Ionic liquids as amphiphile self-assembly media

Ionic Liquid Crystals: Versatile Materials

This Review covers the recent developments (2005-2015) in the design, synthesis, characterization, and application of thermotropic ionic liquid crystals. It was designed to give a comprehensive overview of the "state-of-the-art" in the field. The discussion is focused on low molar mass and dendrimeric thermotropic ionic mesogens, as well as selected metal-containing compounds (metallomesogens), but some references to polymeric and/or lyotropic ionic liquid crystals and particularly to ionic liquids will also be provided. Although zwitterionic and mesoionic mesogens are also treated to some extent, emphasis will be directed toward liquid-crystalline materials consisting of organic cations and organic/inorganic anions that are not covalently bound but interact via electrostatic and other noncovalent interactions.

Understanding the hydrogen bonds in ionic liquids and their roles in properties and reactions

Ionic liquids (ILs) have many potential applications in the chemical industry. In order to understand ILs, their molecular details have been extensively investigated. Intuitively, electrostatic forces are solely important in ILs. However, experiments and calculations have provided strong evidence for the existence of H-bonds in ILs and their roles in the properties and applications of ILs. As a structure-directing force, H-bonds are responsible for ionic pairing, stacking and self-assembling. Their geometric structure, interaction energy and electronic configuration in the ion-pairs of imidazolium-based ILs and protic ionic liquids (PILs) show a great number of differences compared to conventional H-bonds. In particular, their cooperation with electrostatic, dispersion and π interactions embodies the physical nature of H-bonds in ILs, which anomalously influences their properties, leading to a decrease in their melting points and viscosities and thus fluidizing them. Using ILs as catalysts and solvents, many reactions can be activated by the presence of H-bonds, which reduce the reaction barriers and stabilize the transition states. In the dissolution of lignocellulosic biomass by ILs, H-bonds exhibit a most important role in disrupting the H-bonding network of cellulose and controlling microscopic ordering into domains. In this article, a critical review is presented regarding the structural features of H-bonds in ILs and PILs, the correlation between H-bonds and the properties of ILs, and the roles of H-bonds in typical reactions.

Study on the catalytic performance of laccase in the hydrophobic ionic liquid-based bicontinuous microemulsion stabilized by polyoxyethylene-type nonionic surfactants

To formulate a compatible green medium for the conversion of a hydrophobic substrate by a hydrophilic enzyme, we investigated the phase behavior of pseudo ternary hydrophobic ionic liquid (HIL)/buffer/polyoxyethylene-type nonionic surfactant (CnEm)/n-alcohol system and the effects of the components on the formulation of the HIL-based bicontinuous microemulsion. It is found that small head group of the surfactant, high concentration of n-alcohol (medium/long alkyl chain) and low cohesive energy density of the HIL result in low phase transition temperature. In the CnEm stabilized compatible bicontinuous microemulsion, the kinetics of laccase catalyzed oxidation of 2,6-dimethoxyphenol were also investigated. It is found that in addition to temperature, n-alcohol is the key parameter affecting the catalytic performance of laccase, and the optimum n-alcohol depends on the type of HIL as an oil phase. All the kinetic parameters, such as Km, kcat, kcat/Km, and Ea (apparent activation energy), indicate that the bicontinuous microemulsion consisting of [Omim]NTf2/buffer/CnEm/n-hexanol is a suitable medium for the laccase-catalyzed reaction. To the best of our knowledge, this is the first report on the formulation of HIL-based bicontinuous microemulsion for enzyme catalysis.

How the cation 1-butyl-3-methylimidazolium impacts the interaction between the entrapped water and the reverse micelle interface created with an ionic liquid-like surfactant

The behavior of the interfacial water entrapped in reverse micelles (RMs) formed by the ionic liquid-like surfactant 1-butyl-3-methylimidazolium 1,4-bis-2-ethylhexylsulfosuccinate (bmim-AOT) dissolved in benzene (or chlorobenzene) was investigated using noninvasive techniques such as dynamic light scattering (DLS), static light scattering (SLS), FT-IR and (1)H NMR. The DLS and SLS results reveal the formation of discrete spherical and non-interacting water droplets stabilized by the bmim-AOT surfactant. Moreover, since the droplet size increases as the W0 (W0 = [water]/[surfactant]) value increases, water interacts with the RM interface. From FT-IR and (1)H NMR data, a weaker water-surfactant interaction in bmim-AOT RMs in comparison with the RMs created by sodium 1,4-bis-2-ethylhexylsulfosuccinate (Na-AOT) is detected. Consequently, there are less water molecules interacting with the interface in bmim-AOT RMs, and their hydrogen bond network is not completely disrupted as they are in Na-AOT RMs. The results show how the nature of the new cation impacts the interaction between the entrapped water and the RM interface, modifying the interfacial water structure in comparison with the results known for Na-AOT.

