First observation of molecular composition and orientation at the surface of a room-temperature ionic liquid

We report the first measurements of the composition and molecular orientati on at the surface of a room-temperature ionic liquid-1-butyl-3-methylimidaz olium hexafluorophosphate, [bmim][PF6]. Recoil spectrometry using rare gas ions on continuously refreshed liquid surfaces in vacuo shows that neither ion is significantly enriched in the surface. The average orientation of th e cation is with the plane of the ring vertical. The cation ring is rotated about an axis through its center such that the nitrogen atoms and side cha ins are deeper in the surface with the surface normal passing between the t wo nitrogen atoms (with an estimated error of +/-30 degrees).