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

TL;DR

This study shows that ionic liquid films with more asymmetric cations form larger ordered structures.

Contribution

The novel finding is that increasing cation asymmetry enhances long-range ordering in ionic liquid films.

Findings

Ordered domains in ionic liquid films extend up to 0.7 ± 0.2 μm for the most asymmetric cations.

Molecular orientation and ordering are tracked using infrared spectroscopy of vibrational modes.

Symmetric cations result in smaller ordered domains (0.4 ± 0.2 μm) compared to asymmetric ones.

Abstract

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 νas (∼1054 cm–1), SO2 νss (∼1137 cm–1), and SO2 νas (∼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.

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