Orientation of 1-Butyl-3-methylimidazolium Based Ionic Liquids at a Hydrophobic Quartz Interface Using Sum Frequency Generation Spectroscopy

The room-temperature ionic liquid/hydrophobic quartz interface was examined using sum frequency generation vibrational spectroscopy in the C−H stretching region to develop a more detailed model of how ionic liquids absorb to a variety of solid surfaces. Deuterated dodecyltrichlorosilane was synthesized to modify the quartz surface, which produced a hydrophobic quartz interface that had no resonances from C−H stretches. The ionic liquids, 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF 4 ] and 1-butyl-3-methylimidazolium hexafluorophosphate [BMIM][PF 6 ], and neutral derivatives, 1-methylimidazole and 1-butylimidazole, were used in the analysis. The neutral molecules were examined to compare how shape and charge influenced the orientation of the ionic liquids at a hydrophobic interface. The methyl groups of the four compounds studied were oriented much closer to the surface normal than on the hydrophilic quartz surface. This suggested that the methyl groups were arranged toward the alkylsilane monolayer. The sum frequency generation (SFG) spectra suggest that the imidazolium rings of the ionic liquids are lying in the plane of the interface. Ionic liquids at three different interfaces air/liquid, hydrophobic quartz/liquid, and hydrophilic quartz/liquid were compared.