Melting Point and Structure of Ionic Liquid [EMIM][PF₆] on the Surface of Nano-SiO_x Particles

The 1-ethyl-3-methyl imidazolium hexafluorophosphate ionic liquid ([EMIM][PF₆]) was absorbed onto the surfaces of several kinds of nano-SiO_x particles by mechanical grinding using an agate mortar. The samples were investigated by differential scanning calorimetry (DSC), Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, and X-ray diffraction (XRD). We found that the melting point of the [EMIM][PF₆] adsorbed on the surface of nano-SiO_x was significantly less than that of the bulk ionic liquid. For the pure ionic liquid [EMIM][PF₆], the melting point was 62 . After absorption onto SiO_x nanoparticles the melting point decreased to 52 and δT was -10 . For the other two types of nano-SiO_x particles the δT was -20 and -17 , respectively. These results indicate that the melting point depression was dependent on the surface properties of the nano-SiO_x particles. Furthermore, the characteristics of the surface-adsorbed ionic liquid were also found to be quite different from that of the bulk ionic liquid by Raman and XRD analyses. The difference in the density of the surface hydroxyl groups and the specific surface area of the nano-SiO_x particles may induce different interfacial interactions between [EMIM][PF₆] and the nano-SiO_x particles. Our results suggest that the density of the surface hydroxyl groups and specific surface area are the major factors responsible for the behavior of the adsorbed ionic liquid [EMIM][PF₆].