Micelle structure in a deep eutectic solvent: a small-angle scattering study

Choline chloride:urea deep eutectic solvent provides a novel media for surfactant self-assembly with micelle morphology tunable by adding water.

Microstructure of copolymeric micelles modulated by ionic liquids: investigating the role of the anion and cation

The addition of ionic liquids significantly alters the micellization behaviour of, and has a profound effect on, the micelles of T1304.

Dual functionality of amphiphilic 1-alkyl-3-methylimidazolium hydrogen sulfate ionic liquids: surfactants with catalytic function

A series of amphiphilic 1-alkyl-3-methylimidazolium hydrogen sulfate ILs were synthesized. Their co-catalytic activities have been determined and discussed in terms of their structure and surface properties.

Green solvent ionic liquids: structural directing pioneers for microwave-assisted synthesis of controlled MgO nanostructures

Various MgO nanostructures were obtained via one-pot microwave-assisted synthesis in various structural directing ionic liquids.

Illuminating microemulsions: ionic liquid–CdS quantum dots hybrid materials as potential white light harvesting systems

Ionic liquid–CdS quantum dot hybrid materials have been shown to display balanced white light emission with very high quantum efficiency.

Activity and conformation of lysozyme in molecular solvents, protic ionic liquids (PILs) and salt–water systems

The role of solvent features on enzyme stability was investigated by comparing the conformation and activity of lysozyme in molecular solvents, PILs and salt solutions. Electrostatic interactions and the solvophobic effect dominated at low and high solvent concentrations, respectively.

Effect of alkyl chain functionalization of ionic liquid surfactants on the complexation and self-assembling behavior of polyampholyte gelatin in aqueous medium

The interactional behavior of ILSs towards gelatin forming structurally different ILS mediated self-assemblies depending on the nature of the ILS and counterion binding is shown.

Structural effect of glyme–Li+ salt solvate ionic liquids on the conformation of poly(ethylene oxide)

Conformation of poly(ethylene oxide) in solvate ionic liquids is affected by the solvent structure.

Micellization of alkyltrimethylammonium bromide surfactants in choline chloride:glycerol deep eutectic solvent

Cationic surfactant behaviour in choline chloride:glycerol deep eutectic solvent: towards understanding amphiphile self-assembly in the absence of water.

Probing the effects of the ester functional group, alkyl side chain length and anions on the bulk nanostructure of ionic liquids: a computational study

The effects of ester addition on nanostructural properties of biodegradable ILs.

Effect of the environmental humidity on the bulk, interfacial and nanoconfined properties of an ionic liquid

Structural and dynamical properties of ILs are altered by the weakening of ion–ion correlations in the presence of water.

Interaction of imidazolium based ionic liquids with Triton X-100 micelles: investigating the role of the counter ion and chain length

Size and shape of Triton X-100 micelles can easily be controlled by the appropriate selection of ionic liquids with varying hydrophobicity.

Unique Phase Behaviors in the Gemini Surfactant/EAN Binary System: The Role of the Hydroxyl Group

The hydroxyl group in the spacer of a cationic Gemini surfactant (12-3OH-12) caused dramatic changes of the phase behaviors in a protic ionic liquid (EAN). Here, the effects of the hydroxyl group on micellization and lyotropic liquid crystal formation were investigated through the surface tension, small-angle X-ray scattering, polarized optical microscopy, and rheological measurements. With the hydroxyl group in the spacer, the critical micellization concentration of 12-3OH-12 was found to be lower than that of the homologue without hydroxyl (12-3-12) and the 12-3OH-12 molecules packed more densely at the air/EAN interface. It was then interesting to observe a coexistence of two separated phases at wide concentration and temperature ranges in this 12-3OH-12/EAN system. Such a micellar phase separation was rarely observed in the ionic surfactant binary system. With the increase of surfactant concentration, the reverse hexagonal and bicontinuous cubic phases appeared in sequence, whereas only a reverse hexagonal phase was found in 12-3-12/EAN system. But, the hexagonal phases formed with 12-3OH-12 exhibited lower viscoelasticity and thermostability than those observed in 12-3-12/EAN system. Such unique changes in phase behaviors of 12-3OH-12 were ascribed to their enhanced solvophilic interactions of 12-3OH-12 and relatively weak solvophobic interactions in EAN.

