Optical second harmonic generation study of the structure of the interface between water and an ionic liquid based on N-alkylisoquinolinium ions

The orientation of cations at the inter-face 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 (C(12)Iq(+)) 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 C(12)Iq(+) is orientated at the RTIL\water interface with the isoquinolinium moiety tilting at 50 degrees with respect to the surface normal, and with the dodecyl moiety protruding to the RTIL phase.