Ionic Liquid Crystals Research Articles

Nanoconfined water in ionic liquid and lyotropic ionic liquid crystals by small- and wide-angle X-ray and neutron scattering: 1-Decyl-3-methylimidazolium nitrate

Nanoconfined water (water pocket) and ionic liquid crystals (ILCs) were formed in a hydrophilic ionic liquid (IL)–D2O system. Herein, 1-decyl-3-methylimidazolium nitrate ([C10mim][NO3]) was used as the IL. By the complementary use of small- and wide-angle X-ray and neutron scattering, a complicated phase diagram of [C10mim][NO3]–D2O was determined. The lyotropic ILCs of [C10mim][NO3] containing 70–90 mol% D2O appeared at room temperature. At high temperature, the lyotropic ILCs transformed into the water pocket in the nanoheterogeneous liquid.

Ionic Liquid Crystalline Calixarene with Photo‐Switchable Proton Conduction

AbstractWe have developed a new strategy for the preparation of a light‐responsive ionic liquid crystal (LC) that shows photo‐switchable proton conduction. The ionic LC consists of a bowl‐shaped calix[4]arene core ionically functionalized with azobenzene moieties. The non‐covalent architectures were obtained by the formation of ionic salts between the carboxylic acid group of an azo‐derivative and the terminal amine groups of a calixarene core. The presence of ionic salts results in a hierarchical self‐assembly process that extends to the formation of a nanostructured lamellar LC arrangement (smectic A phase). In this LC phase, the ionic LC calixarene is able to display proton conductive properties, since the ionic nanosegregated areas (formed by the ionic pairs) generate the continuous channels that favor proton transport. The optical and photo‐responsive properties were studied by UV‐Vis spectroscopy, demonstrating that the azobenzene moieties of the ionic LC undergo reversible (E)‐to‐(Z) isomerization by irradiation with UV light. Interestingly, this (E)‐to‐(Z) photoisomerization results in a decrease of the proton conductivity values since the bent‐shaped (Z)‐isomer disrupts the lamellar LC phase. This isomerization process is totally reversible and leads to an ionic LC material with unique photo‐switchable proton conductive properties.

Effect of Polymer Hydrophobicity in the Performance of Hybrid Gel Gas Sensors for E-Noses.

Relative humidity (RH) is a common interferent in chemical gas sensors, influencing their baselines and sensitivity, which can limit the performance of e-nose systems. Tuning the composition of the sensing materials is a possible strategy to control the impact of RH in gas sensors. Hybrid gel materials used as gas sensors contain self-assembled droplets of ionic liquid and liquid crystal molecules encapsulated in a polymeric matrix. In this work, we assessed the effect of the matrix hydrophobic properties in the performance of hybrid gel materials for VOC sensing in humid conditions (50% RH). We used two different polymers, the hydrophobic PDMS and the hydrophilic bovine gelatin, as polymeric matrices in hybrid gel materials containing imidazolium-based ionic liquids, [BMIM][Cl] and [BMIM][DCA], and the thermotropic liquid crystal 5CB. Better accuracy of VOC prediction is obtained for the hybrid gels composed of a PDMS matrix combined with the [BMIM][Cl] ionic liquid, and the use of this hydrophobic matrix reduces the effect of humidity on the sensing performance when compared to the gelatin counterpart. VOCs interact with all the moieties of the hybrid gel multicomponent system; thus, VOC correct classification depends not only on the polymeric matrix used, but also on the IL selected, which seems to be key to achieve VOCs discrimination at 50% RH. Thus, hybrid gels' tunable formulation offers the potential for designing complementary sensors for e-nose systems operable under different RH conditions.

Study of the mesomorphic properties and conductivity of n-alkyl-2-picolinium ionic liquid crystals

