Effect of Cation Symmetry on the Long-Range Ordering in Ionic Liquid Films

This work investigates the role of ionic liquid (IL) ion (a)symmetry in promoting ordered structures within liquid films by studying a series of six alkylimidazolium cation isomers of varying symmetry paired with a bis(trifluoromethylsulfonyl)imide anion. The cation symmetry is varied by systematic variations in the alkyl tail lengths on either side of the imidazolium ring. IL films are extruded on a silver substrate using an in situ dynamic wetting apparatus and allowed to thin under shearing force due to gravity. Film thicknesses are monitored via spectroscopic ellipsometry. Infrared reflection absorption spectroscopy (IRRAS) with p-polarized light is used to analyze changes in dipole moments with vector components perpendicular to the substrate and thus report changes to molecular orientations and local chemical environments. Multiple vibrational modes are monitored at varying film thicknesses to deliver chemical insight into the evolving net molecular orientation within the IL films. Specifically, average molecular orientations are tracked by monitoring intensity and energy shifts of vibrational modes, including the S-N-S ν_as (∼1054 cm^-1), SO₂ ν_ss (∼1137 cm^-1), and SO₂ ν_as (∼1330 cm^-1) stretches. This provides a unique ability to determine the extent of ordering in the film. As IRRAS probes the entire film, when changes to the spectral profile cease and only a uniform decrease in absorbance is observed, only the ordered domains of the film remain on the substrate; thus, the film's thickness is equal to the extent of ordering in the film. Ultimately, for the six alkylimidazolium IL isomers examined here, the extent of molecular ordering in the films increases with increasing asymmetry of the cation. IL ordered domains extend to 0.4 ± 0.2 μm for the most symmetric systems and to 0.7 ± 0.2 μm for the most asymmetric systems.