A combined LX-NMR and molecular dynamics investigation of the bulk and local structure of ionic liquid crystals

Abstract

The unique power of NMR spectroscopy in anisotropic media (LX-NMR) as a tool to obtain local and bulk structural information, combined with the effectiveness of molecular dynamics simulations at the atomistic level, shows very attractive potentialities for the study of interesting, even though still poorly understood, materials such as Ionic Liquid Crystals (ILCs). In this work, we focused our attention, in particular, on the orientational ordering of two mesophases: 1-dodecyl-3-methylimidazolium chloride, [C₁₂C₁im]Cl, and 1-dodecyl-3-methylimidazolium tetrafluoroborate, [C₁₂C₁im][BF₄]. Both ILCs were studied by a ²H NMR direct investigation of the molecules forming the phases, suitably deuterated, and by ¹H NMR spectroscopy, using the small rigid probe-solutes 1,4-dichlorobenzene (DCB), dissolved in [C₁₂C₁im][BF₄] and [C₁₂C₁im]Cl, and 1,4-dibromobenzene (DBB) dissolved in [C₁₂C₁im][BF₄], to probe the local, internal structure and organization of the mesophases. The experimental results were then compared with the predictions, by atomistic MD simulations, of the structure of the smectic phase of the two salts, at two selected temperatures, containing a single DCB molecule as a probe. The MD simulations show that the DCB solute is distributed only within the hydrophobic layers of the ILC. Orientational order parameters of the imidazolium cations and of the DCB molecule were obtained and compared with the experiments, showing a general good agreement and allowing a deeper understanding of the microscopic structure of the systems.