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.