Thermal Stability of Protic Ionic Liquids

While protic ionic liquids (ILs) have found great success as solvents for a broad range of applications, little is known about their degradation when exposed to temperatures above ambient for extended periods of time. Here, we report the thermal stability of six protic ILs, namely, ethylammonium nitrate, ethylammonium formate, ethylammonium acetate, ethanolammonium nitrate, ethanolammonium formate, and ethanolammonium acetate. The effect of heating each ionic liquid to 60 °C for 1 h or 1 week (sealed or open to the atmosphere) was evaluated by considering the changes to water content, pH, mass, thermal phase transitions, and molecular structure after each treatment. Heating each of the six ILs when sealed led to measurable shifts in their water content and 10 wt % pH, but there was no significant change in their mass, thermal phase transitions according to differential scanning calorimetry (DSC), or molecular structure using proton nuclear magnetic resonance (1H NMR) spectra, indicating that the samples were largely unchanged. The samples that were heated open to the atmosphere also displayed no significant changes after 1 h but displayed significant changes after 1 week.

Lyotropic liquid crystal phases of monoolein in protic ionic liquids

Monoolein-based liquid crystal phases are established media that are researched for various biological applications, including drug delivery. While water is the most common solvent for self-assembly, some ionic liquids (ILs) can support lipidic self-assembly. However, currently, there is limited knowledge of IL-lipid phase behavior in ILs. In this study, the lyotropic liquid crystal phase behavior of monoolein was investigated in six protic ILs known to support amphiphile self-assembly, namely ethylammonium nitrate, ethanolammonium nitrate, ethylammonium formate, ethanolammonium formate, ethylammonium acetate, and ethanolammonium acetate. These ILs were selected to identify specific ion effects on monoolein self-assembly, specifically increasing the alkyl chain length of the cation or anion, the presence of a hydroxyl group in the cation, and varying the anion. The lyotropic liquid crystal phases with 20-80 wt. % of monoolein were characterized over a temperature range from 25 to 65 °C using synchrotron small angle x-ray scattering and cross-polarized optical microscopy. These results were used to construct partial phase diagrams of monoolein in each of the six protic ILs, with inverse hexagonal, bicontinuous cubic, and lamellar phases observed. Protic ILs containing the ethylammonium cation led to monoolein forming lamellar and bicontinuous cubic phases, while those containing the ethanolammonium cation formed inverse hexagonal and bicontinuous cubic phases. Protic ILs containing formate and acetate anions favored bicontinuous cubic phases across a broader range of protic IL concentrations than those containing the nitrate anion.

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 (SmA_d) for ILCs with 4-alkoxy- and 3,4-dialkoxybenzoates, whereas ILCs with 3,4,5-trisalkoxybenzoates showed hexagonal columnar mesophases (Col_h), 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.

Current Topics in Ionic Liquid Crystals

Ionic liquid crystals (ILCs), that is, ionic liquids exhibiting mesomorphism, liquid crystalline phases, and anisotropic properties, have received intense attention in the past years. Among others, this is due to their special properties arising from the combination of properties stemming from ionic liquids and from liquid crystalline arrangements. Besides interesting fundamental aspects, ILCs have been claimed to have tremendous application potential that again arises from the combination of properties and architectures that are not accessible otherwise, or at least not accessible easily by other strategies. The current review highlights recent developments in ILC research, starting with some key fundamental aspects. Further subjects covered include the synthesis and variations of modern ILCs, including the specific tuning of their mesomorphic behavior. The review concludes with reflections on some applications that may be within reach for ILCs and finally highlights a few key challenges that must be overcome prior and during true commercialization of ILCs.

Ionic-surfactants-based thermotropic liquid crystals

Ionic surfactants can be combined with various functional groups through electrostatic interaction, resulting in a series of thermotropic liquid crystals (TLCs).

Ionic liquids for addressing unmet needs in healthcare

Advances in the field of ionic liquids have opened new applications beyond their traditional use as solvents into other fields especially healthcare. The broad chemical space, rich with structurally diverse ions, and coupled with the flexibility to form complementary ion pairs enables task-specific optimization at the molecular level to design ionic liquids for envisioned functions. Consequently, ionic liquids now are tailored as innovative solutions to address many problems in medicine. To date, ionic liquids have been designed to promote dissolution of poorly soluble drugs and disrupt physiological barriers to transport drugs to targeted sites. Also, their antimicrobial activity has been demonstrated and could be exploited to prevent and treat infectious diseases. Metal-containing ionic liquids have also been designed and offer unique features due to incorporation of metals. Here, we review application-driven investigations of ionic liquids in medicine with respect to current status and future potential.

Separate mechanisms of ion oligomerization tune the physicochemical properties of n-butylammonium acetate: cation-base clusters vs. anion-acid dimers

Ions comprising protic ionic liquids strongly interact with their neutral acid and base forms as exemplified by n-butylammonium acetate in the presence of excess n-butylamine or acetic acid.

A brief overview of the potential environmental hazards of ionic liquids

Over past decades ionic liquids, a promising alternative to traditional organic solvents, have been dramatically expanding in popularity as a new generation of chemicals with potential uses in various areas in industry. In the literature these compounds have often been referred to as environmentally friendly; however, in recent years the perception of their greenness dramatically changed as the scientific community began to proactively assess the risk of their application based on the entire life-cycle. This review gives a brief overview of the current knowledge regarding the potential risks linked to the application of ionic liquids - from preparation to their disposal, with special emphasis on their potential environmental impacts and future directions in designing inherently safer ionic liquids.

