The effects of the position of the ether oxygen atom in pyrrolidinium-based room temperature ionic liquids on their physicochemical properties

Abstract

Room temperature ionic liquids (RTILs) containing ether oxygen atoms have been investigated for a gamut of science and technology applications owing to their superior physicochemical properties. However, the effect of the position of the ether oxygen atom in the side chain on their physicochemical properties is not clearly understood. This study investigates, using both experimental and computational approaches, the effect of ether oxygen atoms on the physicochemical properties of RTILs consisting of bis(trifluoromethylsulfonyl)amide (TFSA⁻) with 1-methyl-1-propylpyrrolidinium (MPP⁺), 1-butyl-1-methylpyrrolidinium (BMP⁺), 1-methoxymethyl-1-methylpyrrolidinium (MOMMP⁺), 1-ethoxymethyl-1-methylpyrrolidinium (EOMMP⁺), and 1-methoxyethyl-1-methylpyrrolidinium (MOEMP⁺). The viscosity of the RTILs containing the ether oxygen atom was lower than that of the alkyl analogues. Moreover, the viscosity of EOMMPTFSA was lower than that of MOEMPTFSA, albeit EOMMPTFSA and MOEMPTFSA have the same molecular weight with ether oxygen atoms at different positions. Ab initio calculations reveal that the number of methylene groups between nitrogen and oxygen atoms in the cation structure profoundly impacts the local stable structure of the cation. Furthermore, we discussed the relationship between the transport properties and the spatial distribution of ions obtained by MD simulations. This result will be valuable in the design of functionalized RTILs, via the judicious tuning of the conformational flexibility of ether oxygen atoms in related ionic liquids.