Ionic liquids as amphiphile self-assembly media

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.

Broadband dielectric spectroscopy of micelles and microemulsions formed in a hydrophilic ionic liquid: the relaxation mechanism and interior parameters

Permittivity, conductivity and volume fraction of continuous and dispersed phases of micelles and non-aqueous microemulsions formed in ionic liquid.

Phase behaviors and characterization of magnetic microemulsions containing pentaalkylguanidinium-based magnetic room-temperature ionic liquids (MRTILs)

Strong magnetic susceptibility and low viscosity magnetic microemulsions containing pentaalkylguanidinium-based magnetic room temperature ionic liquids.

How ionic species structure influences phase structure and transitions from protic ionic liquids to liquid crystals to crystals

Phase behaviour of n-alkylammonium (C6 to C16) nitrates and formates was characterised using synchrotron small angle and wide angle X-ray scattering, differential scanning calorimetry, cross polarised optical microscopy and FTIR.

Phase behaviours of a cationic surfactant in deep eutectic solvents: from micelles to lyotropic liquid crystals

Various aggregates, including micelles and the hexagonal, bicontinuous cubic and lamellar phases, are formed in deep eutectic solvents.

Solvent and concentration effects on highly defined, colloid-like ionic clusters in solution

We characterize the process of ionic self-assembly involving a macrocyclic tetraimidazolium molecular box and small dianionic salts into highly defined, colloid-like ionic clusters called ionoids.

Surfactant-Solvent Interaction Effects on the Micellization of Cationic Surfactants in a Carboxylic Acid-Based Deep Eutectic Solvent

Deep eutectic solvents have been demonstrated to support amphiphile self-assembly, providing potential alternatives as structure-directing agents in the synthesis of nanostructures, and drug delivery. Here we have expanded on this recent research to investigate the self-assembly of alkyltrimethylammonium bromide surfactants in choline chloride:malonic acid deep eutectic solvent and mixtures of the solvent with water. Surface tension and small-angle neutron scattering were used to determine the behavior of the amphiphiles. Surfactants were found to remain active in the solvent, and surface tension measurements revealed changes in the behavior of the surfactants with different levels of hydration. Small-angle neutron scattering shows that in this solvent the micelle shape depends on the surfactant chain length, varying from globular micelles (aspect ratio ∼2) for short chain surfactants to elongated micelles (aspect ratio ∼14) for long chain surfactants even at low surfactant concentration. We suggest that the formation of elongated micelles can be explained through the interaction of the solvent with the surfactant headgroup, since ion-ion interactions between surfactant headgroups and solvent may modify the morphology of the micelles. The presence of water in the deep eutectic solvents promotes an increase in the charge density at the micelle interface and therefore the formation of less elongated, globular micelles.

Quantifying intermolecular interactions of ionic liquids using cohesive energy densities

For ionic liquids (ILs), both the large number of possible cation + anion combinations and their ionic nature provide a unique challenge for understanding intermolecular interactions. Cohesive energy density, ced, is used to quantify the strength of intermolecular interactions for molecular liquids, and is determined using the enthalpy of vaporization. A critical analysis of the experimental challenges and data to obtain ced for ILs is provided. For ILs there are two methods to judge the strength of intermolecular interactions, due to the presence of multiple constituents in the vapour phase of ILs. Firstly, ced_IP, where the ionic vapour constituent is neutral ion pairs, the major constituent of the IL vapour. Secondly, ced_C+A, where the ionic vapour constituents are isolated ions. A ced_IP dataset is presented for 64 ILs. For the first time an experimental ced_C+A, a measure of the strength of the total intermolecular interaction for an IL, is presented. ced_C+A is significantly larger for ILs than ced for most molecular liquids, reflecting the need to break all of the relatively strong electrostatic interactions present in ILs. However, the van der Waals interactions contribute significantly to IL volatility due to the very strong electrostatic interaction in the neutral ion pair ionic vapour. An excellent linear correlation is found between ced_IP and the inverse of the molecular volume. A good linear correlation is found between IL ced_IP and IL Gordon parameter (which are dependent primarily on surface tension). ced values obtained through indirect methods gave similar magnitude values to ced_IP. These findings show that ced_IP is very important for understanding IL intermolecular interactions, in spite of ced_IP not being a measure of the total intermolecular interactions of an IL. In the outlook section, remaining challenges for understanding IL intermolecular interactions are outlined.

