Optical Second Harmonic Generation Study of the Structure of the Interface between Water and an Ionic Liquid Based on N-Alkylisoquinolinium Ions

Abstract

The orientation of cations at the interface between water and a room-temperature ionic liquid (RTIL) has been analyzed using optical second harmonic generation. To detect second harmonic light from the RTIL|water interface with high sensitivity, a RTIL, N -dodecylisoquinolinium bis(pentafluoroethylsulfonyl)imide, which is based on N -dodecylisoquinolinium (C12Iq+) having strong hydrophobicity and high hyperpolarizability, has been used. A polarization-dependence measurement of the light intensity of the second harmonic light has suggested that C12Iq+ is orientated at the RTIL|water interface with the isoquinolinium moiety tilting at 50° with respect to the surface normal, and with the dodecyl moiety protruding to the RTIL phase.