cg4004593_si_001.cif (28.42 kB)
On the Mesophase Formation of 1,3-Dialkylimidazolium Ionic Liquids
dataset
posted on 2013-07-03, 00:00 authored by Mei Yang, Bert Mallick, Anja-Verena MudringA 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 ([CnIMCm][Br]),
where IM = imidazolium and Cn and Cm are varying N-alkyl substituents with n + m = 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 n = 12 is also required. Therefore, a dodecyl group could
be considered as the functional group responsible for liquid crystalline
behavior.