Trace Water as Prominent Factor to Induce Peptide Self-Assembly: Dynamic Evolution and Governing Interactions in Ionic Liquids

The interaction between water and biomolecules including peptides is of critical importance for forming high-level architectures and triggering life's functions. However, the bulk aqueous environment has limitations in detecting the kinetics and mechanisms of peptide self-assembly, especially relating to interactions of trace water. With ionic liquids (ILs) as a nonconventional medium, herein, it is discovered that trace amounts of water play a decisive role in triggering self-assembly of a biologically derived dipeptide. ILs provide a suitable nonaqueous environment, enabling us to mediate water content and follow the dynamic evolution of peptide self-assembly. The trace water is found to be involved in the assembly process of dipeptide, especially leading to the formation of stable noncovalent dipeptide oligomers in the early stage of nucleation, as evident by both experimental studies and theoretical simulations. The thermodynamics of the growth process is mainly governed by a synergistic effect of hydrophobic interaction and hydrogen bonds. Each step of assembly presents a different trend in thermodynamic energy. The dynamic evolution of assembly process can be efficiently mediated by changing trace water content. The decisive role of trace water in triggering and mediating self-assembly of biomolecules provides a new perspective in understanding supramolecular chemistry and molecular self-organization in biology.

"Rigid" Luminescent Soft Materials: Europium-Containing Lyotropic Liquid Crystals Based on Polyoxyethylene Phytosterols and Ionic Liquids

Soft materials of europium β-diketonate complexes constructed in lyotropic liquid crystals (LLCs) mediated by ionic liquids (ILs) are impressive for their excellent luminescence performance and stability. For the aim to further improve their mechanical processability and luminescent tunablility, the polyoxyethylene phytosterols (BPS-n) were introduced here as structure directing agents to prepare relatively "rigid" lamellar luminescent LLCs in 1-butyl-3-methyl-imidazolium hexafluorophosphate by doping europium β-diketonate complexes with different imidazolium counterions. As a result of the solvophobic sterol ring structure of BPS-n, the more effective isolation and confinement effects of europium complexes could be achieved. The longest fluorescence lifetime and the highest quantum efficiency reported so far for europium containing lyotropic organized soft materials were thus obtained. Changing the molecular structures of BPS-n with different oxyethylene chains or doped complexes with imidazolium counterions of different alkyl chain lengths, the spacings of lamellar LLC matrixes and position of dispersed complexes became tunable. The measured luminescent and rheological properties for such composite LLCs showed a dependence on the rigidity and isolation capability afforded by sterol molecules. It was also found that the increase of counterion alkyl chain length would weaken the LLC matrix's confinement and isolation effects and therefore exhibit the deteriorated luminescence performance. The enhanced luminescence efficiency and stability of doped BPS-n LLCs reflected the excellent segregation of europium complexes from each other and therefore the reduced self-quenching process. The obtained results here present the designability of LLC matrixes and their great potential to promote achieving the luminescence tunability of soft materials.

Inhomogeneity in Ethylammonium Nitrate-Acetonitrile Binary Mixtures: The Highest "Low q Excess" Reported to Date

The binary mixtures of the ionic liquid ethylammonium nitrate with acetonitrile have been studied by means of wide- and small-angle X-ray scattering and via two different computational methods, namely, classical molecular dynamics and DFT. The recently debated odd feature in the extreme low q region of some ionic liquid-based binary mixtures is linked to density fluctuations within the system. We show how the "low q excess" is due to some nanoscopic objects which are formed at certain compositions. These structures have different density with respect to the surrounding, thus generating the feature observed. Our results also show how the local arrangement is directly linked to the long-range structure. Moreover, we found once again a similarity in the physicochemical behavior of ethylammonium nitrate and water.

