Comparative Study on Physicochemical Properties of Protic Ionic Liquids Based on Allylammonium and Propylammonium Cations

Abstract

Protic ionic liquids (PILs) were prepared by neutralization of a series of allylamines (allyldimethylamine, allyldiethylamine, and diallylmethylamine) with trifluoromethanesulfonic acid, in order to achieve highly conductive PILs, and their physicochemical properties, including thermal and transport properties, were compared with PILs from a series of propylamines (dimethylpropylamine, diethylpropylamine, and methyldipropylamine). The melting point and ionic conductivity of the allylammonium-based PILs were lower and higher than those of the corresponding propylammonium-based PILs having cations with the same number of carbon atoms, respectively. The origin of the difference in the properties of these two series of PILs was investigated based on the evaluation of the molar concentration, self-diffusion coefficient of the cations and anions, and ionicity. The molar concentration and ionicity of the allylammonium-based PILs were higher than those of the corresponding propylammonium-based PILs, whereas the diffusion coefficient was comparable for both sets of PILs. Ab initio geometrical optimization and calculation of the stabilization energies ( E form ) of the ion pairs of the PILs indicated higher stability of the conformations in which the N–H proton in the cation interacts with the S═O oxygen in the anion via hydrogen bonding interaction, relative to conformations in which such interaction is absent. Further comparison with the ion pairs of tetra-alkylammonium-based aprotic ILs (AILs) having cations with the same number of carbon atoms demonstrated that the interactions in the ion pairs of the PILs were stronger and had higher directionality than those of the AILs. However, the magnitude of the attraction and the directionality of the interactions in the ion pairs of the allylammonium-based PILs did not differ appreciably from those of the propylammonium-based counterparts. It is concluded that the allyl group facilitates the formation of more compact structures of the PILs without enhancing the interionic interactions with consequently higher molar concentration, thereby increasing the ionic conductivity of the allylammonium-based PILs due to an increase in the number of carrier ions. The viscosity increased and the ionicity decreased with increasing numbers of carbon atoms in the alkyl chain of the ammonium cations, due to an increase in dispersion interactions. Thus, among the PILs explored in this study, allyldimethylammonium trifluoromethanesulfonate ([N11a][TfO]) exhibited the highest ionic conductivity of 75 mS·cm –1 at 150 °C, which is the highest conductivity of PILs reported to date, to the best of our knowledge.