Journal article
Effect of Cation Symmetry on the Long-Range Ordering in Ionic Liquid Films
Langmuir, Vol.42(2), pp.1905-1915
01/20/2026
DOI: 10.1021/acs.langmuir.5c03333
PMCID: PMC12825391
PMID: 41234034
Appears in UI Libraries Support Open Access
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 ν
(∼1054 cm
), SO
ν
(∼1137 cm
), and SO
ν
(∼1330 cm
) 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.
Details
- Title: Subtitle
- Effect of Cation Symmetry on the Long-Range Ordering in Ionic Liquid Films
- Creators
- Colleen B Lasar - University of IowaAndrew Horvath - University of IowaMichael B Van Den Top - University of IowaSpyridon Koutsoukos - Imperial College LondonTom Welton - Imperial College LondonScott K Shaw - University of Iowa
- Resource Type
- Journal article
- Publication Details
- Langmuir, Vol.42(2), pp.1905-1915
- DOI
- 10.1021/acs.langmuir.5c03333
- PMID
- 41234034
- PMCID
- PMC12825391
- NLM abbreviation
- Langmuir
- ISSN
- 1520-5827
- eISSN
- 1520-5827
- Publisher
- American Chemical Society
- Grant note
- Bodossaki Foundation: NA Graduate College, University of Iowa: NA National Science Foundation: 1651381
The authors are greatly thankful for funding for this research, which was provided by the National Science Foundation via award number 1651381. C.B.L. is also thankful for the graduate fellowship support provided by the University of Iowa's Graduate College, and S.K. thanks the Bodossaki Foundation for funding his PhD studies. They thank Dr. George Crull for his assistance with NMR data acquisition, as well as Mike Estensen and Benj Revis, electrical engineer and glassblower, respectively, from the University of Iowa College of Liberal Arts and Sciences Shops, for their contributions in designing and enabling the dynamic wetting apparatus used in these experiments.
- Language
- English
- Electronic publication date
- 11/13/2025
- Date published
- 01/20/2026
- Academic Unit
- Chemistry
- Record Identifier
- 9985027361402771
Metrics
1 Record Views