Tunable Gas Sensing Gels by Cooperative Assembly

The cooperative assembly of biopolymers and small molecules can yield functional materials with precisely tunable properties. Here, the fabrication, characterization, and use of multicomponent hybrid gels as selective gas sensors are reported. The gels are composed of liquid crystal droplets self-assembled in the presence of ionic liquids, which further coassemble with biopolymers to form stable matrices. Each individual component can be varied and acts cooperatively to tune gels' structure and function. The unique molecular environment in hybrid gels is explored for supramolecular recognition of volatile compounds. Gels with distinct compositions are used as optical and electrical gas sensors, yielding a combinatorial response conceptually mimicking olfactory biological systems, and tested to distinguish volatile organic compounds and to quantify ethanol in automotive fuel. The gel response is rapid, reversible, and reproducible. These robust, versatile, modular, pliant electro-optical soft materials possess new possibilities in sensing triggered by chemical and physical stimuli.

Nanosegregation and Structuring in the Bulk and at the Surface of Ionic-Liquid Mixtures

Ionic-liquid (IL) mixtures hold great promise, as they allow liquids with a wide range of properties to be formed by mixing two common components rather than by synthesizing a large array of pure ILs with different chemical structures. In addition, these mixtures can exhibit a range of properties and structural organization that depend on their composition, which opens up new possibilities for the composition-dependent control of IL properties for particular applications. However, the fundamental properties, structure, and dynamics of IL mixtures are currently poorly understood, which limits their more widespread application. This article presents the first comprehensive investigation into the bulk and surface properties of IL mixtures formed from two commonly encountered ILs: 1-ethyl-3-methylimidazolium and 1-dodecyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C₂mim][Tf₂N] and [C₁₂mim][Tf₂N]). Physical property measurements (viscosity, conductivity, and density) reveal that these IL mixtures are not well described by simple mixing laws, implying that their structure and dynamics are strongly composition dependent. Small-angle X-ray and neutron scattering measurements, alongside molecular dynamics (MD) simulations, show that at low mole fractions of [C₁₂mim][Tf₂N], the bulk of the IL is composed of small aggregates of [C₁₂mim]⁺ ions in a [C₂mim][Tf₂N] matrix, which is driven by nanosegregation of the long alkyl chains and the polar parts of the IL. As the proportion of [C₁₂mim][Tf₂N] in the mixtures increases, the size and number of aggregates increases until the C12 alkyl chains percolate through the system and a bicontinuous network of polar and nonpolar domains is formed. Reactive atom scattering-laser-induced fluorescence experiments, also supported by MD simulations, have been used to probe the surface structure of these mixtures. It is found that the vacuum-IL interface is enriched significantly in C12 alkyl chains, even in mixtures low in the long-chain component. These data show, in contrast to previous suggestions, that the [C₁₂mim]⁺ ion is surface active in this binary IL mixture. However, the surface does not become saturated in C12 chains as its proportion in the mixtures increases and remains unsaturated in pure [C₁₂mim][Tf₂N].

Effects of a Spacer on the Phase Behavior of Gemini Surfactants in Ethanolammonium Nitrate

The aggregation behavior of quaternary ammonium gemini surfactants (12-s-12) in a protic ionic liquid, ethanolammonium nitrate (EOAN), was investigated by small-angle X-ray scattering, freeze-fracture transmission electron microscopy, polarized optical microscopy, and rheological measurements. The rarely reported nonaqueous two phases in the ionic liquid were observed at lower 12-s-12 concentrations. The upper phase was composed of micelles, whereas only the surfactant unimers or multimers were detected in the low phase. At higher 12-s-12 concentrations, different aggregates were formed. The lamellar phase was observed in the 12-2-12/EOAN system, whereas the normal hexagonal phases in 12-s-12/EOAN (s = 3, 4, 5, 6, 8) systems and the micellar phase in the 12-10-12/EOAN system were observed. Such a dramatic phase transition induced by the spacer chain length was due to the unique solvent characteristics of EOAN compared to those of water and its counterpart ethylammonium nitrate.

