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Issue 23, 2001
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Molecular states of water in room temperature ionic liquids

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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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Publication details

The article was received on 31 Jul 2001, accepted on 03 Oct 2001 and first published on 26 Nov 2001

Article type: Paper
DOI: 10.1039/B106900D
Citation: Phys. Chem. Chem. Phys., 2001,3, 5192-5200
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    Molecular states of water in room temperature ionic liquids

    L. Cammarata, S. G. Kazarian, P. A. Salter and T. Welton, Phys. Chem. Chem. Phys., 2001, 3, 5192
    DOI: 10.1039/B106900D

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