Abstract
We report on molecular and local orientational order of a series of imidazolium-based ionic liquid crystals exhibiting layered smectic A mesophase. Materials constituting of 1-dodecyl-3-methylimidazolium cation, and different counter-ions, were investigated. We apply two-dimensional 13C-1H dipolar NMR spectroscopy to quantify orientational order of C-H bonds of the organic cation. The experimental data supported the structural model of the interdigitated chains aligned with the smectic layer normal. Molecular order parameter S was found to increase in the anion sequence BF4− < I− < Br− < Cl−. This trend correlates well with ionic radius, negative charge delocalization, and hydrogen-bonding properties of the anions.
Highlights
Ionic liquids (ILs) are considered today as key materials in a broad range of emerging technological applications in the areas of electrochemistry, synthesis, catalysis and separation, and lubrication
Mesophase formation is predetermined by a pronounced anisotropy of interparticle interactions, which in the case of molecular liquid crystals (LCs) with prevailing van der Waals interaction is a consequence of molecular anisotropic shape
We report detailed investigation of the molecular and local bond orientational ordering in a series of
Summary
Ionic liquids (ILs) are considered today as key materials in a broad range of emerging technological applications in the areas of electrochemistry, synthesis, catalysis and separation, and lubrication. Theoretical and experimental studies of orientational order in ILCs [1,10,11,12,13,14] revealed that, due to the contribution of the electrostatic interactions to the stabilization of layered structures, smectic phases in ILCs exhibit significantly lower value of the molecular orientational order parameter S as compared to that in its non-ionic counterparts. We report detailed investigation of the molecular and local bond orientational ordering in a series of ILCs with a fixed imidazolium-based cation and number of anions varying in structure, ionic radius, negative charge localization, and hydrogen bonding properties. We apply two-dimensional (2D) 13 C-1 H dipolar NMR spectroscopy to quantify orientational order of C-H bonds of a long-chain imidazolium-based cation in an ionic smectic phase. The observed trends of the measured bond and molecular order parameter values depending on counter-ion are discussed
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