Efficient Ionic-Liquid-Promoted Chemical Fixation of CO2 into α-Alkylidene Cyclic Carbonates

The efficient conversion of CO₂ into value-added chemicals under metal-free conditions is of significant importance from the viewpoint of sustainable chemistry. In this work, ionic liquids (ILs) with different properties were used to promote the reaction between CO₂ and propargylic alcohol for the synthesis of α-alkylidene cyclic carbonates. The protic IL 1,8-diazabicyclo-[5.4.0]-7-undecenium 2-methylimidazolide ([DBUH][MIm]) was prepared by simple neutralization of the superbase with a weak proton donor and could efficiently promote the reactions in high yields. After the reactions, the IL was separated from the reaction mixtures by simply adding water, and then reused after drying without an observable decrease in the catalytic activity and selectivity. NMR spectroscopy and detailed density functional theory analysis were used to propose a reaction mechanism. Both the cation and anion of the IL played a key synergistic role in promoting the reaction. These findings may be useful for the rational design of novel metal-free and recyclable routes for the reaction between CO₂ and propargylic alcohols.

Proteins in Ionic Liquids: Current Status of Experiments and Simulations

In the last two decades, while searching for interesting applications of ionic liquids as potent solvents, their solvation properties and their general impact on biomolecules, and in particular on proteins, gained interest. It turned out that ionic liquids are excellent solvents for protein refolding and crystallization. Biomolecules showed increased solubilities and stabilities, both operational and thermal, in ionic liquids, which also seem to prevent self-aggregation during solubilization. Biomolecules can be immobilized, e.g. in highly viscous ionic liquids, for particular biochemical processes and can be designed to some extent by the proper choice of the ionic liquid cations and anions, which can be characterized by the Hofmeister series.

Solvation of apolar compounds in protic ionic liquids: the non-synergistic effect of electrostatic interactions and hydrogen bonds

Protic ionic liquids can dissolve apolar compounds better than aprotic ionic liquids.

Dielectric relaxation of nonaqueous ionic liquid microemulsions: polarization, microstructure, and phase transition

Two nonaqueous ionic liquid (IL) microemulsions (toluene/TX-100/[bmim][PF₆] and [bmim][BF₄]/TX-100/benzene) were studied by dielectric spectroscopy covering a wide frequency range (40 Hz to 110 MHz).

Interplay of Noncovalent Interactions in Ionic Liquid/Sodium Bis(2-ethylhexyl) Sulfosuccinate Mixtures: From Lamellar to Bicontinuous Cubic Liquid Crystalline Phase

Phase transitions in mixtures of imidazolium based ionic liquid ([C₁₂mim]Br) and anionic double tail surfactant, sodium bis(2-ethylhexyl) sulfosuccinate (AOT), were studied using a multitechnique approach. The system was primarily chosen for its expected ability to form a variety of lamellar and nonlamellar liquid crystalline phases which can transform into each other via different mechanisms. Depending on the bulk composition and total surfactant concentration, mixed micelles, coacervates, and lamellar and inverse bicontinuous cubic liquid crystalline phase were observed. Along with electrostatic attractions and geometric packing constraints, additional noncovalent interactions (hydrogen bonding, π-π stacking) enhanced attractive interactions and stabilized low curvature aggregates. At stoichiometric conditions, coexistence of coacervates and vesicles was found at lower, while bicontinuous cubic phase and vesicles were present at higher total surfactant concentrations. The phase transitions from a dispersed lamellar to inverse cubic bicontinuous phase occur as a consequence of charge shielding and closer packing of oppositely charged headgroups followed by a change in bilayer curvature. Transition is continuous with both phases coexisting over a relatively broad range of concentrations and very likely involves a sponge-like phase as a structural intermediate. To the best of our knowledge, this type of phase transition has not been observed before in surface active ionic liquid/surfactant mixtures.

