Surface structure of a neat ionic liquid investigated by grazing-incidence x-ray diffraction.

Grazing-incidence x-ray diffraction measurements were carried out to observe the surface structure of [bmim][PF(6)]. At an incident angle of less than the critical angle, a diffraction pattern from 10 nm beneath the surface was obtained. A broad Debye-Scherrer ring was observed in the diffraction patterns, indicating that the [bmim][PF(6)] molecules near the surface are randomly orientated, similar to those in the bulk liquid. The distance of the nearest anions was estimated to be 4.2 A, which is very close to that for bulk [bmim][I], suggesting that the liquid structure of [bmim][PF(6)] is determined by the packing of the cation.