Abstract
In this work we report on the evolution of the structure of two model ionic liquid families, N-alkyl-N-methylpyrrolidinium (Pyr1n-TFSI) and 1-alkyl-3-methylimidazolium (CnMIm-TFSI) (n = 3, 4, 6 and 8) both containing the bis(trifluoromethanesulfonyl)imide (TFSI) anion, upon the addition of LiTFSI using small angle X-ray scattering (SAXS). The introduction of a lithium salt (Li-salt) tunes the interactions through the substitution of the large cation in the ionic liquid with the small and charge localized lithium ion, thus increasing the coulombic contribution from ion-ion interactions. We find that the introduction of lithium ions results in a restructuring of the polar groups in the ionic liquids. These changes are manifested as an increase in the correlation lengths related to charge alternation of the ions and a more disordered structure. This restructuring is interpreted as a reconfiguration of the anions as they coordinate to the small and ionic lithium. In contrast, the length scale of the mesoscopic heterogeneities related to the clustering of alkyl chains is virtually unchanged with lithium doping. Moreover, the correlation corresponding to alkyl chain domains becomes more well defined with increasing salt concentration, suggesting that Li-salt doping, i.e. an increased columbic interaction in the system, promotes clustering of the alkyl tails.
Highlights
Tailoring the properties of ionic liquids is typically achieved by the synthesis of new anions and cations or by the addition of functional groups to the base ions
In this work we report on the influence of lithium salt (Li-salt) doping on the structural heterogeneities in two model ionic liquids based on pyrrolidinium (Pyr) and imidazolium (Im) cations and the TFSIÀ ([N(CF3SO2)2]À) anion by small angle X-ray scattering (SAXS) experiments as a function of salt concentration (x[LiTFSI]/(1Àx)[Ionic liquids (ILs)] 0 o x o 0.4 x = mole fraction) and length of the alkyl chains on the cations (CnMImTFSI and Pyr1n-TFSI with n = 3, 4, 6 and 8)
In order to better quantify the effects of the addition of the Li-salt on the structure of the ionic liquids, the SAXS patterns were fitted with 3 Lorentzian functions accounting for the prepeak, the charge alternation peak, and the peak associated with inter- and intra-molecular distances, just entering the measured window at high Q, and a linear baseline to account for the background
Summary
We find that the introduction of lithium ions results in a restructuring of the polar groups in the ionic liquids. These changes are manifested as an increase in the correlation lengths related to charge alternation of the ions and a more disordered structure. This restructuring is interpreted as a reconfiguration of the anions as they coordinate to the small and ionic lithium. The correlation corresponding to alkyl chain domains becomes more well defined with increasing salt concentration, suggesting that Li-salt doping, i.e. an increased columbic interaction in the system, promotes clustering of the alkyl tails
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