The impact of the alkyl size moiety in the mesomorphic and conductivity properties was evaluated in 2-picolinium ([Cn-2-Pic]+, n = 6 (n-hexyl), 12 (n-dodecyl) and 16 (n-hexadecyl)) organic salts. While the lengthier chains imprint crystalline order, the shortest [C6-2-Pic][Br] easily bypasses crystallisation, vitrifying on cooling. Furthermore, both [C12-2-Pic][Br] and [C16-2-Pic][Br] were found to be ionic liquid crystals (ILCs) due to the emergence of mesophases identified by Polarised Optical Microscopy and confirmed by X-Ray Diffraction, whereas the high mobility of the shortest member impairs the manifestation of LC behaviour. The temperature dependence of conductivity, between −90 to 160 °C, disclosed multiple types of charge transport, ranging from glass transition assisted mechanism in [C6-2-Pic][Br] to thermally activated mechanisms for the two ILCs. The Arrhenian activation plots of the direct current (dc) conductivity for both [C12-2-Pic][Br] and [C16-2-Pic][Br] are sensitive to the materials’ phase transitions and the respective slopes allowed to determine the activation energies. Moreover, ionic diffusion coefficients were estimated in the T-range where dc conductivity is detected, which, for [C12-2-Pic][Br] and [C16-2-Pic][Br], includes room temperature, contrary to [C6-2-Pic][Br] that is highly affected by electrode polarisation starting at cryogenic temperatures. Nyquist and Bode plots were simulated for [C12-2-Pic][Br] and [C16-2-Pic][Br] at 24 °C by electrical RC circuits, showing a strong dependence of the resistance (R) on the chain length, while the capacitance (C) maintains almost invariant. This supports the hypothesis that charge transport is made through defects, longitudinally to the chains, in the crystalline lattice and between layers, transversally to the chain alignment, in the LC phase. The established correlation between conductivity response and material’s physical state should be considered for a rational design of electronic materials tuned by the size of the alkyl tail.

Highly redispersible CNT dough for better processiblity

Carbon nanotubes (CNTs) have received considerable attention for their excellent thermal and electrical conductivity as well as scalable production. However, CNT dispersions are prone to settling and have a short shelf time, especially under high concentration, which significantly hinders their further processing and increases transportation costs. Here, we report a highly concentrated CNT dough enabled by ionic liquid crystal (ILC) as auxiliaries. Benefiting from the temperature-controlled physical transformation of the ILC, the CNTs of the powder state are successfully transferred to highly processable dough with excellent electrical conductivity, flame retardancy, and outstanding redispersibility even after 180 days of storage. In particular, the CNT dough exhibits excellent self-healing properties and good reshapable capability. Various bulk form CNT derived from the ILC armored CNT dough are realized by facile processing technique. Hybrid nanocomposite papers with ANF nanofiber exhibited excellent photothermal conversion and Joule heating properties. The redispersible CNT doughs presented here promise to revolutionize traditional CNT powder and dispersions as the primary raw material for building CNT-based architectures and facilitate the large-scale application of CNTs.

Competition between enzymatic and non-enzymatic electrochemical determination of cholesterol

Cholesterol is considered an essential component in animal bodies that plays a vital role in hormone production and creating vitamin D. The elevated blood cholesterol level increases the possibility of heart disease. Consequently, the clinical diagnosis of cholesterol concentration is critical for human health care. The electrochemical sensor is one of the leading techniques for detecting cholesterol in biological fluids with high accuracy. Therefore, this review included comparative insights into the enzymatic and non-enzymatic techniques for electrochemical sensing. We discussed the recent progress in the electrochemical determination of cholesterol via various materials, e.g., metal composites, carbonaceous materials, ionic liquid crystals, and polymers. We explained electrochemical sensing techniques like amperometric, potentiometric, conductometric, and impedimetric methods. Furthermore, various enzyme-immobilization approaches were reported, like surface binding and encapsulation. The synergistic effect between the electrode surface and enzymes was demonstrated to clarify the detection mechanism. The review article summarized the outcome detection limits and linear range of detection for various surfaces for enzymatic and non-enzymatic electrochemical techniques within the last twenty years.

Development of ionic liquid crystals based on pyridinium and picolinium cations

ABSTRACT The search for novel materials and technologies with improved performance increased in the last decades. In particular, ionic liquid crystals (ILCs), a modern class of advanced materials that benefits from the synergistic properties of ionic liquids and liquid crystals, are playing an important role in different areas of knowledge, attracting the interest of the academia and industry, driven by their peculiar tailoring ability. Herein, it comprises a brief synopsis of several pyridinium and picolinium ILCs previously prepared to understand how their mesomorphism and conductivity profiles are affected by small structural changes in terms of the methyl group position and alkyl chain length, providing important insights into future design of functional materials by a simple anionic exchange. Moreover, microwave-assisted reactions were emphasised as a simple and greener synthetic method to prepare ionic liquid crystals through alkylation, which can also be applied to other organic salts.

Does thermotropic liquid crystalline self-assembly control biological activity in amphiphilic amino acids? - tyrosine ILCs as a case study.