Surfactant Behavior of Sodium Dodecylsulfate in Deep Eutectic Solvent Choline Chloride/Urea

Deep eutectic solvents (DES) resemble ionic liquids but are formed from an ionic mixture instead of being a single ionic compound. Here we present some results that demonstrate that surfactant sodium dodecyl sulfate (SDS) remains surface-active and shows self-assembly phenomena in the most commonly studied DES, choline chloride/urea. X-ray reflectivity (XRR) and small angle neutron scattering (SANS) suggest that the behavior is significantly different from that in water. Our SANS data supports our determination of the critical micelle concentration using surface-tension measurements and suggests that the micelles formed in DES do not have the same shape and size as those seen in water. Reflectivity measurements have also demonstrated that the surfactants remain surface-active below this concentration.

The origin of surfactant amphiphilicity and self-assembly in protic ionic liquids

The nature of amphiphilic self-assembly in alkylammonium protic ionic liquids (PILs) is examined by systematically varying the ionic structure and composition, H-bonding capacity, and nanostructure of both the PIL and micelle-forming cationic surfactant, and contrasted with self-assembly in water. Using small-angle neutron scattering, micelle structure and concentrations are determined for primary - quaternary dodecylammonium salts in nitrate and thiocyanate PILs. While the solvophobic driving force depends only on the average polarity of the PIL, surprisingly strong, specific interactions of the head group and counterion with the PIL H-bond network are found. This suggests the importance of developing designer amphiphiles for assembling soft matter structures in PILs.

Strong Acid-Nonionic Surfactant Lyotropic Liquid-Crystalline Mesophases as Media for the Synthesis of Carbon Quantum Dots and Highly Proton Conducting Mesostructured Silica Thin Films and Monoliths

Lyotropic liquid-crystalline (LLC) materials are important in designing porous materials, and acids are as important in chemical synthesis. Combining these two important concepts will be highly beneficial to chemistry and material science. In this work, we show that a strong acid can be used as a solvent for the assembly of nonionic surfactants into various mesophases. Sulfuric acid (SA), 10-lauryl ether (C12E10), and a small amount of water form bicontinuous cubic (V1), 2D-hexagonal (H1), and micelle cubic (I1) mesophases with increasing SA/C12E10 mole ratio. A mixture of SA and C12E10 is fluidic but transforms to a highly ordered LLC mesophase by absorbing ambient water. The LLC mesophase displays high proton conductivity (1.5 to 19.0 mS/cm at room temperature) that increases with an increasing SA content up to 11 SA/C12E10 mole ratio, where the absorbed water is constant with respect to the SA amount but gradually increases from a 2.3 to 4.3 H2O/C12E10 mole ratio with increasing SA/C12E10 from 2 to 11, respectively. The mixture of SA and C12E10 slowly undergoes carbonization to produce carbon quantum dots (c-dots). The carbonization process can be controlled by simply controlling the water content of the media, and it can be almost halted by leaving the samples under ambient conditions, where the mixture slowly absorbs water to form photoluminescent c-dot-embedded mesophases. Over time the c-dots grow in size and increase in number, and the photoluminescence frequency gradually shifts to a lower frequency. The SA/C12E10 mesophase can also be used as a template to produce highly proton conducting mesostructured silica films and monoliths, as high as 19.3 mS/cm under ambient conditions. Aging the silica samples enhances the conductivity that can be even larger than for the LLC mesophase with the same amount of SA. The presence of silica has a positive effect on the proton conductivity of SA/C12E10 systems.

Irreversible structural change of a dry ionic liquid under nanoconfinement

Studies of 1-hexyl-3-methyl-imidazolium ethylsulfate ([HMIM] EtSO4) using an extended surface forces apparatus show, for the first time, an ordered structure within the nanoconfined ionic liquid (IL) between mica surfaces that extends up to ∼60 nm from the surface. Our measurements show the growth of this ordered IL-film upon successive nanoconfinements-the structural changes being irreversible upon removal of the confinement-and the response of the structure to shear. The compressibility of this system is lower than that typically measured for ILs, while creep takes place during shear, both findings supporting a long-range liquid-to-solid transition. AFM (sharp-tip) studies of [HMIM] EtSO4 on mica only reveal ∼2 surface IL-layers, with order extending only ∼3 nm from the surface, indicating that confinement is required for the long-range IL-solidification to occur. WAXS studies of the bulk IL show a more pronounced ordered structure than is the case for [HMIM] with bis(trifluoromethylsulfonyl)imide as anion, but no long-range order is detected, consistent with the results obtained with the sharp AFM tip. These are the first force measurements of nanoconfinement-induced long-range solidification of an IL.

