Phase Transition and Dynamics in Imidazolium-Based Ionic Liquid Crystals through a Metastable Highly Ordered Smectic Phase

Phase transitions and dynamics in ionic liquid crystals confined in nanopores

We investigate the phase-transition behavior of ionic liquid crystals, namely 1-methyl-3-alkylimidazolium tetrafluoroborate, [Cnmim]BF4, confined in cylindrical nanopores using differential scanning calorimetry, x-ray scattering, and dielectric relaxation spectroscopy. Here, n is the number of carbon atoms in the alkyl part of this ionic liquid crystal. For n = 10 and 12, the isotropic liquid phase changes to the smectic phase and then to a metastable phase for the cooling process. During the subsequent heating process, the metastable phase changes to the isotropic phase via crystalline phases. The transition temperatures for this ionic liquid crystal confined in nanopores decrease linearly with the increase in the inverse pore diameter, except for the transitions between the smectic and isotropic phases. In the metastable phase, the relaxation rate of the α-process shows the Vogel-Fulcher-Tammann type of temperature dependence for some temperature ranges. The glass transition temperature evaluated from the dynamics of the α-process decreases with the decrease in the pore diameter and increases with the increase in the carbon number n. The effect of confinement on the chain dynamics can clearly be observed for this ionic liquid crystal. For n = 10, the melting temperature of the crystalline phase is slightly higher than that of the smectic phase for the bulk, while, in the nanopores, the melting temperature of the smectic phase is higher than that of the crystalline phase. This suggests that the smectic phase can be thermodynamically stable, thanks to the confinement effect.

Various Extraction Techniques of Curcumin-A Comprehensive Review

Curcumin, the active component of the rhizome of Curcuma longa, is a safe substance whose applications are extensively used in medicinal, biological, pharmacological activities, and food cosmetic additives. In the field of medicine, curcuminoids have a greater impact; they have been associated with the suppression of neuropathic pain, depression, angiogenesis, tumorigenesis, diabetes, and diseases of the liver, skin, and pulmonary systems, as well as cardiovascular and nervous systems. These are in high demand and have high market potential and inflated costs. For the aforementioned uses, as well as for basic research, it is crucial to get pure curcumin from plant sources. There is a need for effective extraction and purification techniques that adhere to standards for process efficiency, environmental friendliness, and safety. Scope: This account offers an accurate and thorough explanation of the many techniques used to extract and purify curcumin from plant sources, as well as a look at its various roles in the pharmaceutical, cosmetic, medical, and other industries. Curcumin's prospective and commercial roles are also discussed. Key findings: Curcuminoids have been extracted and purified by using a broad range of techniques that are utilized extensively across the world. Extraction of curcuminoids includes both traditional and contemporary approaches, of which a handful include Soxhlet extraction, maceration, solvent extraction, ultrasound-assisted extraction, microwave-assisted extraction, enzyme-assisted extraction, and supercritical liquid extraction. The other process called purification can be performed alone or in combination with techniques. The use of column chromatography and semipreparative high-performance liquid chromatography are examples of traditional purification procedures, and other innovative methods include high-speed counter-current chromatography and supercritical fluid chromatography.

Bridging the crystal and solution structure of a series of lipid-inspired ionic liquids

A series of 1,2-dimethylimidazolium ionic liquids bearing a hexadecyl alkyl chain are thoroughly examined via X-ray crystallography. The crystal structures reveal several key variations in the non-covalent interactions in the lipid-like salts. Specifically, distinct cation-cation π interactions are observed when comparing the bromide and iodide structures. Changing the anion to bis(trifluoromethane)sulfonimide (Tf₂N^-) changes these cation-cation π interactions with anion⋯π interactions. Additionally, several well-defined geometries of the cations are noted based on torsion and core-plane angles of the alkyl chains. Hirshfeld surface analysis is used to distinguish the interactions and geometries in the solid state, helping to reveal characteristic structural fingerprints for the compounds. The solid-state structures of the ionic liquids are correlated with the solution-state structures through UV-vis spectroscopic studies, further emphasizing the importance of the π interactions in the formation of aggregates. Finally, we investigated the thermal properties of the ionic liquids, revealing complex phase transitions for the iodide-containing species. These phase transitions are further rationalized via the analysis of the data gathered from the structures of the other crystallized salts.

