Observation of surface structure of 1-butyl-3-methylimidazolium hexafluorophosphate using high-resolution Rutherford backscattering spectroscopy.

The surface structure of 1-butyl-3-methylimidazolium hexafluorophosphate is studied by high-resolution Rutherford backscattering spectroscopy (HRBS) at room temperature. Elemental depth profiles are derived from the observed HRBS spectrum through spectrum simulation. While the obtained carbon profile has a sharp peak at the surface, the nitrogen profile shows a broader peak at a depth approximately 0.3 nm. These observations indicate that the butyl chain protrudes from the bulk liquid to the vacuum at the surface. The profiles of phosphorous and fluorine also have a broad peak at almost the same depth as the nitrogen profile, indicating that the anions are located near the imidazolium rings. These results are in good agreement with recent molecular dynamics simulations.