Dielectric relaxation spectra of three members of the alkylammonium formate family of protic ionic liquids (PILs), namely, ethylammonium formate (EAF), n-butylammonium formate (BuAF), and n-pentylammonium formate (PeAF), as well as the pseudo-PIL triethylamine + formic acid (molar ratio 1:2; TEAF) have been studied over a wide frequency (50 MHz to 89 GHz) and temperature range (5–65 °C), complemented by measurements of their density, viscosity, and conductivity. It turned out that the dominating relaxation of EAF, BuAF, and PeAF arises from both cation and anion reorientations which are synchronized in their dynamics due to hydrogen bonding. Amplitudes and relaxation times of this mode reflect the—compared to nitrate—different nature of H bonding between the formate anion and ethylammonium cation, as well as increasing segregation of the PIL structure into polar and non-polar domains. The TEAF data suggest that its dominating relaxation is due to the rotation of the complex triethylamine⋅(formic acid)2 in which no significant proton transfer to an ion pair occurred. Weak dissociation of this complex into ions was postulated to account for the high conductivity of TEAF.
Closer inspection of the NMR spectrum published by Patil and Dagade (their Fig. S1) revealed very narrow integration intervals for their signals at 8.086 ppm and 11.869 ppm.24 Application of these intervals to the corresponding peaks of our spectrum reproduced the intensity ratio of Ref. 24 [Fig. S2(b)]. However, this choice of integration limits is not justified.
After immersion of the sample-filled cell in the thermostat, T was increased in 10 K steps from 278.15 to 338.15 K with ∼30 min equilibration time for each step. Thus, samples were exposed to prolonged thermal stress in these experiments.
Defined here by the data of a 1M aqueous KCl solution, which was assumed to be fully dissociated.