Coarse-grained simulations of ionic liquid materials: from monomeric ionic liquids to ionic liquid crystals and polymeric ionic liquids

Ionic liquid (IL) materials are promising electrolytes with striking physicochemical properties for energy and environmental applications. Heterogeneous structures and transport quantities of monomeric and polymeric ILs are intrinsically intercorrelated and span multiple spatiotemporal scales, which is more feasible for coarse-grained (CG) simulations than atomistic modelling. Herein we constructed a novel CG model for ethyl-imidazolium tetrafluoroborate ILs with varied cation alkyl chains ranging from C₂ to C₂₀, and the interaction parameters were validated against representative static and dynamic properties that were obtained from atomistic reference simulations and experimental characterizations at relevant thermodynamic states. This CG model was extended to study thermotropic phase behaviors of monomeric ILs and to explore ion association structures and ion transport quantities in polymeric ILs with different architectures. A systematic analysis of structural and dynamical quantities identifies an evolution of liquid morphology from homogeneous to nanosegregated structures and then a smectic mesomorphism via a gradual lengthening of cation alkyl chains, and thereafter a distinct structural transition characterized by a monotonic decrease in orientational and translational order parameters in a sequential heating cascade. Backbone and pendant polymeric ILs exhibit evident anion association structures with cation monomers and polymer chains, and striking intra- and interchain coordinations between cation monomers owing to an intrinsic polymer architecture effect. Such a peculiar ion pairing association leads to a progressive increase in anion intrachain hopping probabilities, and a concomitant decrease in anion interchain hopping events with a gradual lengthening of polymeric ILs. The anion diffusivities in polymeric ILs are intrinsically correlated with ion pairing association lifetimes and ion structural relaxation times via a universal power law correlation D ∼ τ^-1, irrespective of polymer architectures.

Ionic Liquid-Based Surfactants: Recent Advances in Their Syntheses, Solution Properties, and Applications

The impetus for the expanding interest in ionic liquids (ILs) is their favorable properties and important applications. Ionic liquid-based surfactants (ILBSs) carry long-chain hydrophobic tails. Two or more molecules of ILBSs can be joined by covalent bonds leading, e.g., to gemini compounds (GILBSs). This review article focuses on aspects of the chemistry and applications of ILBSs and GILBSs, especially in the last ten years. Data on their adsorption at the interface and micelle formation are relevant for the applications of these surfactants. Therefore, we collected data for 152 ILBSs and 11 biamphiphilic compounds. The head ions of ILBSs are usually heterocyclic (imidazolium, pyridinium, pyrrolidinium, etc.). Most of these head-ions are also present in the reported 53 GILBSs. Where possible, we correlate the adsorption/micellar properties of the surfactants with their molecular structures, in particular, the number of carbon atoms present in the hydrocarbon "tail". The use of ILBSs as templates for the fabrication of mesoporous nanoparticles enables better control of particle porosity and size, hence increasing their usefulness. ILs and ILBSs form thermodynamically stable water/oil and oil/water microemulsions. These were employed as templates for (radical) polymerization reactions, where the monomer is the "oil" component. The formed polymer nanoparticles can be further stabilized against aggregation by using a functionalized ILBS that is co-polymerized with the monomers. In addition to updating the literature on the subject, we hope that this review highlights the versatility and hence the potential applications of these classes of surfactants in several fields, including synthesis, catalysis, polymers, decontamination, and drug delivery.

