Surface induced smectic order in ionic liquids - an X-ray reflectivity study of [C22C1im]+[NTf2]-

Surface induced smectic order was found for the ionic liquid 1-methyl-3-docosylimidazolium bis(tri-fluoromethlysulfonyl) imide by X-ray reflectivity and grazing incidence scattering experiments. Near the free liquid surface, an ordered structure of alternating layers composed of polar and non-polar moieties is observed. This leads to an oscillatory interfacial profile perpendicular to the liquid surface with a periodicity of 3.7 nm. Small angle X-ray scattering and polarized light microscopy measurements suggest that the observed surface structure is related to fluctuations into a metastable liquid crystalline SmA(2) phase that was found by supercooling the bulk liquid. The observed surface ordering persists up to 157 degrees C, i.e. more than 88 K above the bulk melting temperature of 68.1 degrees C. Close to the bulk melting point, we find a thickness of the ordered layer of L = 30 nm. The dependency of L(tau) = Lambda ln(tau/tau(1)) vs. reduced temperature tau follows a logarithmic growth law. In agreement with theory, the pre-factor Lambda is governed by the correlation length of the isotropic bulk phase.