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 ([C16mim][PF6]) and 1-methyl-3-tetradecylimidazolium hexafluorophosphate [C14mim][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 [C16mim][PF6] suggest that the higher-temperature crystalline phase (CII) has greatly increased freedom in its long alkyl chain and anion than the lowertemperature crystalline phase (CI). This conclusion is supported by single-crystal X-ray diffraction results for [C14mim][PF6]. In both crystalline phases, as well as in the highertemperature 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. Introduction Ionic liquid (IL) is the term now widely applied to salts that are liquid at or below 100 °C. ILs show great promise as environmentally benign reaction media for many types of chemical processes.1 The key to sustainable technology is achieving an economic benefit with environmental improvements: the use of ILs offers improved performances and greater flexibility for a variety of processes such as biphasic catalysis and organic synthesis,2 separations,3 electrochemistry,4 photochemistry, 5 and liquid crystals,6 potentially leading to economic advantages. ILs are also environmentall