Using ionic liquids to formulate microemulsions: Current state of affairs

Electropolymerization of EDOT in an anionic surfactant-stabilized hydrophobic ionic liquid-based microemulsion

Here, an anionic surfactant [AOT]^- (bis-(2-ethylhexyl) sulfosuccinate)-stabilized H₂O/[Omim][PF₆] (1-octyl-3-methylimidazolium hexafluorophosphate) microemulsion has been tested for the first time as a medium for the electropolymerization of 3,4-ethylenedioxythiophene (EDOT). To formulate AOT-stabilized [Omim][PF₆]-based microemulsions of different water contents, the phase triangle was determined at 35 °C. Measurements of the conductivities of the microemulsions, their solubilization capacities toward EDOT and their catalytic effects on EDOT electrooxidation show that the present [AOT]^--stabilized ionic liquid microemulsion is a good medium for EDOT electropolymerization. Studies on the process of the electropolymerization of EDOT in this [Omim][PF₆]-based microemulsion indicate that the water content (i.e., microstructure) of the microemulsion medium is an important factor affecting the onset potential and the deposition rate of the PEDOT. The morphology and the doping level of the as-prepared PEDOT are also found to be correlated with the water content of the ionic liquid microemulsion. The microemulsion with higher water content results in a PEDOT with better electroactivity and higher doping levels. FTIR spectra and XPS analysis show that the PEDOT electrosynthesized in the microemulsion is co-doped by both [AOT]^- and [PF₆]^-. Compared with the neat [Omim][PF₆], the use of the ionic liquid microemulsions can reduce not only the consumption of the expensive ionic liquid, but also the onset potential for the electrooxidation of EDOT. Moreover, by tuning the water content of the medium, the electropolymerization of PEDOT and its electrochemical properties could be regulated accordingly. Under the identical deposited charge, the PEDOT originated from the high water content microemulsion (50% H₂O μE) has a higher specific capacitance (124 F g^-1) than that from neat [Omim][PF₆] (117 F g^-1). It follows that the present ionic liquid microemulsion is a good medium for EDOT electropolymerization. The present study opens up a new route for the green and low-cost electrochemical preparation of high-performance PEDOT.

Phase studies of ethyl ammonium nitrate (EAN)/sugar surfactant microemulsions: effect of chain length of alkanes and length of the hydrophobic chain of the non-ionic surfactant

Microemulsions were formulated with the ionic liquid ethylammonium nitrate (EAN) used instead of water as the polar phase, hydrocarbon solvents (n-alkanes) and sugar-based non-ionic surfactants, and their phase behaviour and microstructure were investigated. The sugar-based non-ionic surfactants used are non-toxic, biodegradable and environmentally friendly. Due to these properties, their use in microemulsion systems is a clear alternative to the conventionally used non-ionic surfactants from the class of alkyl polyoxyethylene ethers (C i E j ). The influence of n-alkanes with different chain lengths and of sugar-based nonionic surfactants with hydrophobic chains of different lengths on the microemulsion system was also investigated. The results obtained for the microemulsions with EAN described here are similar to those obtained for microemulsion systems formulated with water as the polar solvent. Liquid crystalline (LC) phases were observed in microemulsion systems with sugar-based nonionic surfactants having longer hydrocarbon chains, at lower temperatures and higher surfactant mass fraction.

Good's buffer ionic liquid tunes the phase behavior of an anionic surfactant SDBS-stabilized n-octane-water microemulsion and the stability of the solubilized horseradish peroxidase

