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MAS NMR investigation of molecular order in an ionic liquid crystal
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Mann, Sarah K., Devgan, Mohit K., Franks, W. Trent, Huband, Steven, Chan, Chi Long, Griffith, Jeraime, Pugh, David, Brooks, Nicholas J., Welton, Tom, Pham, Tran N., McQueen, Lisa L., Lewandowski, Józef R. and Brown, Steven P. (2020) MAS NMR investigation of molecular order in an ionic liquid crystal. Journal of Physical Chemistry B, 124 (24). pp. 4975-4988. doi:10.1021/acs.jpcb.0c02328 ISSN 1520-6106.
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WRAP-MAS-NMR-investigation-molecular-ionic-liquid-crystal-Brown-2020.pdf - Accepted Version - Requires a PDF viewer. Download (6Mb) | Preview |
Official URL: https://doi.org/10.1021/acs.jpcb.0c02328
Abstract
The structure and molecular order in the thermotropic ionic liquid crystal (ILC), [choline][geranate(H)octanoate], an analogue of Choline And GEranate (CAGE), which has potential for use as a broad-spectrum antimicrobial and transdermal and oral delivery agent, were investigated by magic-angle spinning (MAS) nuclear magnetic resonance (NMR), polarizing optical microscopy, small-angle X-ray scattering (SAXS), and mass spectrometry. Mass spectrometry and the 1H NMR chemical shift reveal that CAGE-oct is a dynamic system, with metathesis (the exchange of interacting ions) and hydrogen exchange occurring between hydrogen-bonded/ionic complexes such as [(choline)(geranate)(H)(octanoate)], [(choline)(octanoate)2(H)], and [(choline)(geranate)2(H)]. These clusters, which are shown by mass spectrometry to be significantly more stable than expected for typical electrostatic ion clusters, involve hydrogen bonding between the carboxylic acid, carboxylate, and hydroxyl groups, with rapid hydrogen bond breaking and re-formation observed to average the 1H chemical shifts. The formation of a partial bilayer liquid crystal (LC) phase was identified by SAXS and polarizing optical microscopy at temperatures below ∼293 K. The occurrence of this transition close to room temperature could be utilized as a potential temperature-induced “switch” of the anisotropic properties for particular applications. The presence of an isotropic component of approximately 23% was observed to coexist with the LC phase, as detected by polarizing optical microscopy and quantified by both 1H–13C dipolar-chemical shift correlation (DIPSHIFT) and 1H double-quantum (DQ) MAS NMR experiments. At temperatures above the LC-to-isotropic transition, intermediate-range order (clustering of polar and nonpolar domains), a feature of many ILs, persists. Site-specific order parameters for the LC phase of CAGE-oct were obtained from the MAS NMR measurement of the partially averaged 13C–1H dipolar couplings (DCH) by cross-polarization (CP) build-up curves and DIPSHIFT experiments, and 1H–1H dipolar couplings (DHH) by double-quantum (DQ) build-up curves. The corresponding order parameters, SCH and SHH, are in the range 0–0.2 and are lower compared to those for smectic (i.e., layered) phases of conventional nonionic liquid crystals, resembling those of lamellar phases formed by lyotropic surfactant–solvent systems.
| Item Type: | Journal Article | |||||||||||||||
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| Subjects: | Q Science > QC Physics Q Science > QD Chemistry T Technology > TA Engineering (General). Civil engineering (General) |
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| Divisions: | Faculty of Science, Engineering and Medicine > Science > Chemistry Faculty of Science, Engineering and Medicine > Science > Physics |
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| Library of Congress Subject Headings (LCSH): | Ionic solutions , Liquid crystals, Nuclear magnetic resonance spectroscopy | |||||||||||||||
| Journal or Publication Title: | Journal of Physical Chemistry B | |||||||||||||||
| Publisher: | American Chemical Society | |||||||||||||||
| ISSN: | 1520-6106 | |||||||||||||||
| Official Date: | 18 June 2020 | |||||||||||||||
| Dates: |
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| Volume: | 124 | |||||||||||||||
| Number: | 24 | |||||||||||||||
| Page Range: | pp. 4975-4988 | |||||||||||||||
| DOI: | 10.1021/acs.jpcb.0c02328 | |||||||||||||||
| Status: | Peer Reviewed | |||||||||||||||
| Publication Status: | Published | |||||||||||||||
| Reuse Statement (publisher, data, author rights): | “This document is the Accepted Manuscript version of a Published Work that appeared in final form in The Journal of Physical Chemistry B, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see [insert ACS Articles on Request author-directed link to Published Work, see http://pubs.acs.org/page/policy/articlesonrequest/index.html].” | |||||||||||||||
| Access rights to Published version: | Restricted or Subscription Access | |||||||||||||||
| Date of first compliant deposit: | 11 June 2020 | |||||||||||||||
| Date of first compliant Open Access: | 15 May 2021 | |||||||||||||||
| RIOXX Funder/Project Grant: |
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