Liquid Structure of and Li+ Ion Solvation in Bis(trifluoromethanesulfonyl)amide Based Ionic Liquids Composed of 1-Ethyl-3-methylimidazolium and N-Methyl-N-propylpyrrolidinium Cations

Liquid structures of the bis (trifluoromethanesulfonyl) amide based ionic liquids composed of 1-ethyl-3-methylimidazolium and N-methyl-N-propylpyrrolidinium (C (2) mIm (+) TFSA (-) and C (3) mPyrro (+) TFSA (-), respectively) and Li (+) ion solvation structure in their lithium salt solutions were studied by means of high-energy X-ray diffraction (HEXRD) technique with the aid of MD simulations. With regard to neat ionic liquids, a small but significant difference was found at around 3. 5 Å in the intermolecular radial distribution functions G (inter) (r) s for these two ionic liquids; i. e. , G (inter) (r) for C (2) mIm (+) TFSA (-) was positioned at a slightly shorter region relative to that for C (3) mPyrro (+) TFSA (-), which suggests that the nearest neighboring cation-anion interaction in the imidazolium ionic liquid is slightly greater than that in the other. With regard to Li (+) ion solvation structure, G (inter) (r) s for C (2) mIm (+) TFSA (-) dissolving Li (+) ion exhibited additional small peak of about 1. 9 Å attributable to the Li (+) -O (TFSA (-) ) atom-atom correlation, though the corresponding peak was unclear in C (3) mPyrro (+) TFSA (-) due to overlapping with the intramolecular atom-atom correlations in C (3) mPyrro (+). In addition, the long-range density fluctuation observed in the neat ionic liquids diminished with the increase of Li (+) ion concentration for both ionic liquid solutions. These observations indicate that the large scale Li (+) ion solvated clusters are formed in the TFSA based ionic liquids, and well support the formation of Li (TFSA) (2) (+) cluster clarified by previous Raman spectroscopic studies. MD simulations qualitatively agree with the experimental facts, by which the decrease in the long-range oscillation amplitude of r (2) G (r) - 1 for the Li (+) containing ionic liquids can be ascribed to the variation in the long-range anion-anion correlations caused by the formation of the Li (+) ion solvated clusters.