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

Abstract

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.

Specific Solvation of Benzyl Methacrylate in 1-Ethyl-3-methylimidazolium Bis(trifluoromethanesulfonyl)amide Ionic Liquid

“…4 The ion-ion interaction in ILs strongly depends on the composing ions (cation and/or anion), the alkylchain length of the cation and the hydrogen bonding capability to give a specific solvation environment of such solutes as metal ion, low-weight molecule and polymer in the ILs. [5][6][7][8] The miscibility of polymer and solvent IL varies as the temperature changes, and polymer/IL solutions often exhibit an upper critical solution temperature (UCST) or lower critical solution temperature (LCST) phase behavior. 9 It is well-known that poly(N-isopropylacrylamide) (PNIPAm) exhibits the LCST phase behavior at near room-temperatures in aqueous solutions.…”

Design and Materialization of Ionic Liquids Based on an Understanding of Their Fundamental Properties

“…Unfortunately, the addition of lithium salts into ILs significantly increases the viscosity of the mixtures because the strongly Lewis acidic Li + cations require strong coordination by the anions to form bulky ion-multiplets. [85][86][87][88] Furthermore, the Li + -transference number is low due to the presence of at least two different cations (Li + and IL cation) and one anion (the common anion of Li-salt and IL). [85][86][87][88] The Li + -conducting IL electrolytes may improve the safety issue of lithium batteries, but their performance is not good due to poor transport properties.…”

Structural Aspect on Li Ion Solvation in Room-Temperature Ionic Liquids

“…The cation-cation correlations are also found at around 8 Å in the G MD inter (r), although they are significantly weak in the intensity. This is because [C 2 mim] + is constructed by H, C, and N atoms with small X-ray scattering factor, f, unlike [FSI] − with large ones of F and S. It is thus concluded that [36] and [62] the anion-anion correlations mainly contribute to the 8.5 Å peak observed in the total G exp inter (r), whereas the cation-cation ones less contribute to the total G exp inter (r). With regard to the first neighboring cation-anion interaction, we further analyzed the MD results to evaluate space distribution function, SDF.…”