Photon Upconversion and Molecular Solar Energy Storage by Maximizing the Potential of Molecular Self-Assembly

The self-assembly of functional molecules into ordered molecular assemblies and the fulfillment of potentials unique to their nanotomesoscopic structures have been one of the central challenges in chemistry. This Feature Article provides an overview of recent progress in the field of molecular self-assembly with the focus on the triplet-triplet annihilation-based photon upconversion (TTA-UC) and supramolecular storage of photon energy. On the basis of the integration of molecular self-assembly and photon energy harvesting, triplet energy migration-based TTA-UC has been achieved in varied molecular systems. Interestingly, some molecular self-assemblies dispersed in solution or organogels revealed oxygen barrier properties, which allowed TTA-UC even under aerated conditions. The elements of molecular self-assembly were also introduced to the field of molecular solar thermal fuel, where reversible photoliquefaction of ionic crystals to ionic liquids was found to double the molecular storage capacity with the simultaneous pursuit of switching ionic conductivity. A future prospect in terms of innovating molecular self-assembly toward molecular systems chemistry is also discussed.

Induction of bicontinuous cubic liquid-crystalline assemblies for polymerizable amphiphiles via tailor-made design of ionic liquids

The lyotropic liquid-crystalline behavior of polymerizable amphiphiles has been tuned by using tailor-made ionic liquids as solvents so as to induce the formation of bicontinuous cubic assemblies having 3D interconnected nanochannels. The fixation of the amphiphilic assemblies has been successfully achieved by in situ photopolymerization.

Surfactant and Gelation Properties of Acetylsalicylate Based Room Temperature Ionic Liquid in Aqueous Media

An amphiphilic room temperature ionic liquid (RTIL) containing acetylsalicylate anion of type 1-dodecyl-1-methylpiperidinium acetylsalicylate, [C₁₂mpip][AcSa], is synthesized from the precursor [C₁₂mpip][Cl] by an ion exchange process. The sample is characterized, and its surface active and aggregation behavior in water has been studied and explained. The critical aggregation concentrations (CACs) are determined by a variety of methods, namely, electrical conductivity, surface tension, steady state florescence, and isothermal titration calorimetry (ITC) at different temperatures. As compared to its precursor, [C₁₂mpip][AcSa] has low CAC values, indicating enhanced favorable interactions between the [alkylmpip]⁺ cation···bulky [AcSa]^- anion and also hydrogen bonding of both of the ions with water. The free energy of aggregation ΔG⁰_a is always negative, and both enthalpy and entropy of aggregation drive the aggregation process. The micelle-like aggregates are ellipsoidal in shape. The aggregation numbers are determined from translational diffusion coefficients and florescence quenching measurements. Aggregates of [C₁₂mpip][AcSa] are larger than those of its precursor IL with chloride anion. Therefore, it is evident that the close interactions between the ion pairs of [C₁₂mpip]⁺···[AcSa]^- facilitate packing of more molecules in an aggregate. The steady state and oscillatory rheology measurements in aqueous solutions consisting of mixtures of [C₁₂mpip][AcSa] and sodium salicylate (SS), an hydrotope additive, were carried out. The analysis of zero shear viscosity and moduli properties as a function of concentration and temperature reveals that the addition of SS promotes the growth of small ellipsoid aggregates into large worm-like structures with a typical viscoelastic gel behavior. The moduli properties vs temperature profiles are complex and no hysteresis was produced in heating and cooling modes, suggesting the thermoirreversibile and complex nature of the network structures. The release of the acetylsalicylate anion from the gels could be triggered by simple dilution, and the release occurs due to surface erosion and demicellization.

