Interactions of Silica Nanoparticles and Ionic Liquids Probed by High Pressure Vibrational Spectroscopy

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 (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.