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

Abstract

We have used direct recoil spectrometry (DRS) to investigate the composition and molecular orientation at the surfaces of a variety of room-temperature ionic liquids of the general type [C_nmim]X where [C_nmim] = 1-C_nH_2n+1-3-methylimidazolium cation (n = 4 (bmim), 8 (omim), 12 (C₁₂mim)) and X is [PF₆]⁻, [BF₄]⁻, Cl⁻ or Br⁻. Comparing experimental atomic F/C and H/C ratios for [C_nmim][PF₆] (n = 4, 8, 12) or [C_nmim][BF₄] (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 perpendicular rather than parallel to the surface. The data for the cations of fluorine-containing liquids support a common orientation with the N atoms of the ring uppermost. The cations of the shorter chain hexafluorophosphate liquids appears to stay within a rotation angular spread of about ± 30° from this orientation but increasing the alkyl chain length to 12 carbon atoms favors a tilt of ∼45° to bring the Me group closer to the surface. Changing to the smaller [BF₄]⁻ anion has a similar effect. For [omim][Cl], the data fit best for a cation orientation with either N-up or N-down and the methyl group tilted towards the surface in a similar manner to [omim][BF₄]. In the case of [omim][Br], a number of cation orientations are compatible with the data.