A Good's buffer ionic liquid (GB-IL) composed of quaternary ammonium cations and Good's buffer anions is first introduced into a microemulsion system as a self-buffering and biocompatible electrolyte. The effects of the constituting ions of a GB-IL and their concentrations on the phase behavior of the anionic surfactant SDBS stabilized n-octane-H₂O microemulsion system were studied for the first time using the ε-β fish-like phase diagram method. The result indicates that the phase behavior of the above microemulsion system is greatly affected by GB-IL cations with a longer alkyl chain on the cation being more favorable for phase inversion. Compared with NaCl, a GB-IL of the same concentration is more efficient for achieving phase inversion, due to the dual role of an electrolyte and a co-alcohol. In addition to the phase behavior, the stability of horseradish peroxidase (HRP) solubilized in an SDBS stabilized bicontinuous microemulsion is also affected by a GB-IL. It is found that the variation of the cationic alkyl chain has a negligible effect on the microemulsion microstructure, but has a significant influence on the stability of the solubilized HRP. At a fixed concentration of the GB-IL, the quaternary ammonium cation with a longer alkyl chain is better for the stabilization of the HRP activity. For a given GB-IL, a higher level of the GB-IL results in a better HRP stability. More importantly, the GB-IL-buffered microemulsion, at the same level of the buffering salt, is more advantageous than the phosphate-buffered one for the stabilization of the HRP activity. This advantage is more pronounced for higher concentrations of the GB-IL. This difference in the HRP stability, caused by the buffering salts, should be ascribed to the microemulsion microstructure effect as well as the Hofmeister effect. The present study provides a guideline for the construction of a bicontinuous microemulsion with a simplified composition and stabilizing effect on the solubilized enzyme.

Ionic Liquids Based on the Concept of Melting Point Lowering Due to Ethoxylation

Most of the commonly used Ionic Liquids (ILs) contain bulky organic cations with suitable anions. With our COMPLET (Concept of Melting Point Lowering due to Ethoxylation), we follow a different approach. We use simple, low-toxic, cheap, and commercially available anions of the type C_x(EO)_yCH₂COO^- to liquefy presumably any simple metal ion, independently of its charge. In the simplest case, the cation can be sodium or lithium, but synthesis of Ionic Liquids is also possible with cations of higher valences such as transition or rare earth metals. Anions with longer alkyl chains are surface active and form surface active ionic liquids (SAILs), which combine properties of ionic and nonionic surfactants at room temperature. They show significant structuring even in their pure state, i.e., in the absence of water or any other added solvent. This approach offers new application domains that go far beyond the common real or hypothetical use of classical Ionic Liquids. Possible applications include the separation of rare earth metals, the use as interesting media for metal catalysis, or the synthesis of completely new materials (for example, in analogy to metal organic frameworks).

Using Microemulsions: Formulation Based on Knowledge of Their Mesostructure

Microemulsions, as thermodynamically stable mixtures of oil, water, and surfactant, are known and have been studied for more than 70 years. However, even today there are still quite a number of unclear aspects, and more recent research work has modified and extended our picture. This review gives a short overview of how the understanding of microemulsions has developed, the current view on their properties and structural features, and in particular, how they are related to applications. We also discuss more recent developments regarding nonclassical microemulsions such as surfactant-free (ultraflexible) microemulsions or ones containing uncommon solvents or amphiphiles (like antagonistic salts). These new findings challenge to some extent our previous understanding of microemulsions, which therefore has to be extended to look at the different types of microemulsions in a unified way. In particular, the flexibility of the amphiphilic film is the key property to classify different microemulsion types and their properties in this review. Such a classification of microemulsions requires a thorough determination of their structural properties, and therefore, the experimental methods to determine microemulsion structure and dynamics are reviewed briefly, with a particular emphasis on recent developments in the field of direct imaging by means of electron microscopy. Based on this classification of microemulsions, we then discuss their applications, where the application demands have to be met by the properties of the microemulsion, which in turn are controlled by the flexibility of their amphiphilic interface. Another frequently important aspect for applications is the control of the rheological properties. Normally, microemulsions are low viscous and therefore enhancing viscosity has to be achieved by either having high concentrations (often not wished for) or additives, which do not significantly interfere with the microemulsion. Accordingly, this review gives a comprehensive account of the properties of microemulsions, including most recent developments and bringing them together from a united viewpoint, with an emphasis on how this affects the way of formulating microemulsions for a given application with desired properties.

