Application of Complementary Experimental Techniques to Characterization of the Phase Behavior of [C₁₆mim][PF₆] and [C₁₄mim][PF₆]

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][PF 6 ]) and 1-methyl-3-tetradecylimidazolium hexafluorophosphate [C 14 mim][PF 6 ]. 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][PF 6 ] 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][PF 6 ]. 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.