A series of different 1,3-dialkylimidazolium bromides ([CnIMCm]), where IM = imidazolium and Cn and Cm are varying N-alkyl substituents with n + m = 13,  with the same molecular weight and size, but different symmetries was synthesized. The effect of symmetry on the physicochemical properties, such as thermal transitions, densities and viscosities and particularly mesophase formation was investigated. This allows to draw the conclusion that for liquid crystal phase of an ionic liquid  to exist, not only the calamitic shape and integral length of a molecule are important: A minimal alkyl chain length of n = 12 is required. Therefore, a dodecyl group could be considered as the functional group responsible for liquid crystalline behavior. To verify this, a library of 1-alkyl-3-dodecylimidazolium bromides was investigated. Remarkably, the arrangement of imidazolium cores in the structure is very complicated, due to multiple non-classical hydrogen bonds between bromide anions and imidazolium head groups. In this arrangement neighboring imidazolium rings are forced by hydrogen bonds to form a 'face-to-face' conformation. This seems to be responsible for the elimination of a mesophase. To conclude, the general view of a dodecyl chain being a functional group to generate a mesophase is not entirely valid.

Mesophase formation in ionic liquids - what renders an ionic liquid and ionic liquid crystal

248th National Meeting (2014)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.