Amphiphilic amino acids represent promising scaffolds for biologically active soft matter. In order to understand the bulk self-assembly of amphiphilic amino acids into thermotropic liquid crystalline phases and their biological properties a series of tyrosine ionic liquid crystals (ILCs) was synthesized, carrying a benzoate unit with 0-3 alkoxy chains at the tyrosine unit and a cationic guanidinium head group. Investigation of the mesomorphic properties by polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (WAXS, SAXS) revealed smectic A bilayers (SmAd) for ILCs with 4-alkoxy- and 3,4-dialkoxybenzoates, whereas ILCs with 3,4,5-trisalkoxybenzoates showed hexagonal columnar mesophases (Colh), while different counterions had only a minor influence. Dielectric measurements revealed a slightly higher dipole moment of non-mesomorphic tyrosine-benzoates as compared to their mesomorphic counterparts. The absence of lipophilic side chains on the benzoate unit was important for the biological activity. Thus, non-mesomorphic tyrosine benzoates and crown ether benzoates devoid of additional side chains at the benzoate unit displayed the highest cytotoxicities (against L929 mouse fibroblast cell line) and antimicrobial activity (against Escherichia coli ΔTolC and Staphylococcus aureus) and promising selectivity ratio in favour of antimicrobial activity.

Phase coexistence in [C22/C1MIm]+[NO3]- ionic-liquid mixtures and first-order phase transitions from homogeneous liquid to smectic B by varying the cation ratio.

We perform molecular dynamics simulations to investigate the transition processes of [C22/C1MIm]+[NO3]- binary mixtures by varying the cation ratio of C22 to C1 at a fixed temperature of 400 K. The cation ratio is tuned by ranging C22 percentage from 0% to 100% with a fixed number of 4096 total simulated ion pairs. Our simulated-annealing results indicate that, at 400 K, pure C1 is a homogeneous liquid whilst pure C22 is an ionic liquid crystal (ILC) of smectic-B (SmB) type. With increasing C22 percentage, the system goes through a first-order phase transition from homogeneous liquid to nano-fragment liquid in the range from 15% to 17.5%, during which some of the individual cationic alkyl side chains locally aggregate to form small bundles "floating" in the polar "solvent" composed of anions and cationic head groups. Although the side chains in each bundle are parallelly aligned, the bundles distribute randomly without a global orientation. As the C22 percentage further increases, another first-order phase transition occurs to bring the system into the SmB ILC phase. Particularly, when the C22 percentage is in the range from 45% to 50%, the SmB phase coexists with the liquid phase containing both individual and bundled alkyl side chains.

Phase behaviour of mixtures of charged soft disks and spheres.

We have investigated the phase behaviour of mixtures of soft disks (Gay-Berne oblate ellipsoids, GB) and soft spheres (Lennard-Jones, LJ) with opposite charge as a model of ionic liquid crystals and colloidal suspensions. We have used constant volume Molecular Dynamics simulations and fixed the stoichiometry of the mixture in order to have electroneutrality; three systems have been selected GB : LJ = 1 : 2, GB : LJ = 1 : 1 and GB : LJ = 2 : 1. For each system we have selected three values of the scaled point charge q* of the GB particles, namely 0.5, 1.0 and 2.0 (and a corresponding negative scaled charge of the LJ particles that depends on the stoichiometric ratio). We have found a very rich mesomorphism with the formation, as a function of the scaled temperature, of the isotropic phase, the discotic nematic phase, the hexagonal columnar phase and crystal phases. While the structure of the high temperature phases was similar in all systems, the hexagonal columnar phases exhibited a highly variable morphology depending on the scaled charge and stoichiometry. On the one hand, GB : LJ = 1 : 2 systems form lamellar structures, akin to smectic phases, with an alternation of layers of disks (exhibiting an hexagonal columnar phase) and layers of LJ particles (in the isotropic phase). On the other hand, for the 2 : 1 stoichiometry we observe the formation of a frustrated hexagonal columnar phase with an alternating tilt direction of the molecular axis. We rationalize these findings based on the structure of the neutral ion pair dominating the behaviour at low temperature and high charge.

Ionic Chiral Ferrocene Doped Cholesteric Liquid Crystal with Electronically Tunable Reflective Bandwidth performance.

Cholesteric liquid crystals (CLC) were widely used in optical devices as one-dimensional photonic crystals. However, their reflective bands cannot be adjusted, which limits their wide application in many fields. In this paper, a series of ionic chiral ferrocene derivatives (CD-Fc+) as dopants were designed and prepared, and their doping into negative liquid crystal matrix was investigated to develop cholesteric response liquid crystal composites with electrically tunable reflective bands. The effects of electric field frequency, voltage, retention time of voltage and molecular structure on the broadening of reflection bandwidth were investigated in detail.

