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

The branched ionic liquids (ILs) 1-(iso-alkyl)-3-methylimidazolium bis[(trifluoromethane)sulfonyl]amide ([(N-2)mC_N-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 ([C_NC₁im][NTf₂] with N = 3-7). For N = 4-7, the density of the branched IL [(N-2)mC_N-1C₁im][NTf₂] is the same as that of its linear analogue [C_NC₁im][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 T_g, melting temperature T_m, and viscosity n are higher for the branched IL than for the linear IL. [2mC₃C₁im][NTf₂] is an exception in that its T_m 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.