Fluorenone imidazolium salts as novel de Vries materials

In ionic liquid crystals (ILCs) tilted mesophases such as SmC required for electro-optic devices are quite rare. We report a design concept that induced the SmC phase and enabled de Vries-like behaviour in ILCs. For this purpose, we synthesized and characterized a library of ILC derivatives ImR(On,Ym)X which consist of a rigid central fluorenone core containing an alkoxy or thioether side chain and connected via a flexible spacer to an imidazolium head group. The mesomorphic properties were studied by differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffraction (XRD). Temperature-dependent measurements of smectic layer spacing d by small-angle X-ray scattering (SAXS) and of optical tilt angles by POM demonstrate that ILCs ImR(On,Ym)X undergo SmA–SmC phase transitions with maximum layer contraction values between 0.4% and 2.1%. The lowest reduction factor R of 0.2 at the reduced temperature T − T_AC = −10 K was calculated for Im(O12,S14)Br. Electron density calculations indicated a bilayer structure. Furthermore, temperature dependent emission studies show that self-assembling has a strong influence on the emission intensity of these ILCs.

ILCs consisting of cationic head group–spacer–fluorenone central core–side chain show de Vries-like behaviour.

Influence of the ion size on the stability of the smectic phase of ionic liquid crystals

The thermotropic phase behavior of ionic liquids and ionic liquid crystals based on novel N-alkyl-3-methylpyridinium halides, trihalides and dichloroiodates was experimentally studied by polarized optical spectroscopy (POM) and differential scanning calorimetry (DSC) as well as by molecular dynamics (MD) simulation. In the experiments, the existence and thermal range of stability of the smectic phase of these ionic liquid crystals are found to strongly depend on the volume ratio between the cation and anion, that is their relative size. Only compounds with a relatively large volume ratio of the cation to anion, i.e., those with longer cationic alkyl chains and monoatomic halide anions, have a stable smectic A phase. Both melting points and clearing points increase with such a ratio. The MD simulation results qualitatively agree very well with the experimental data and provide molecular details which can explain the experimentally observed phenomena: the stronger van der Waals interactions from the longer alkyl chains and the stronger electrostatic interactions from the smaller anions with a higher charge density increase the stability of both the crystal phase and the smectic phase; this also prevents the ionic layers from easily mixing with the hydrophobic regions, a mechanism that ultimately leads to a nanosegregated isotropic liquid phase.

Percolation Phase Transition from Ionic Liquids to Ionic Liquid Crystals

Due to their complex molecular structures and interactions, phase behaviors of complex fluids are quite often difficult to be identified by common phase transition analysis methods. Percolation phase transition, on the other hand, only monitors the degree of connection among particles without strict geometric requirements such as translational or orientational order, and thus suitable for pinpointing phase transitions of complex fluids. As typical complex fluids, ionic liquids (ILs) exhibit phases beyond the description of simple liquid theories. In particular, with an intermediate cationic side-chain length, ILs can form the nanoscale segregated liquid (NSL) state, which will eventually transform into the ionic liquid crystal (ILC) structure when the side chains are adequately long. However, the microscopic mechanism of this transformation is still unclear. In this work, by means of coarse-grained molecular dynamics simulation, we show that, with increasing cationic side-chain length, some local pieces of non-polar domains are gradually formed by side chains aligned in parallel inside the NSL phase, before an abrupt percolation phase transition happens when the system transforms into the ILC phase. This work not only identifies that the NSL to ILC phase transition is a critical phenomenon, but also demonstrates the importance of percolation theory to complex fluids.

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

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.

Phase Behaviors of Ionic Liquids Heating from Different Crystal Polymorphs toward the Same Smectic-A Ionic Liquid Crystal by Molecular Dynamics Simulation

Five distinct crystal structures, based on experimental data or constructed manually, of ionic liquid [C14Mim][NO3] were heated in NPT molecular dynamics simulations under the same pressure such that they melted into the liquid crystal (LC) phase and then into the liquid phase. It was found that the more entropy-favored structure had a higher solid-LC transition temperature: Before the transition into the LC, all systems had to go through a metastable state with the side chains almost perpendicular to the polar layers. All those crystals finally melted into the same smectic-A LC structure irrelevant of the initial crystal structure.

