Phase transition of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide thin films on highly oriented pyrolytic graphite.

Thin glassy films of a room-temperature ionic liquid, 1-ethyl-3-methylimidazolium bis[trifluoromethylsulfonyl]imide ([emim][Tf(2)N]), were deposited by thermal evaporation onto a substrate of highly oriented pyrolytic graphite. Their crystallization and fusion kinetics are discussed on the basis of results of time-of-flight secondary-ion mass spectrometry (TOF-SIMS) by measuring sputtered secondary-ion intensities as a function of temperature. Multilayer films crystallize at 205 K and then fuse at 255 K, as determined from temperature-programmed TOF-SIMS measurements, whereas crystallization occurs at around the glass-transition temperature (175-180 K) within 10 min, as shown by isothermal TOF-SIMS measurements. The ionic pairs in the [emim][Tf(2)N] monolayer tend to align over a wide temperature range of 180-220 K and retain crystal-like alignment up to 285 K. The weak van der Waals interaction at the interface is thought to be prerequisite for the aligned monolayer formation, because the ionic pairs on a Ni(111) substrate tend to be disordered. Thus, it is demonstrated that the alignment and wettability of the first monolayer, as well as the crystallization and fusion kinetics of thin films, are influenced by the substrate.