Nuclear magnetic resonance studies of translational diffusion in thermotropic ionic liquid crystals

Abstract

The NMR methodologies employed for investigating translational diffusion in anisotropic fluids and the results of their applications to ionic liquid crystals are reviewed. Experiments on ionic liquid crystals are preferably performed using oriented samples and require magnetic field gradients in orthogonal directions. Diffusion experiments in anisotropic systems with broad NMR lines are performed using line narrowing techniques and by application of strong static or pulsed field gradients for efficient gradient encoding/decoding of the spatial locations of molecules. Self-diffusion studies on various thermotropic ion-conductive materials exhibiting smectic, cubic, and columnar phases have been reported. Diffusion rates and anisotropy characterise the translational dynamics of ions in nanostructures and reflect the molecular ordering and ion pairing/dissociation processes. Distinct diffusion behaviours were observed for cations and anions. The knowledge of molecular mobility in ionic liquid crystals is important for the understanding their dynamic properties and is, therefore, valuable for the development of anisotropic soft materials for ion transport.

NMR Spectroscopic Study of Orientational Order in Imidazolium-Based Ionic Liquid Crystals

“…Thus, the increasing interlayer spacing contributes to more extended chains with higher ordering. While the order parameter characterizes the re-orientational molecular dynamics, anisotropy of translational mobility in LCs is also directly affected by the orientational order [30,31]. Having obtained in the present work, the data on the order parameters in studied ILC series, we are now in the position to explain some previously found trends in translation diffusion results.…”

Understanding ionic mesophase stabilization by hydration: a solid-state NMR study

“…This held also for diffusion in a chlorine salt, reported in our previous work. 75 Increasing diffusional anisotropy stabilized the smectic layers of less orientationally ordered cations. In hydrated materials, the H-bond network, localized in ionic sublayers, is denser but it has a more dynamic nature.…”

“…Also, anion diffusion is highly anisotropic, as has been demonstrated in an analogous ILC with BF 4 counterions. 60,75…”

Ion conformation and orientational order in a dicationic ionic liquid crystal studied by solid-state nuclear magnetic resonance spectroscopy

“…High translational anisotropy is also indicative of phase stability. Director alignment in the magnetic field makes it possible to access the diffusion anisotropy using pulsed-field-gradient NMR techniques 50 . Diffusion studies of ILCs are currently in progress in our laboratory.…”