Kenneth Richard Seddon

Molecular composition and orientation at the surface of room-temperature ionic liquids: Effect of molecular structure

We n have used direct recoil spectrometry (DRS) to investigate the composition and molecular orientation n at the surfaces of a variety of room-temperature ionic liquids of the general type [Cnmim]X where n [Cnmim] n= 1-CnH2n+1-3-methylimidazolium cation (n n= 4 (bmim), 8 (omim), 12 (C12mim)) and X is n [PF6]−, [BF4]−, Cl− or Br−. Comparing experimental atomic F/C and H/C ratios for [Cnmim][PF6] (n n= 4, 8, 12) n or [Cnmim][BF4] (n n= 4, 8) with predicted ratios for various surface compositions suggests that the surface is populated by both anions and cations with no segregation. The data best fit the cation ring being n perpendicular rather than parallel to the surface. The data for the cations of fluorine-containing n liquids support a common orientation with the N atoms of the ring uppermost. The cations of the shorter n chain hexafluorophosphate liquids appears to stay within a rotation angular spread of about ± 30° from this n orientation but increasing the alkyl chain length to 12 carbon atoms favors a tilt of ∼45° to bring the Me n group closer to the surface. Changing to the smaller [BF4]− n anion has a similar effect. For [omim][Cl], n the data fit best for a cation orientation with either N-up or N-down and the methyl group tilted towards the n surface in a similar manner to [omim][BF4]. n In the case of [omim][Br], n a number of cation orientations are compatible with the data.