Triprotic Ammonium Oleate Ionic Liquid Crystal Lubricant for Copper-Copper Friction and Wear Reduction

The triprotic ammonium carboxylate ionic liquid crystal (2-hydroxyethyl)ammonium oleate (MO) has been studied as a neat lubricant and as a lubricant additive in two base oils, PAO6 and 100N. The lubricants have been used in commercially pure copper-OFHC copper balls on disk reciprocating sliding contact at room temperature. Neat MO presents a very good lubricating performance, with a friction coefficient of 0.06 and a wear rate of OFHC copper disk of 4.15 × 10−7 mm3/N·m. These results are, respectively, 94% and 98% lower than those obtained for PAO6, with similar reductions obtained with respect to 100N. MO has also been studied as an additive in 2wt.% proportion. The severe abrasive wear mechanism that takes place in the presence of neat base oils is reduced by the MO additive.

Towards Nematic Phases in Ionic Liquid Crystals - A Simulation Study.

Ionic liquid crystals (ILCs) are soft matter materials with broad liquid crystalline phases and intrinsic electric conductivity. They typically consist of a rod-shaped mesogenic ion and a smaller spherical counter-ion. Their mesomorphic properties can be easily tuned by exchanging the counter ion. ILCs show a strong tendency to form smectic A phases due to the segregation of ionic and the non-ionic molecular segments. Nematic phases are therefore extremely rare in ILCs and the question of why nematic phases are so exceptional in existing ILCs, and how nematic ILCs might be obtained in the future is of vital interest for both the fundamental understanding and the potential applications of ILCs. Here, we present the result of a simulation study, which highlights the crucial role of the location of the ionic charge on the rod-like mesogenic ions in the phase behaviour of ILCs. We find that shifting the charge from the ends towards the centre of the mesogenic ion destabilizes the liquid crystalline state and induces a change from smectic A to nematic phases.

Ionic liquid-doped liquid crystal/polymer composite for multifunctional smart windows

In this work, a facile approach to fabricating multifunctional smart windows by employing an ionic liquid-doped liquid crystal (LC)/polymer composite was reported. The composite was prepared via an established method of polymerization-induced phase separation while a commercial ionic liquid was doped into the composite as a supporting electrolyte. The smart window, which was prepared by sandwiching the as-prepared composite in two indium tin oxide (ITO) glasses, can exhibit a multifunctional mode. Specifically, the light transmittance and the coloration of the windows were regulated by different external stimuli. The electrically switchable transmittance of the window was achieved by an alternating current (AC) electric field and the window transformed into a transparent state from an opaque state under the AC field. The temperature was also used to realize the heat controllable transmittance, in which the window was turbid at a lower temperature and transparent at a high temperature. The electrically induced coloration of the window was manipulated by a direct current (DC) electric field and the window was changed into a green state from a colorless state under the DC field. This work is expected to open avenues to achieve a promising candidate for multifunctional smart windows with practical applications.

Intensity-dependent optical nonlinearities of composite materials made of ionic liquid crystal glass and bimetallic nanoparticles

ABSTRACT Although isotropic glass materials are ubiquitous in everyday life, the properties and applications of anisotropic liquid crystal glasses stable at room temperature are still insufficiently explored. In this paper, nonlinear-optical properties of unconventional glass nanocomposites made of mesogenic cadmium octanoate and bimetallic nanoparticles are reported. Two types of bimetallic nanoparticles (Ag/Au core – Au shell and Ag/Au homogeneous alloy) are synthesised utilising an ionic liquid crystal phase of cadmium octanoate as a nanoreactor. A nonlinear-optical characterisation of the studied samples is performed by means of a Z-scan technique and using nanosecond laser pulses at 1064 nm excitation wavelength. The studied nanocomposites exhibit a strong nonlinear-optical response with the magnitude of the third-order nonlinear optical susceptibility within a 2.81 × 10−8 - 2.19 × 10−7 esu range. The measured values of the effective nonlinear refractive indices and nonlinear absorption coefficients depend on the laser beam intensity. The physical mechanisms of the observed intensity-dependent optical nonlinearities are discussed.

