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(+)] [C4C4N-]) and trihexyltetradecylphosphonium bis(nonafluorobutanesulfonyl)amide ([THTDP+][C4C4N-]), using X-ray reflectivity measurements at 285,300, and 315 K. Both [TOMA(+)] [C4C4N-] and [THTDP+][C4C4N-] develop multilayers at the surface. The structure of the multilayers at the [TOMA(+)][C4C4N-] surface shows little temperature-dependent change, whereas that at the [THTDP+][C4C4N-] 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(+)] [C4C4N-] and [THTDP+][C4C4N-] at the ionic liquid vertical bar water interface.