Fast cation dynamics in the crystalline state of an imidazolium-based room temperature ionic liquid due to the presence of a tiny amount of H2O

• Cation dynamics of a typical imidazolium-based room temperature ionic liquid (RTIL) in the crystalline state is investigated by 1H NMR spectroscopy.
• The RTIL with a slight amount of H2O (200 ppm) shows minor liquid-like component in the crystalline state, the fraction of which increases with increasing temperature.
• Cation in the liquid-like phase has faster dynamics than in the supercooled liquid state at the same temperature because the presence of H2O reduces local viscosity in the phase.

We report the results of a 1H NMR spectroscopic study of cation dynamics in the RTIL, 1-butyl-3-methylimidazolium bromide, in its crystalline state with two different but considerably low water contents, 30 ppm and 200 ppm. The results indicate the presence of a minor liquid-like (or solution) component in the crystalline state in samples with the higher water contents. The relative fraction of this “liquid-like” phase in the crystal is estimated to be ca. 1% around room temperature and increases with increasing temperature up to the melting (liquidus) point, the behavior of which is well explained in terms of a two component phase diagram. It is also indicated based on the lever rule that H2O molecules exist at substantially high concentration in the liquid-like phase compared to in the supercooled liquid state, which can significantly change both microscopic and macroscopic properties of the RTIL. 1H NMR spectral line width and spin–lattice relaxation time measurements indicate that the cations in the liquid-like phase are characterized by a rotational dynamics that is distinct from that of the supercooled liquid state with significantly higher mobility. Such high cation mobility results from the interaction between the cations and H2O molecules in the liquid-like phase that reduces the Coulombic interaction between the cations and anions and decreases local viscosity.