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FDIONIC18 How Ionic Species Structure influences Phase Structure and Transitions from Protic Ionic Liquids to Liquid Crystals to Crystals

Abstract

The phase behaviour of n-alkylammonium (C6 to C16) nitrates and formates have been characterised by using synchrotron small angle and wide angle X-ray scattering (SAXS/WAXS), differential scanning calorimetry (DSC), cross polarised optical microscopy (CPOM), and Fourier Transform Infrared Spectroscopy (FTIR). The protic salts may exist as crystalline, liquid crystalline and ionic liquid materials depending on the alkyl chain length and temperature. n-alkylammonium nitrates with n ≥ 6 form thermotropic liquid crystalline (LC) lamellar phases, whereas n ≥ 8 was required for the formate series to form this LC phase. The protic ionic liquid phase showed an intermediate length scale nanostructure resulting from segregation of the polar and nonpolar components of the ionic liquid. This segregation was enhanced for longer n-alkyl chains, with a corresponding increase in the correlation length scale. The crystalline and liquid crystalline phases were both lamellar. Phase transition temperatures, lamellar d-spacings, and liquid correlation lengths for the n-alkylammonium nitrates and formates were compared with those for n-alkylammonium chlorides and n-alkylamines. Plateau regions in the liquid crystalline to liquid phase transition temperatures as a function of n for the n-alkylammonium nitrates and formates are consistent with hydrogen-bonding and cation-anion interaction between ionic species dominating alkyl chain-chain van der Waals interactions, with the exception of the mid chained hexyl- and heptylammonium formates. The d-spacings of the lamellar phases for both the n-alkylammonium nitrates and formates were consistent with an increase in chain-chain layer interdigitation within the bilayer-based lamellae with increasing alkyl chain length, and they were comparable to the n-alkylammonium chlorides.

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Publication details

The article was received on 26 Apr 2017, accepted on 12 May 2017 and first published on 12 May 2017


Article type: Paper
DOI: 10.1039/C7FD00148G
Citation: Faraday Discuss., 2017, Accepted Manuscript
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    FDIONIC18 How Ionic Species Structure influences Phase Structure and Transitions from Protic Ionic Liquids to Liquid Crystals to Crystals

    T. L. Greaves, H. Broomhall, A. Weerawardena, D. A. Osborne, B. Canonge and C. J. Drummond, Faraday Discuss., 2017, Accepted Manuscript , DOI: 10.1039/C7FD00148G

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