Orientation of 1-Dodecyl-4-phenylpyridinium Ions Constituting an Ionic Liquid at the Ionic Liquid|Water Interface Studied by Second Harmonic Generation

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|water and RTIL|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|air interface are orientated with the main axis of 4-phenylpyridinium moiety (from phenyl to pyridinium direction) tilted at 53° from the surface normal (from RTIL to air direction) and that the dodecyl chain is protruding to the air. At the RTIL|water interface, it was suggested that C 12 ppy + is orientated at the tilt angle of 131° from the surface normal (from RTIL to water direction) with the dodecyl chain protruding away from the aqueous phase.