Water content and related physical properties of aliphatic quaternary ammonium imide-type ionic liquid containing metal ions

The ionic liquid, trimethyl-n-hexylammonium bis((trifluoromethyl)sulfonyl)amide (TMHA-Tf₂N), has a wide electrochemical window of more than 5 V and is considered to be hydrophobic because of two -CF₃ groups in its Tf₂N^- anion. However, a small amount of water remains in the ionic liquid even after dehydration in vacuo, which causes some problems in metal electrodeposition when using the ionic liquid as a solvent. We measured the water content of the ionic liquids, TMHA-Tf₂N containing M(Tf₂N)_n (M=H, Li, Mg, Ni, Cu, Zn, La, and Dy; n=1, 2, or 3), as well as their kinematic viscosity and electrical conductivity in the temperature range of 25-130 °C. Furthermore, differential scanning calorimetry was performed for these ionic liquids to find their glass transition temperature, crystallization temperature, and melting temperature. The water content of TMHA-Tf₂N containing M(Tf₂N)_n salts decreased with an increase in temperature. At the same time, the kinematic viscosity decreased and the electrical conductivity increased. By measuring the optical absorption spectrum, it was found that the metal ions in TMHA-Tf₂N were hydrated. The addition of M(Tf₂N)_n to TMHA-Tf₂N, increased the water content at a constant temperature, which resulted in slight increases in the kinematic viscosity and decrease in the electrical conductivity. A Walden-like plot of the electrical conductivities against the kinematic viscosities, measured over various temperatures and water contents, gave a single straight line.