Sum frequency generation study on the orientation of room-temperature ionic liquid at the graphene–ionic liquid interface

The sum frequency generation (SFG) vibrational spectrum and contact angle of a room-temperature ionic liquid, 1-butyl-3-methylimidazolium methane sulfate, [BMIM][MS], at the graphene interface has been measured. The SFG spectra are dominated by the CH3 resonance of the anion. The methyl group of methane sulfate is oriented at an angle greater than 40° from the surface normal. The SFG data suggests the cation, [BMIM]+, is weakly oriented with the alkyl chain parallel to the graphene plane and the aromatic ring somewhat tilted from the surface plane. As the ionic liquid [BMIM][MS] wets the surface with a contact angle of 58 ± 2°, suggesting that the non-polar interaction of the alkyl chain and π–π interaction of the ring, to graphene dominate the surface interactions.