New Water-Ethylene Glycol Lubricants with Stearate Ionic Liquid Crystal Additive

The main purpose of the present study is to improve the tribological performance of aqueous lubricants with the use of ecofriendly, fatty acid-derived additives. The protic ionic liquid crystal bis(2-hydroxyethyl)ammonium stearate (DES) has been added to 50:50 water+ethylene glycol (W–EG) to obtain (W–EG)+0.5%DES; (W–EG)+1%DES and (W–EG)+2%DES emulsions. The new lubricants have been studied in sapphire-AISI (American Iron and Steel Institute) 316L stainless-steel pin-on-disk sliding contacts. The addition of DES reduces the friction coefficient by up to 76% and wear rate by up to 80%, with respect to (W–EG). The best performance is found for the emulsions with the lower proportion of DES (0.5 and 1 wt.%). These results have been related to viscosity and turbidity values. Wear mechanisms have been studied by Scanning Electron Microscopy/Energy Dispersive X-ray Spectroscopy (SEM/EDX) and by Raman microscopy. While W–EG shows a severe abrasive mechanism, no abrasion marks are present inside the wear track after lubrication with (W–EG)+0.5%DES, the emulsion with the lowest wear rate. After lubrication with W–EG, an increase in oxygen content is observed inside the wear track, as determined by EDX and confirmed by Raman microscopy, which shows the presence of iron oxides. The addition of DES reduces these oxidation processes.

Main‐chain liquid‐crystalline polymers bearing different anion

ABSTRACT In this paper, d-camphoryl chloride, terephthaloyl chloride, hydroquinone and 2,3-dibromosuccinyl chloride were used as raw materials to synthesise the main-chain liquid crystalline polymers by condensation polymerisation. A series of main-chain ionic liquid crystalline polymers with triphenylphosphine as the cationic group were successfully prepared by grafting triphenylphosphine onto the liquid crystalline polymer. By characterisation, we found that the main chain ionic liquid crystal polymer containing quaternary phosphonium ions had low glass transition temperature, high ionic conductivity and strong antibacterial properties. The quaternary phosphonium ions-containing main-chain ionic liquid-crystalline polymers show Smectic A phase, and the main-chain liquid crystalline polymers show nematic phase. The antibacterial experiments showed that the quaternary phosphonium ions-containing main-chain ionic liquid-crystalline polymers had good antibacterial properties, and the antibacterial ability of the polymer was also enhanced with the increase of phosphorus content. In addition, the orientation structure of liquid crystal also had a significant influence on its antibacterial properties.

Self‐assembly, mesomorphic behavior, and ionic conductivity of polymerized ionic liquid crystals with a threshold switching characteristic

AbstractA series of pyridinium‐based polymerized ionic liquid crystals (PILCs) with different anions were synthesized, including poly(pyridinium bromide)s and poly(pyridinium trifluoroacetate)s, and their chemical structure, liquid‐crystalline properties and conductivity properties were investigated by many experimental techniques. At low temperature, some PILCs containing less cholesteryl mesogens presented smectic A phases; while the PILCs containing more cholesteryl mesogens displayed bicontinuous cubic phases. At high temperature, all the PILCs showed nematic phases. Compared with the corresponding poly(pyridinium bromide)s based on identical polymer cations, the poly(pyridinium trifluoroacetate)s showed lower glass transition and isotropic transition temperature. The PILCs displayed excellent temperature‐dependent ionic conductivity, making it attractive for applications in temperature‐controlled electronic switches. In cubic and smectic A phases, the PILCs presented Arrhenius‐like ionic conductivity behavior, and they exhibited Vogel–Fulcher–Tammann‐like behavior in nematic phases.

Anisotropic polymer membranes retaining nanolayered hydrogen sulfate anions for enhanced anhydrous proton conduction

Ionic liquids bearing acidic anions are intrinsically proton-conductive but mechanically weak for device application. Maintaining ionic conductivities after solidifying ionic liquids by polymerization remains a challenge due to the restriction of ion mobility. In this work, nanolayered ionic membranes bearing aligned acidic hydrogen sulfate anions are prepared from ionic liquid crystals for enhanced anhydrous proton conduction. To obtain polymerizable smectic assemblies, 3-tetradecyl-1-vinyl-imidazolium hydrogen sulfate as monomer and 3, 3'-(decane-1,10-diyl) bis(1-vinyl-imidazolium) hydrogen sulfate as cross-linker are synthesized. In situ polymerization of the ionic assemblies in the smectic phase results in robust membranes retaining nanolayered acidic anions. The nanolayered ionic membranes exhibit tensile strengths up to 16.6 MPa and keep thermally stable at temperatures up to 220 °C. Anhydrous conductivity of the aligned ionic membrane reaches a maximum of 22 mS cm−1 and follows the Arrhenius law. A peak power density of 71.5 mW cm−2 is achieved at 110 °C for the fuel cell constructed from the nanolayered ionic membrane under non-humidified conditions.

Hydrogen-bonded ionic liquid crystals based on multi-armed structure: synthesis and characterization

Hydrogen-bonded ionic liquid crystals based on multi-armed structure: synthesis and characterization