About the nanostructure of the ternary system water - [BMIm]PF6 - TX-100

HYPOTHESIS

Many efforts have been made to formulate water-IL microemulsions. One of the most intensely studied systems is H₂O - 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIm]PF₆) - 4-octylphenol polyethoxylate (TX-100) and it is not questioned that this system forms microemulsions. The nanostructures observed for traditional microemulsions are postulated with the surfactant being adsorbed at the interface such that the hydrophilic EO sides intrude into the water domains, while the hydrophobic hydrocarbon chains are immersed into [BMIm]PF₆. However, the high polarity of [BMIm]PF₆ and the observation that [BMIm]PF₆ mixes well with oligoethylene oxides but hardly with non-polar solvents like toluene or alkanes are not in line with this picture.

EXPERIMENTS

We re-studied the ternary system H₂O - [BMIm]PF₆ - TX-100 by measuring phase diagrams, determining tie-lines, and carrying out ROESY NMR and PFG NMR measurements.

FINDINGS

We found that the hydrophobic part of the surfactant interacts neither with water nor with [BMIm]PF₆, while both solvents interact with the hydrophilic part of the surfactant. We suggest that the surfactant is not adsorbed at the interface between water and the IL, but forms normal spherical or elongated micelles or even continuous aggregates with the hydrocarbon chains forming the interior of the aggregates.

Micelle formation of a non-ionic surfactant in non-aqueous molecular solvents and protic ionic liquids (PILs)

Many ionic liquids and low molecular weight polar solvents have been reported to support amphiphile self-assembly, with most of these reported for the first time in the last decade. This phenomenon is attributed to the solvophobic effect (analogous to the hydrophobic effect in water). However, to date there has been no systematic study which evaluates micelle formation in a large library of non-aqueous solvents. Here we investigate micelle formation of a non-ionic amphiphile, hexa-ethyleneglycol mono n-dodecyl ether, C12E6, in a diverse range of molecular solvents and protic ionic liquids (PILs). Nine of the 19 non-aqueous molecular solvents investigated, and all four of the PILs, were found to support micelle formation. A link was investigated between the solvent cohesive energy density (as estimated using the Gordon parameter) and both the critical micelle concentration and the related free energy of micellization . In addition, the chemical structure and liquid mesostructure of the solvent were found to be important factors in the ability of the solvents to support micelle formation.

Tailored ionic liquid-based surfactants for the formation of microemulsions with water and a hydrophobic ionic liquid

Microemulsions (μe) with water and a hydrophobic ionic liquid (IL) usually require 45-60 wt% surfactant to solubilize equal amounts of water and IL. To increase the efficiency we designed a new class of surfactants by combining a hydrophilic but IL-ophobic carbohydrate-based part with a hydrophobic but IL-ophilic IL-based part. These surfactants allow formulating microemulsions with 20 wt% surfactant only which opens up a new arena for efficient water-IL μes.

Micelle co-assembly in surfactant/ionic liquid mixtures

HYPOTHESIS

The phase behavior of amphiphiles is known to depend on their solvent environment. The organic character of ionic liquids suggested the possibility to tune surfactant aggregation, even in the absence of water, by selection of appropriate ionic liquid chemistry. To that end the behavior of the surfactant sodium dodecylsulfate in a chemically similar imidazolium ionic liquid, 1-ethyl-3-methyl imidazolium ethylsulfate, was explored.

EXPERIMENTS

The solubility of sodium dodecylsulfate in 1-ethyl-3-methyl imidazolium ethylsulfate was determined, establishing the Krafft temperature. Tensiometry was performed to obtain interfacial properties such as the surface excess and area per molecule. Pulsed-field gradient spin-echo NMR was used to determine the diffusion coefficients of all the major species, including micelles, as a function of surfactant concentration. Importantly, all three methods provided consistent values for the critical micelle concentration.

FINDINGS

Analysis of tensiometry data suggests, and is confirmed by NMR results, that the ionic liquid ions are incorporated along with surfactants into micelles, revealing a complex micellization behavior. In light of these findings past studies with ternary mixtures of surfactants, ionic liquids, and water may merit additional scrutiny. Given the large number of ionic liquids, this work suggests opportunities to further control micelle formation and properties.