Orientation of 1-dodecyl-4-phenylpyridinium ions constituting an ionic liquid at the ionic liquid vertical bar water interface studied by second harmonic generation

摘要

A hydrophobic room-temperature ionic liquid (RTIL) of 1-dodecyl-4-phenylpyridinium (C(12)ppy(+)) ions, the cations with high first hyperpolarizability, was prepared in combination with bis(pentafluoroethylsulfonyl)imide, and the orientation of C(12)ppy(+) at the RTIL vertical bar water and RTIL vertical bar air interfaces was analyzed using optical second harmonic generation (SHG). From polarization-dependence measurements and absolute phase measurements, it was found that C(12)ppy(+) ions at the RTIL vertical bar air interface are orientated with the main axis of 4-phenylpyridinium moiety (from phenyl to pyridinium direction) tilted at 53 degrees from the surface normal (from RTIL to air direction) and that the dodecyl chain is protruding to the air. At the RTIL vertical bar water interface, it was suggested that C(12)ppy(+) is orientated at the tilt angle of 131 degrees from the surface normal (from RTIL to water direction) with the dodecyl chain protruding away from the aqueous phase.