## 3 Results and Discussion
In contrast to the results obtained under ambient pressure (Figure 1), the local structures of both the imidazolium C-H and alkyl C-H groups are disturbed under high pressures for the mixture after five washes, shown in Figure 5, when compared to the pressure dependence of the [BMIM][TFS]/Al\({}_{2}\)O\({}_{3}\) mixture (Figure 3). Therefore, high pressures may have the potential to control the order and strength of ionic liquid/surface interactions in this system. Figure 6 shows the infrared spectra of the mixture after five washes in the spectral range between 1000 and 1350 cm\({}^{-1}\) 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 absence of drastic band-narrowing and splitting in Figure 6 suggests that interfacial ionic liquid/Al\({}_{2}\)O\({}_{3}\) interactions of the mixture after five washes may be enhanced by the application of pressure. As revealed in Figure 6g, the absorption bands display mild blue-shifts in frequency as the mixture is compressed to 2.5 GPa. Figure 5: IR spectra of the mixture after five washes with ethanol 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). All curves were baseline-corrected by subtracting straight lines. 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. To illustrate the frequency shift, the pressure dependence of C-H stretching frequencies of the [BMIM][TFS]/Al\({}_{2}\)O\({}_{3}\) mixture, the mixture after two washes with ethanol, and the mixture after five washes is plotted in Figure 7. The C-H bands corresponding to the [BMIM][TFS]/Al\({}_{2}\)O\({}_{3}\) mixture display significant blue-shifts in frequency as the pressure increases to 0.4 GPa, and then undergo mild shifts in frequency when the pressure increases from 0.4 to 2.5 GPa. The discontinuous jump occurring around 0.4 GPa becomes less obvious for the mixture after two washes with ethanol. In Figure 7, we observe no discontinuous jump in frequency for the mixture after five washes. It is known that the hydrogen bond cooperativity due to concerted charge transfer can greatly enhance the strength of the individual hydrogen bonds involved in the coupling [25]. Hydrogen-bonding non-additivity and the size of clusters are suggested to be responsible for the enhancement of hydrogen bonding [25; 26]. Thus, the anomalous jump in frequency at 0.4 GPa can be attributed to both the cooperative and geometric effects of blue-shifting hydrogen bonds. The disappearance of the discontinuous jump upon the reduction of ionic liquid through washing with ethanol may indicate the decrease in strength of blue-shifting hydrogen bonding as C-H/anion interactions are replaced by C-H/Al\({}_{2}\)O\({}_{3}\) interactions. The loss of the frequency jump at 0.4 GPa due to washing is related to a reduction of the amount of ionic liquid along the interface. At thicker values, there is likely enough IL to form crystallites. If there is just a monolayer (or about that), crystallization might be difficult and hydrogen bond cooperativity may be reduced. Figure 6: IR spectra of the mixture after five washes with ethanol 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). Relative intensities of the IR bands are given in increments of 0.2; the scale bar is displayed in the upper left corner of the figure. This study indicates that high pressures can serve as a useful tool to probe the strength of blue-shifting C-H hydrogen bonding in ILs deposited along the surface of nano-Al2O3 surfaces. Furthermore, the surfaces of Al2O3 appear to be capable of breaking or weakening the interactions of the cation/anion clusters, presumably in favor of stable ionic liquid/Al2O3 interactions under high pressures.