Neutron scattering studies on short- and long-range layer structures and related dynamics in imidazolium-based ionic liquids

Alkyl-methyl-imidazolium ionic liquids CnmimX (n: alkyl-carbon number, X: anion) have short-range layer structures consisting of ionic and neutral (alkylchain) domains. To investigate the temperature dependences of the interlayer, interionic group, and inter-alkylchain correlations, we have measured the neutron diffraction (ND) of C16mimPF₆, C9.5mimPF₆, and C8mimPF₆ in the temperature region from 4 K to 470 K. The quasielastic neutron scattering (QENS) of C16mimPF₆ was also measured to study the dynamics of each correlation. C16mimPF₆ shows a first-order transition between the liquid (L) and liquid crystalline (LC) phases at T_c = 394 K. C8mimPF₆ exhibits a glass transition at T_g = 200 K. C9.5mimPF₆, which is a 1:3 mixture between C8mimPF₆ and C10mimPF₆, has both transitions at T_c = 225 K and T_g = 203 K. In the ND experiments, all samples exhibit three peaks corresponding to the correlations mentioned above. The widths of the interlayer peak at ca. 0.2 Å^-1 changed drastically at the L-LC transitions, while the interionic peaks at ca. 1 Å^-1 exhibited a small jump at T_c. The peak position and area of the three peaks did not change much at the transition. The structural changes were minimal at T_g. The QENS experiments demonstrated that the relaxation time of the interlayer motion increased tenfold at T_c, while those of other motions were monotonous in the whole temperature region. The structural and dynamical changes mentioned above are characteristic of the L-LC transition in imidazolium-based ionic liquids.

High-pressure glass formation of a series of 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide homologues

[C_nmim][TFSI] resists external pressure and retains the local liquid structure, as if a sponge absorbs a stimulus.

Metastable State during Melting and Solid-Solid Phase Transition of [CnMim][NO3] (n = 4-12) Ionic Liquids by Molecular Dynamics Simulation

We simulate the heating process of ionic liquids [C_nMim][NO₃] (n = 4, 6, 8, 10, 12), abbreviated as C_n, by means of molecular dynamics (MD) simulation starting from a manually constructed triclinic crystal structure composed of polar layers containing anions and cationic head groups and nonpolar regions in between containing cationic alkyl side chains. During the heating process starting from 200 K, each system undergoes first a solid-solid phase transition at a lower temperature, and then a melting phase transition at a higher temperature to an isotropic liquid state (C₄, C₆, and C₈) or to a liquid crystal state (C₁₀ and C₁₂). After the solid-solid phase transition, all systems keep the triclinic space symmetry, but have a different set of lattice constants. C₄ has a more significant structural change in the nonpolar regions which narrows the layer spacing, while the layer spacings of other systems change little, which can be qualitatively understood by considering that the contribution of the effective van der Waals interaction in the nonpolar regions (abbreviated as EF1) to free energy becomes stronger with increasing side-chain length, and at the same time the contribution of the effective electrostatic interaction in the polar layers (abbreviated as EF2) to free energy remains almost the same. The melting phase transitions of all systems except C₆ are found to be a two-step process with an intermediate metastable state appeared during the melting from the crystal state to the liquid or liquid crystal state. Because the contribution of EF2 to the free energy is larger than EF1, the metastable state of C₄ has the feature of having higher ordered polar layers and lower ordered side-chain orientation. By contrast, C₈-C₁₂ have the feature of having lower ordered polar layers and higher ordered side-chain orientation, because for these systems, the contribution of EF2 to the free energy is smaller than EF1. No metastable state is found for C₆ because the free-energy contribution of EF1 is balanced with EF2.