Impact of nanostructuration on the transport properties of ionic liquids

The impact of nanostructuration on the transport properties of ionic liquids (ILs) was explored by a systematic and high-resolution study of the temperature dependence of the viscosity and electrical conductivity of seven ILs homolog series: [C n C 1 im]BF 4 , [C n C 1 im]PF 6 , [C n C 1 im][OTf], [C n C 1 im][FAP], [C n C 1 im][NTf 2 ], [C n C 1 pyr][NTf 2 ], [C n py][NTf 2 ]. The increase of the alkyl chain length was found to increase the viscosity and decrease the molar conductivity due to a reduction of the overall mobility of the liquid and enhancement of the van der Waals interactions. The temperature dependency of transport properties was fitted to the Vogel-Fulcher-Tammann equation (VFT), and the energy barrier and pre-exponential coefficients were derived. The obtained results highlight the trendshift ( n = 6–7) in the profile of the transport properties, which is a reflection of the intensification of nanostructuration and describes the transition from a liquid with a strong ionic character to a nanostructured liquid dominated by the hydrophobic domain. The derived energy barriers were found to correspond to around 0.2–0.35 of the cohesive interactions of the ionic liquids, with the spherical anions BF 4 − and PF 6 − showing a higher fraction than the more stretched and larger anions, such as NTf 2 . This fraction was found to not be affected by the alkyl chain length. The increase of the nonpolar region was also reflected in a more pronounced deviation from the ideal Walden relation. This highlights the increased complexity of the electric conductivity when compared with viscosity due to the heterogeneity of charge distribution, revealing the impact of ionic surface-volume ratio and anion-cation size ratio.