Adsorption of Polyether Block Copolymers at Silica-Water and Silica-Ethylammonium Nitrate Interfaces

Atomic force microscope (AFM) force curves and images are used to characterize the adsorbed layer structure formed by a series of diblock copolymers with solvophilic poly(ethylene oxide) (PEO) and solvophobic poly(ethyl glycidyl ether) (PEGE) blocks at silica-water and silica-ethylammoniun nitrate (EAN, a room temperature ionic liquid (IL)) interfaces. The diblock polyethers examined are EGE109EO54, EGE113EO115, and EGE104EO178. These experiments reveal how adsorbed layer structure varies as the length of the EO block varies while the EGE block length is kept approximately constant; water is a better solvent for PEO than EAN, so higher curvature structures are found at the interface of silica with water than with EAN. At silica-water interfaces, EGE109EO54 forms a bilayer and EGE113EO115 forms elongated aggregates, while a well-ordered array of spheres is present for EGE104EO178. EGE109EO54 does not adsorb at the silica-EAN interface because the EO chain is too short to compete with the ethylammonium cation for surface adsorption sites. However, EGE113EO115 and EGE104EO178 do adsorb and form a bilayer and elongated aggregates, respectively.

Effects of Water on Solvation Layers of Imidazolium-Type Room Temperature Ionic Liquids on Silica and Mica

Effects of the addition of water on solvation layers of imidazolium-type room temperature ionic liquids (RT-ILs) have been studied through force curve measurements of atomic force microscopy (AFM). Two kinds of RT-ILs were employed in this study; one is a hydrophilic RT-IL (1-butyl-3-methylimidazolium tetrafluoroborate, BmimBF4), and the other is a hydrophobic one (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, EmimTFSI). These RT-ILs form solvation layers on hydrophilic solid substances (i.e., silica and mica) in the absence of added water. The addition of water into BmimBF4 resulted in the disruption of the solvation layers and then the formation of an interfacial water phase on silica. In contrast, the formation of the interfacial water phase was not evidenced on mica because of the absence of hydrogen-bonding sites on the mica surface. Interestingly, the addition of water into EmimTFSI induced the formation of the interfacial water phase on the two solid surfaces. In the EmimTFSI system, importantly, significantly greater adhesion forces were observed on silica than on mica. This reflects the different formation mechanisms of the interfacial water phase on the two solid surfaces. We conclude that the hydrogen bonding is a key factor in determining whether water molecules can be adsorbed on the solid surfaces, but it is also necessary to take into account the hydrophilic/hydrophobic nature of the RT-ILs.

Hydrogen bonding in ionic liquids

Ionic liquids (IL) and hydrogen bonding (H-bonding) are two diverse fields for which there is a developing recognition of significant overlap. Doubly ionic H-bonds occur when a H-bond forms between a cation and anion, and are a key feature of ILs. Doubly ionic H-bonds represent a wide area of H-bonding which has yet to be fully recognised, characterised or explored. H-bonds in ILs (both protic and aprotic) are bifurcated and chelating, and unlike many molecular liquids a significant variety of distinct H-bonds are formed between different types and numbers of donor and acceptor sites within a given IL. Traditional more neutral H-bonds can also be formed in functionalised ILs, adding a further level of complexity. Ab initio computed parameters; association energies, partial charges, density descriptors as encompassed by the QTAIM methodology (ρBCP), qualitative molecular orbital theory and NBO analysis provide established and robust mechanisms for understanding and interpreting traditional neutral and ionic H-bonds. In this review the applicability and extension of these parameters to describe and quantify the doubly ionic H-bond has been explored. Estimating the H-bonding energy is difficult because at a fundamental level the H-bond and ionic interaction are coupled. The NBO and QTAIM methodologies, unlike the total energy, are local descriptors and therefore can be used to directly compare neutral, ionic and doubly ionic H-bonds. The charged nature of the ions influences the ionic characteristics of the H-bond and vice versa, in addition the close association of the ions leads to enhanced orbital overlap and covalent contributions. The charge on the ions raises the energy of the Ylp and lowers the energy of the X-H σ* NBOs resulting in greater charge transfer, strengthening the H-bond. Using this range of parameters and comparing doubly ionic H-bonds to more traditional neutral and ionic H-bonds it is clear that doubly ionic H-bonds cover the full range of weak through to very strong H-bonds.