## 3 Results and Discussion  
Figure 1 shows the infrared spectra of pure [BMIM][TFS], the [BMIM][TFS]/Al\({}_{2}\)O\({}_{3}\) mixture, the mixture after two washes with ethanol, and the mixture after five washes obtained under ambient pressure. The amount of [BMIM][TFS] are adjusted by controlling the numbers of washes in this study; the amount of [BMIM][TFS] becomes almost depleted after five washes, as shown in Figure 1d. The IR spectrum of pure [BMIM][TFS] in Figure 1a shows two imidazolium C-H bands at 3116 and 3155 cm\({}^{-1}\), a shoulder to the 3155 cm\({}^{-1}\) band at 3098 cm\({}^{-1}\), and three alkyl C-H bands at 2877, 2940, and 2967 cm\({}^{-1}\), respectively [21]. As shown in Figure 1, no appreciable changes in spectral features and band frequencies of C-H vibrations occurred as [BMIM][TFS] was mixed with nano-Al\({}_{2}\)O\({}_{3}\), except for a noticeable absorption caused by O-H groups from the surfaces of nano-Al\({}_{2}\)O\({}_{3}\) that appears in Figure 1d. These results indicate that IR measurements recorded under ambient pressure may not be sensitive enough to monitor the interfacial interactions between the BMIM\({}^{+}\) cation and nano-Al\({}_{2}\)O\({}_{3}\). Figure 1: Infrared (IR) spectra of pure 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([BMIM][TFS]) (curve a), the [BMIM][TFS]/Al\({}_{2}\)O\({}_{3}\) mixture (curve b), the mixture after two washes with ethanol (curve c), and the mixture after five washes (curve d), recorded under ambient pressure. Curve d was baseline-corrected by subtracting a straight line. Relative intensities of the IR bands are given in increments of 0.05; the scale bar is displayed in the upper left corner of the figure. Figure 2 displays the IR spectra of samples in the spectral range between 1000 and 1350 cm\({}^{-1}\). This region of the spectrum is predominately dominated by absorption bands due to vibrations of the TFS\({}^{-}\) anions. The bands at ca. 1165 and 1226 cm\({}^{-1}\) are assigned to CF\({}_{3}\) asymmetric and symmetric stretching vibrations, respectively. The absorption bands located near 1032 and 1270 cm\({}^{-1}\) (doublet) are assigned to the symmetric and asymmetric stretching vibration of the SO\({}_{3}\) group of the TFS\({}^{-}\) anion, respectively. Comparing the spectral features of Figure 2d with those of Figure 2a reveal mild frequency shifts and narrowing for the bands at ca. 1165 and 1270 cm\({}^{-1}\), caused by the presence of nano-Al\({}_{2}\)O\({}_{3}\). The absorption of CaF\({}_{2}\) windows (below 1050 cm\({}^{-1}\)) also becomes obvious in Figure 2d. Figure 3 displays the IR spectra of a sample from a [BMIM][TFS]/Al\({}_{2}\)O\({}_{3}\) mixture obtained under ambient pressure (curve a) and at 0.4 (curve b), 0.7 (curve c), 1.1 (curve d), 1.5 (curve e), 1.8 (curve f), and 2.5 GPa (curve g). The elevation of pressure to 0.4 GPa leads to a phase transition, and the imidazolium C-H bands are blue-shifted to 3102, 3132, and 3168 cm\({}^{-1}\) in Figure 3b. The sharper structures of imidazolium C-H bands revealed in Figure 3b-g are in part due to the anisotropic environment in a solid structure. We note that spectral features of the [BMIM][TFS]/Al\({}_{2}\)O\({}_{3}\) mixture in Figure 3 are almost identical to those of pure [BMIM][TFS], as revealed in our previous studies [21]. The results of Figure 3 indicate that the presence of nano-Al\({}_{2}\)O\({}_{3}\) does not perturb ion/ion interactions from the perspective of the C-H stretching motions of the cations. Instead, the blue-shifts revealed in Figure 3 may originate from the combined effect of the overlap repulsion enhanced by hydrostatic pressure, C-H--O contacts, and so forth. The increased understanding of weak interactions, such as hydrogen bonding between the C-H groups on the imidazolium cation and oxygen atoms of the  
Figure 2: IR spectra of pure [BMIM][TFS] (curve a), the [BMIM][TFS]/Al\({}_{2}\)O\({}_{3}\) mixture (curve b), the mixture after two washes with ethanol (curve c), and the mixture after five washes (curve d), recorded under ambient pressure.