and a cosolvent, water or methanol, over the entire concentration range have been investigated by methods of attenuated total reflectance infrared spectroscopy, 1H 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.