The deviations from Stokes-Einstein hydrodynamics of small solutes are more pronounced in ionic liquids than in conventional solvents (J. Phys. Chem. B 2013 117 (39), 11697). Small neutral solutes diffuse much faster than expected, whereas small charged solutes diffuse much slower. This article attempts to establish a link between the local friction experienced by tracer solutes and the polar/apolar structure of ionic liquids. We find that small neutral solutes probe locally "stiff" (mostly charged, high electrostriction) regions and locally "soft" (mostly apolar, low electrostriction) regions. These regions of high and low friction are associated with cage and jump regimes. Enhanced neutral tracer mobility in the low friction regions associated with the cationic apolar component has an important bearing on the large positive deviations from Stokes-Einstein behavior. In contrast, diminished charged tracer mobility involves long caging dynamics separated by jump events often triggered by the loss and recovery of counterions.

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 N C1im]­[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 N C1im]­[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 η 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.