Application of complementary experimental techniques to characterization of the phase behavior of [C(16)mim][PF6] and [C(14)mim][PF6]

J De Roche, C M Gordon, C T Imrie, M D Ingram, A R Kennedy, F Lo Celso, A Triolo

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Abstract

A range of analytical techniques (DSC, conductivity measurement, Raman spectroscopy, small- and wide-angle X-ray diffraction (S-WAXS), quasi-elastic neutron scattering (QENS), and single-crystal X-ray diffraction) are applied to the characterization of the phase behavior of the low-melting-point liquid crystalline salts 1-hexadecyl-3-methylimidazolium hexafluorophosphate ([C(16)mim][PF6]) and 1-methyl-3-tetradecylimidazolium hexafluorophosphate [C(14)mim][PF6]. This is the first time that QENS has been applied to the structural analysis of this type of ionic liquid crystal. For the first time in this class of salts, a low-temperature phase transition is identified, which is assigned to a crystal-crystal transition. Conductivity and QENS data for [C(16)mim] [PF6] suggest that the higher-temperature crystalline phase (C-II) has greatly increased freedom in its long alkyl chain and anion than the lower-temperature crystalline phase (C-I). This conclusion is supported by single-crystal X-ray diffraction results for [C(14)mim][PF6]. In both crystalline phases, as well as in the higher-temperature mesophase, the structure maintains a monodispersed layer structure with interdigitated alkyl chains. The structure of the mesophase is confirmed as smectic A by the S-WAXS and Raman spectroscopy results. Detailed analysis suggests that in this phase the alkyl chains undergo complete conformational melting.

Original languageEnglish
Pages (from-to)3089-3097
Number of pages9
JournalChemistry of Materials
Volume15
DOIs
Publication statusPublished - 2003

Keywords

  • TEMPERATURE IONIC LIQUIDS
  • POLYMER ELECTROLYTES
  • SALTS
  • CRYSTALS
  • CHAIN
  • ASSEMBLIES
  • CATALYSIS
  • TRANSPORT
  • STATE

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