Effect of Alkyl Chain Branching on Physicochemical Properties of Imidazolium-Based Ionic Liquids

Lianjie Xue, Eshan Gurung, George Tamas, Yung P. Koh, Michael Shadeck, Sindee L. Simon, Mark Maroncelli, Edward L. Quitevis

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The branched ionic liquids (ILs) 1-(iso-alkyl)-3-methylimidazolium bis[(trifluoromethane)sulfonyl]amide ([(N-2)mCN-1C1im][NTf2] with N = 3-7) were synthesized and their physicochemical properties characterized and compared with the properties of linear ILs 1-(n-alkyl)-3-methylimidazolium bis[(trifluoromethane)sulfonyl]amide ([CNC1im][NTf2] with N = 3-7). For N = 4-7, the density of the branched IL [(N-2)mCN-1C1im][NTf2] is the same as that of its linear analogue [CNC1im][NTf2] within the standard uncertainty of the measurements. In the case of the N = 3 [1mC2C1im][NTf2]/[C3C1im][NTf2] pair, the density of the branched IL is 0.13% higher than that of the linear IL. For a branched/linear IL pair with a given N, the glass transition temperature Tg, melting temperature Tm, and viscosity n are higher for the branched IL than for the linear IL. [2mC3C1im][NTf2] is an exception in that its Tm is lower than that of [C4C1im][NTf2]. Moreover, the viscosity of [2mC3C1im][NTf2] is anomalously higher than what would be predicted based on the trend of the other branched ILs. These trends in the viscosities of the linear and branched ILs are consistent with recent molecular dynamics simulations. Thermal gravimetric analysis indicates that linear ILs are thermally more stable than branched ILs. Pulsed-gradient spin-echo (PGSE) NMR diffusion measurements show that the self-diffusion coefficients of the ions vary inversely with the viscosities according to the Stokes-Einstein (SE) equation. The hydrodynamic radii of the cations and anions of linear ILs calculated from the SE equation however are consistently higher than those of the corresponding branched ILs.

Original languageEnglish (US)
Pages (from-to)1078-1091
Number of pages14
JournalJournal of Chemical and Engineering Data
Issue number3
StatePublished - Mar 10 2016

All Science Journal Classification (ASJC) codes

  • General Chemistry
  • General Chemical Engineering


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