Temperature dependence of multilayering at the free surface of ionic liquids probed by X-ray reflectivity measurements

The effect of the temperature on the surface layering of ionic liquids has been studied for two ionic liquids, trioctylmethylammonium bis(nonafluorobutanesulfonyl)amide([TOMA(+)][C(4)C(4)N(-)]) and trihexyltetradecylphosphonium bis(nonafluorobutanesulfonyl)amide ([THTDP(+)][C(4)C(4)N(-)]), using X-ray reflectivity measurements at 285, 300, and 315 K. Both [TOMA(+)][C(4)C(4)N(-)] and [THTDP(+)][C(4)C(4)N(-)] develop multilayers at the surface. The structure of the multilayers at the [TOMA(+)][C(4)C(4)N(-)] surface shows little temperature-dependent change, whereas that at the [THTDP(+)][C(4)C(4)N(-)] surface clearly becomes diffused with increasing temperature. The different temperature dependence seems to be related to the difference in the recently reported ultraslow dynamics of the interfacial structure of [TOMA(+)][C(4)C(4)N(-)] and [THTDP(+)][C(4)C(4)N(-)] at the ionic liquid|water interface.