On the Mesophase Formation of 1,3-Dialkylimidazolium Ionic Liquids

A series of seven different 1,3-dialkylimidazolium-based ion-pair salts with the same molecular weight and size but different symmetries was synthesized. For all salts, bromide was chosen as the counterion, giving the series ([C<i>n</i>IMC<i>m</i>]­[Br]), where IM = imidazolium and C<i>n</i> and C<i>m</i> are varying <i>N</i>-alkyl substituents with <i>n</i> + <i>m</i> = 13. Thus, the effect of symmetry on the physicochemical properties, such as thermal transitions, densities and viscosities and particularly mesophase formation, is investigated herein. All salts are fully characterized by NMR spectroscopy and mass spectrometry, and their physicochemical properties such as thermal transitions, densities, and viscosities are reported. Single crystal X-ray structure analysis is reported for 1-tridecylimidazolium bromide ([C0IMC13]­[Br]) and 1-ethyl-3-undecylimidazolium bromide ([C2IMC11]­[Br]). Salts 1-tridecylimidazolium bromide ([C0IMC13]­[Br]) and 1-dodecyl-3-methylimidazolium bromide ([C1IMC12]­[Br]) exhibit thermotropic liquid crystal behavior, confirmed by differential scanning calorimetry, polarized optical microscopy, and small-angle X-ray diffraction to be the SmA mesophase. A structure with interdigitation of alkyl chains is observed for all of [C0IMC13]­[Br], [C1IMC12]­[Br], and [C2IMC11]­[Br], despite the absence of thermotropic liquid crystalline behavior for the latter (and all other isomers with an alkyl chain length less than 12 carbon atoms). This allows us to draw the conclusion that for the liquid crystal phase of an ionic liquid to exist, not only are the calamitic shape and integral length of a molecule important but a minimal alkyl chain length of <i>n</i> = 12 is also required. Therefore, a dodecyl group could be considered as the functional group responsible for liquid crystalline behavior.