Molecular states of water in room temperature ionic liquidsElectronic Supplementary Information available. See http://www.rsc.org/suppdata/cp/b1/b106900d/

TLDR

ATR and transmission IR spectroscopy were used to probe water and methanol interactions in a series of 1‑alkyl‑3‑methylimidazolium‑based room‑temperature ionic liquids with diverse anions. Water absorbed from air in these ionic liquids exists mainly as free molecules forming symmetric 1:2 H‑bonded complexes with the anions, with H‑bond strength increasing from [PF6]⁻ to [CF3CO2]⁻ and enthalpies of 8–13 kJ mol⁻¹, while more basic anions such as [NO3]⁻ and [CF3CO2]⁻ allow higher water concentrations and liquid‑like associations.

Abstract

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.