Sum Frequency Generation Spectroscopy of Dicyanamide Based Room-Temperature Ionic Liquids. Orientation of the Cation and the Anion at the Gas−liquid Interface

The orientation of the cation and the anion of room-temperature ionic liquids using sum frequency generation vibrational spectroscopy is reported. The ionic liquids are based on butyl-methyl imidazolium [BMIM]+ and hexyl-tributyl ammonium [N6444]+ together with dicyanamide [DCA]- as the anion. The tilt angle of the C3 axis of the methyl group from the alkyl chain in the cations was found to vary from 52 degrees to 80 degrees as a function of the distribution width sigma (which ranges from 0 degrees to 30 degrees with respect to the surface normal) for [BMIM][DCA] and similarly for [N6444][DCA]. The orientation of the C2 axis in the dicyanamide anion as a function of the twist angle phi, varied between 46 degrees and 90 degrees for [BMIM][DCA] and from 53 degrees to 90 degrees for [N6444][DCA]. These results suggest the presence of both ionic species at the gas-liquid interface and help describe the behavior of a simple inorganic anion at the surface.