Solvent nanostructure, the solvophobic effect and amphiphile self-assembly in ionic liquids

The ability of ionic liquids (ILs) to support amphiphile self-assembly into a range of mesophase structures has been established as a widespread phenomenon. From the ILs evaluated as self-assembly media, the vast majority have supported some lyotropic liquid crystal phase formation. Many neat ionic liquids have been shown to segregate into polar and non-polar domains to form a nanostructured liquid. A very strong correlation between the nanostructure of the ionic liquid and its characteristics as an amphiphile self-assembly solvent has been found. In this review we discuss ionic liquids as amphiphile self-assembly media, and identify trends that can be used to distinguish which ionic liquids are likely to have good promotion properties as self-assembly media. In particular these trends focus on the nanostructure of neat ionic liquids, their solvent cohesive energy density, and the related solvophobic effect. We forecast that many more ILs will be identified as amphiphile self-assembly solvents in the future.

Theoretical Design Study on Photophysical Properties of Light-emitting Pyrido[3,4-b]pyrazine-based Oligomers

The electronic structures, charge injection and transport, and absorption and emission properties of four series of dimethylpyrido[3,4-b]pyrazine-based oligomers (5-(5,5-dimethyl-5H-dibenzo[b,d]silol-3-yl)-2,3-dimethylpyrido[3,4-b]pyrazine)n (SPP)n, (5-(dibenzo[b,d]thiophen-3-yl)-2,3-dimethylpyrido[3,4-b]pyrazine)n (TPP)n, (5-(9,9-dimethyl-9H-fluoren-2-yl)-2,3-dimethylpyrido[3,4-b]pyrazine)n (FPP)n, (2-(2,3-dimethylpyrido[3,4-b]pyrazin-5-yl)-9-methyl-9H-carbazole)n (PPC)n were investigated by the density functional theory approach. The ground-state geometries of (SPP)n, (TPP)n, (FPP)n and (PPC)n (n = 1–4) were optimized at the B3LYP/6–31G(d) level. The energies of the HOMO, LUMO and HOMO–LUMO energy gaps of (SPP)n, (TPP)n, (FPP)n and (PPC)n (n = 1–4) were obtained by a linear extrapolation method. Further, calculations of ionization potential, electronic affinity and reorganization energy were used to evaluate charge injection and transport abilities. For (SPP)n, (TPP)n, (FPP)n and (PPC)n (n = 1–4), the time-dependent density functional theory (TDDFT) calculation results revealed that the absorption peaks can be characterized as π–π* transitions and are coupled with the location of electron density distribution change in different repeat units. All the primary theoretical investigations are intended to establish structure–property relationships, which can provide guidance in designing and preparing novel efficient organic light-emitting materials with a high performance.

Effect of protic ionic liquids (PILs) on the formation of non-ionic dodecyl poly(ethylene oxide) surfactant self-assembly structures and the effect of these surfactants on the nanostructure of PILs

The ability of a series of non-ionic dodecyl poly(ethylene oxide) surfactants to form micelles in a variety of protic ionic liquids (PILs) was investigated using small and wide angle X-ray scattering (SAXS/WAXS). The C(12)E(n) surfactants with n = 3-8 were examined in PILs which contained either an ethyl, diethyl, triethyl, butyl, pentyl, ethanol or pentanol-ammonium cation in conjunction with either a nitrate or formate anion. The ability of the PILs to support micelles of these surfactants was highly dependent on their liquid nanostructure. The PILs containing hydroxyl groups on the cations were not nanostructured and had very low surfactant solubility (<1 wt%). The highly nanostructured PILs with butylammonium or pentylammonium cations contain large non-polar domains, and had excellent surfactant solubility, but due to the greater hydrocarbon solubility they had insufficient drive from the "solvophobic effect" to enable micelle formation. The PILs of ethylammonium nitrate (EAN), propylammonium nitrate (PAN), diethylammonium formate (DEAF) and triethylammonium formate (TEAF) had smaller non-polar domains, and all supported micelle formation below 20 wt% surfactant. The critical micelle concentration (CMC) of surfactants in EAN were two orders of magnitude greater than in water. The minimum molecular areas of the poly(ethylene oxide) head groups at the air/ionic liquid interface, A(min), were significantly larger in EAN than in water. The SAXS patterns from the micelles present in EAN fitted well to ellipsoids, whereas the micelles present in PAN fitted well to spheres. The nanostructure of select PILs was also influenced by the presence of surfactants.

Nanostructure and amphiphile self-assembly in polar molecular solvents: amides and the "solvophobic effect"

The ability of low molecular weight amides to support amphiphile self-assembly is shown to be a general feature for this class of solvents. This report extends the number of known polar solvents which can support amphiphile self-assembly by five new amides; more than doubling the number of known amides able to serve as amphiphile self-assembly media. The formation of lyotropic liquid crystalline phases by cationic and non-ionic surfactants in these liquid amides is reported. The ability of a solvent to promote amphiphile self-assembly is governed by the "solvophobic effect" and is linked to the solvent cohesiveness. The Gordon parameter which is a measure of the solvent cohesiveness was found to provide a guide to an amides capacity to support lyotropic liquid crystalline phase diversity and thermal stability ranges of those phases. The "solvophobic effect" and steric hindrance factors were compared between amide's and protic ionic liquids possessing analogous chemical structures and also being able to promote amphiphile self-assembly.