Proton transport properties in an ionic liquid having a hydroxylgroup

Abstract

An ionic liquid having a hydroxyl group, choline bis(trifluoromethylsulfonyl)amide ([N_111(2OH)][N(Tf)₂]), was synthesized to investigate the effect of hydroxyl groups on the proton transport. 1,1,1-Trifluoro-N-(trifluoromethylsulfonyl)methanesulfoneamide (HN(Tf)₂) as a proton source was mixed with the choline derivative at various molar ratios. Their thermal properties, viscosities, and ionic conductivities were investigated. [N_111(2OH)][N(Tf)₂] showed a melting point at 27 °C, and its thermal stability was higher than 400 °C. The viscosity of [N_111(2OH)][N(Tf)₂]/HN(Tf)₂ mixtures increased as the acid molar fraction increased. The ionic conductivity of [N_111(2OH)][N(Tf)₂] was 2.1 × 10⁻³ S cm⁻¹ at 25 °C; the ionic conductivity monotonously decreased as the acid molar fraction increased. There was a clear correlation between the ionic conductivity and the viscosity for the mixtures of the choline derivative and the acid. PFG-NMR measurements were carried out to investigate the diffusion behavior of protons. Although the acid and the hydroxyl group were indistinguishable by ¹H NMR, the self-diffusion coefficient of the ¹H of the hydroxyl group and the acid was larger than those of other ¹H nuclei. This difference suggests that a fast intermolecular proton transfer exists between the hydroxyl group and the acid.