Journal of Physical Chemistry B, Vol.111, No.33, 9733-9740, 2007
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 C-3 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 C-2 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.