Effect of Cation Symmetry on the Morphology and Physicochemical Properties of Imidazolium Ionic Liquids

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• 作者：Wei Zheng ; Ali Mohammed ; Larry G. Hines ; Jr. ; Dong Xiao ; Omar J. Martinez ; Richard A. Bartsch ; Sindee L. Simon ; Olga Russina ; Alessandro Triolo ; Edward L. Quitevis
• 刊名：Journal of Physical Chemistry B
• 出版年：2011
• 出版时间：May 26, 2011
• 年：2011
• 卷：115
• 期：20
• 页码：6572-6584
• 全文大小：1247K
• 年卷期：v.115,no.20(May 26, 2011)
• ISSN：1520-5207

文摘

In this paper, the morphology and bulk physical properties of 1,3-dialkylimidazolium bis{(trifluoromethane)sulfonyl}amide ([(C_N/2)₂im][NTf₂]) are compared to that of 1-alkyl-3-methylimidazolium bis{(trifluoromethane)sulfonyl}amide ([C_N鈥?C₁im][NTf₂]) for N = 4, 6, 8, and 10. For a given pair of ionic liquids (ILs) with the same N, the ILs differ only in the symmetry of the alkyl substitution on the imidazolium ring of the cation. Small-wide-angle X-ray scattering measurements indicate that, for a given symmetric/asymmetric IL pair, the structural heterogeneities are larger in the asymmetric IL than in the symmetric IL. The correlation length of structural heterogeneities for the symmetric and asymmetric salts, however, is described by the same linear equation when plotted versus the single alkyl chain length. Symmetric ILs with N = 4 and 6 easily crystallize, whereas longer alkyl chains and asymmetry hinder crystallization. Interestingly, the glass transition temperature is found to vary inversely with the correlation length of structural heterogeneities and with the length of the longest alkyl chain. Whereas the densities for a symmetric/asymmetric IL pair with a given N are nearly the same, the viscosity of the asymmetric IL is greater than that of the symmetric IL. Also, an even鈥搊dd effect previously observed in molecular dynamics simulations is confirmed by viscosity measurements. We discuss in this paper how the structural heterogeneities and physical properties of these ILs are consistent with alkyl tail segregation.