Abstract
The structure and molecular order in the thermotropic ionic liquid crystal (ILC), [choline][geranate(H)octanoate], an analogue of Choline And GEranate (CAGE), which has potential for use as a broad-spectrum antimicrobial and transdermal and oral delivery agent, were investigated by magic-angle spinning (MAS) nuclear magnetic resonance (NMR), polarizing optical microscopy, small-angle X-ray scattering (SAXS), and mass spectrometry. Mass spectrometry and the 1H NMR chemical shift reveal that CAGE-oct is a dynamic system, with metathesis (the exchange of interacting ions) and hydrogen exchange occurring between hydrogen-bonded/ionic complexes such as [(choline)(geranate)(H)(octanoate)], [(choline)(octanoate)2(H)], and [(choline)(geranate)2(H)]. These clusters, which are shown by mass spectrometry to be significantly more stable than expected for typical electrostatic ion clusters, involve hydrogen bonding between the carboxylic acid, carboxylate, and hydroxyl groups, with rapid hydrogen bond breaking and re-formation observed to average the 1H chemical shifts. The formation of a partial bilayer liquid crystal (LC) phase was identified by SAXS and polarizing optical microscopy at temperatures below ∼293 K. The occurrence of this transition close to room temperature could be utilized as a potential temperature-induced “switch” of the anisotropic properties for particular applications. The presence of an isotropic component of approximately 23% was observed to coexist with the LC phase, as detected by polarizing optical microscopy and quantified by both 1H–13C dipolar-chemical shift correlation (DIPSHIFT) and 1H double-quantum (DQ) MAS NMR experiments. At temperatures above the LC-to-isotropic transition, intermediate-range order (clustering of polar and nonpolar domains), a feature of many ILs, persists. Site-specific order parameters for the LC phase of CAGE-oct were obtained from the MAS NMR measurement of the partially averaged 13C–1H dipolar couplings (DCH) by cross-polarization (CP) build-up curves and DIPSHIFT experiments, and 1H–1H dipolar couplings (DHH) by double-quantum (DQ) build-up curves. The corresponding order parameters, SCH and SHH, are in the range 0–0.2 and are lower compared to those for smectic (i.e., layered) phases of conventional nonionic liquid crystals, resembling those of lamellar phases formed by lyotropic surfactant–solvent systems.
Highlights
As the name suggests, the liquid crystalline state of matter is an intermediate between liquids and solids; there is long-range order, but the molecules themselves are mobile
We investigate the short-range and long-range structure, and molecular order of Choline And GEranate (CAGE)-oct, by magic-angle spinning (MAS) nuclear magnetic resonance (NMR), small-angle X-ray scattering (SAXS), and mass spectrometry
The layer formation of smectic ionic LCs (ILCs) is driven by Coulombic forces and nanosegregation of the charged moieties from the hydrophobic chains:[5] forces that are expected to dominate the intermolecular interactions of CAGE-oct
Summary
The liquid crystalline state of matter is an intermediate between liquids and solids; there is long-range order, but the molecules themselves are mobile. If there is a dominant dipolar interaction, the polarization transfer shows oscillatory behavior where the frequency of oscillations is directly proportional to the dipolar coupling constant.[25] CP build-up curves or variants such as Lee−Goldburg (LG) CP have been used to obtain RDCs and the related SCH order parameters in various dynamic systems, including polymers[26] and LCs.[27,28]. The MAS NMR techniques used ([NO3]−, Cl−, and in this paper have not, to our knowledge, far been used to investigate ILCs. In this work, we investigate the short-range (interactions between ions) and long-range structure (the occurrence of the partial bilayer LC phase), and molecular order of CAGE-oct, by MAS NMR, SAXS, and mass spectrometry. Residual 13C−1H and 1H−1H dipolar couplings are measured by means of 1H−13C CP and 1H doublequantum (DQ) MAS NMR build-up curves, so as to obtain order parameters, SCH and SHH, which quantitatively describe the amplitude of motion of the ions
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