Water content and properties of aliphatic ammonium imide-type room temperature ionic liquid containing metal ions

The ionic liquid, trimethyl-n-hexylammonium bis((trifluoromehyl)sulfonyl) amide (TMHA-Tf₂N), with a wide electrochemical window of more than 5 V, 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. Therefore, we measured the water content of the ionic liquids, TMHA-Tf₂N and TMHA-Tf₂N containing M(Tf₂N)_n(M = Li, Mg, Ni, Zn, Al, La, and Dy; n = 2, 3), as well as their kinematic viscosities and electrical conductivities in the temperature range of 25 to 130°C. The water content of TMHA-Tf₂N decreased with an increase in temperature from about 2300 ppm at 25°C to about 200 ppm at 130°C when water vapor pressure of the atmosphere was kept constant at 1600 Pa. At the same time, the kinematic viscosity decreased and the electrical conductivity increased. On the other hand, the water content of TMHA-Tf₂N containing M(Tf ₂N)_n also decreased with an increase in temperature. 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 electrical conductivity. Walden's plot of the conductivities against the viscosities, measured over various temperatures and water contents, gave a single straight line.