Gel-like ionic liquid/nanosilica mixtures have been prepared with the addition of 10 wt % nanosilica to 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (EMI+TFSA–), but no changes in C–H spectral features of EMI+TFSA– were observed in the presence of nanosilica under ambient pressure. As the gel-like mixture of EMI+TFSA–/nanosilica was compressed, a new alkyl C–H band at ca. 3017 cm–1 arose. This new spectral feature at ca. 3017 cm–1 may be attributed to the structural reorganization of alkyl C–H groups induced by the interactions between ionic clusters and nanosilica. The imidazolium C–H bands display nonmonotonic shifts as the pressure is elevated. They blue shift slightly initially, but red shifts occur upon further increase of the pressure. The compression also leads to an increase in the relative intensity of the imidazolium C–H band at ca. 3105 cm–1. This observation suggests that some associated species are switched to the isolated form due to the presence of nanosilica under high pressures. In contrast to EMI+TFSA–/nanosilica, the band frequencies and band shapes of 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (BMI+TFSA–)/nanosilica are similar to those of pure BMI+TFSA–. These results indicate that the length of alkyl C–H groups plays a nonnegligible role in isolated-associated equilibrium.

The local structures between nano-TiO2 and 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EMI+TFS–) and 1-butyl-3-methylimidazolium trifluoromethanesulfonate (BMI+TFS–) were investigated using high-pressure infrared spectroscopy. No significant changes in C–H spectral features of EMI+TFS– were observed in the presence of nano-TiO2 under ambient pressure. As the EMI+TFS–/nano-TiO2 mixture was compressed to 0.3 GPa, the imidazolium C–H absorptions became two sharp bands at 3108 and 3168 cm–1, respectively, and the alkyl C–H stretching absorption exhibits a new band at 3010 cm–1 associated with a weaker band at 3028 cm–1. It appears that pressure stabilizes the isolated conformations due to pressure-enhanced imidazolium C–H---nano-TiO2 interactions. Our results also reveal that alkyl C–H groups play non-negligible roles at the conditions of high pressures. The results of BMI+TFS–/nano-TiO2 are remarkably different from what is revealed for EMI+TFS–/nano-TiO2. The spectral features and band frequencies of BMI+TFS–/nano-TiO2 are almost identical to those of pure BMI+TFS– under various pressures. This study demonstrates that changes to the alkyl chain length of the cation could be made to control the order and strength of ionic liquid/nano-TiO2 interactions.

We have investigated the effect of pressure on imidazolium C-H---O interactions in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (EMI(+)TFSA(-))/L64 and EMI(+)TFSA(-)/1,4-dioxane mixtures. The addition of Pluronic L64 to EMI(+)TFSA(-) leads to appreciable changes in band frequencies and shapes of the imidazolium C-H stretching bands. A possible explanation is the formation of C-H---O interactions between imidazolium C-H groups and oxygen atoms of polyethylene oxides (PEOs). In other words, L64 can be added to change the relative contribution of the isolated and associated components of EMI(+)TFSA(-). In contrast to L64, the oxygen atoms of 1,4-dioxane cannot perturb the local structures of imidazolium C-H groups of EMI(+)TFSA(-) and the association configuration is still favored in the presence of 1,4-dioxane. As the pressure is elevated, 1,4-dioxane molecules tend to associate with themselves and TFSA(-) interacts with EMI(+) to form associated configurations. Our results suggest the formation of association between EMI(+) cation and L64 and the complexes are stable up to the pressure of 2.5 GPa.

The interactions between ionic liquid ([EMI][TFS]) and gold surfaces have been investigated via the application of pressures up to ca. 2 GPa. Comparing the spectral features of [EMI][TFS]/gold with those of pure [EMI][TFS], no appreciable changes of C-H bands in the presence of gold powders were observed under ambient pressure. Nevertheless, the imidazolium C-H bands display red shifts in frequency as the [EMI][TFS] / Au mixture was compressed to the pressure above 1.4 GPa and a new alkyl C-H band at ca. 3016 cm−1 was also revealed. These spectral changes, being related to the addition of gold powders and pressure elevation, should be attributed to the local structural changes of C-H groups caused by pressure-enhanced interfacial interactions between [EMI][TFS] and Au. Gold powders tend to induce the changes in hydrogen bonding structures of imidazolium C2-H group under high pressures. The pressure-dependent spectral features in the asymmetric SO3 stretching region display band-narrowing and minor local structural changes induced by the presence of gold particles under high pressures. These observations suggest that Au powders perturb structural equilibrium of C-H groups of cations under high pressures.