ATR and transmission IR spectroscopy have been used to investigate the state of water in room temperature ionic liquids (RTILs) based on the 1-alkyl-3-methylimidazolium cation with the anions: [PF6]−, [SbF6]−, [BF4]−, [ClO4]−, [CF3SO3]−, [(CF3SO2)2N]−, [NO3]− and [CF3CO2]−. It has been shown that in these RTILs water molecules absorbed from the air are present mostly in the “free”
(not self-associated) state, bound via H-bonding with [PF6]−, [BF4]−, [SbF6]−, [ClO4]−, [CF3SO3]−, [(CF3SO2)2N]− with the concentrations of dissolved water in the range 0.2–1.0 mol dm−3. It has been concluded that most of the water molecules at these concentrations exist in symmetric 1 : 2 type H-bonded complexes: anion...HOH...anion. Additional evidence that the preferred sites of interaction with water molecules are the anions has been obtained from the experiments with RTILs of the 1-butyl-2,3-dimethylimidazolium and 1-butyl-2,3,4,5-tetramethylimidazolium cations. Water molecules can also form associated liquid-like formations in RTILs with anions of stronger basicity such as [NO3]− and [CF3CO2]−. When these RTILs are exposed to air the water concentrations exceed 1.0 mol dm−3. The strength of H-bonding between water molecules and anions increases in the order [PF6]−<[SbF6]−<[BF4]−<[(CF3SO2)2N]−<[ClO4]−<[CF3SO3]−<[NO3]−<[CF3CO2]−. The energies of this H-bonding were estimated from spectral shifts, with the resulting enthalpies being in the range 8–13 kJ mol−1. ATR-IR spectroscopy has also been used to study H-bonding between methanol and RTILs.
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