A time-of-flight secondary ion mass spectroscopy study of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide RT-ionic liquid

Time-of-flight secondary ion mass spectroscopy (TOF-SIMS) has been used to investigate the surface structural transformation of room temperature ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imid ([EMIM][Tf2N]), in the temperature range between 300 and 160 K. The intensity of the [EMIM]+ cation from the crystal surface becomes about twice as large as that from the glassy and liquid surfaces, whereas the fragment ions from the [Tf2N]− moiety are almost unchanged upon crystallization. This phenomenon can be ascribed to the steric effect of the cation relative to the counter anion at the topmost surface layer rather than their surface compositions: a specific layered structure of the crystal surface, in which the imidazolium ring of [EMIM]+ is aligned parallel to the surface plane, is thought to be responsible for the enhancement of the [EMIM]+ ion emission. The smaller [EMIM]+ intensity from the glassy and liquid films evidences that the imidazolium ring is not parallel to the surface.