Evidence for mesoscopic local structures in ionic liquids: CARS signal spatial distribution of Cnmim[PF6] (n = 4, 6, 8)

We measure the spatial distribution of the coherent anti-Stokes Raman scattering (CARS) signal from three room-temperature ionic liquids, 1-alkyl-3-methylimidazolium hexafluorophosphates, where alkyl is butyl, hexyl, and octyl. The spatial distribution pattern becomes narrower as the alkyl-chain length increases. This result suggests that, although these ionic liquids are uniform and transparent macroscopically, specific local structures are formed with their size increasing with the alkyl-chain length. From the broadening mechanism of the CARS spatial distribution, it is most likely that the size of local structures in these ionic liquids is in the range of several tens of nanometers.