Physicochemical Properties of Pyrrolidinium-based Room Temperature Ionic Liquids with Propoxyethyl, Ethoxypropyl, or (Methoxymethoxy)ethyl Group

Room temperature ionic liquids (RTILs) with ether oxygen atoms in the side chain have been investigated for electrochemical applications because they show favorable transport properties compared with alkyl analogs. However, the influence of the position and number of ether oxygen atoms on physical properties has not yet been fully clarified. In the present study, the physicochemical properties of RTILs consisting of bis(trifluoromethanesulfonyl)amide (TFSA-) with 1-methy-1-propoxyethylpyrrolidinium (Pyr1,2O3+), 1-methyl-1-ethoxypropylpyrrolidinium (Pyr1,3O2+), and 1-methyl-1-(methoxymethoxy)ethylpyrrolidinium (Pyr1,2O1O1+) was investigated using experimental and computational approach. Pyr1,2O3TFSA exhibited the lowest viscosity and highest ionic conductivity. Pyr1,2O1O1TFSA with two oxygen atoms in the side chain showed relatively high viscosity, indicating the physicochemical properties were affected sensitively by the position and number of ether oxygen atoms. Furthermore, by comparing with the structural isomers reported so far, we were able to systematically discuss the introduction effect of ether oxygen on the physicochemical properties of RTILs.