Composition and Solubilization of the Microemulsion Systems Containing Triton X-100: Effects of Aqueous Composition and Oil-Water-Ratios

The composition and solubilization of the optimum microemulsion systems containing Triton X-100/pentan-1-ol/aliphatic hydrocarbon/water were studied with the ∊-β fishlike phase diagram method. The solubilities of the alkanol (S A), the mass fractions of the alkanol in the interfacial layer (AS ) and the optimum solubilization parameters (SP∗) of the microemulsion systems with different aqueous phases (salt, acid, alkali) and with different α values were obtained and discussed. The SP∗ values increase significantly with the cation radius of the chlorides (NaCl, KCl, CsCl), but decrease slightly with the anion radius of the sodium halides (NaCl, NaBr, NaI). However, SP∗ values decrease with the increased salinities and NaOH contents. The trend was reversed for the acid systems. When the oil-water ratio (α) increases, the S A and A S values significantly increase, while the SP∗ values increase slowly. As the length of the carbon chain of the alkane molecules increases, both the S A and A S values increase, while the SP∗ values decrease significantly.

Thermally Stable Ionic Liquid-Based Microemulsions for High-Temperature Stabilization of Lysozyme at Nanointerfaces

The development of new strategies for thermal stability and storage of enzymes is very important, considering the nonretention of catalytic activity by enzymes under harsh conditions of temperature. Following this, herein, a new approach based on the interfacial adsorption of lysozyme (LYZ) at nanointerfaces of ionic liquid (IL)-based microemulsions, for enhanced thermal stability of LYZ, is reported. Microemulsions (MEs) composed of dialkyl imidazolium-based surface active ILs (SAILs) as surfactants, ILs as the nonpolar phase, and ethylene glycol (EG) as the polar phase, without any cosurfactants, have been prepared and characterized in detail. Various regions corresponding to polar-in-IL, bicontinuous, and IL-in-polar phases have been characterized using conductivity measurements. Dynamic light scattering (DLS) measurements have provided insights into the size distribution of microdroplets, whereas temperature-dependent DLS measurements established the thermal stability of the MEs. Nanointerfaces formed by SAILs with EG in thermally stable reverse MEs act as fluid scaffolds to adsorb and provide thermal stability, up to 120 °C, to LYZ. Thermally treated LYZ upon extraction into a buffer shows enzyme activity owing to negligible change in the active site of LYZ, as marked by retention of microenvironment of Trp residues present in the active site of LYZ. The present work is expected to establish a new platform for the development of novel nanointerfaces utilizing biobased components for other biomedical applications.

Nonaqueous Microemulsion in the Bmim Tf2N/Brij 30/ n-Nonane System: Structural Investigation and Application as Gold Nanoparticle Microreactor

Microemulsions based on ionic liquids (ILs) are being increasingly studied in many different areas of physical chemistry because of the attractive properties of ILs. In particular, waterless microemulsions where the IL represents the polar phase can be of interest for those applications that demand the nanosegregation of polar substances, but in which the absence of water is a strict requirement. In this work, we prepared a reverse, nonaqueous microemulsion based on the low-viscosity room-temperature IL, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide , the surfactant Brij 30, and n-nonane. The systems were characterized by dynamic light scattering and small-angle X-ray scattering; the IL/oil microemulsion was further employed as a templating system for the synthesis of gold nanoparticles from hydrogen tetrachloroaurate(III), HAuCl₄, by UV-photoreduction technique.

Nanoreactors stable up to 200 °C: a class of high temperature microemulsions composed solely of ionic liquids

A class of microemulsions solely consisting of ionic liquids was shown to maintain nanoscale droplets up to about 200 °C, and this unique property was used to prepare porous Pt where the microemulsions played the roles of a solvent, a template and a reductant.

Phase Behavior and Solubilization of Microemulsions Containing C16mimBr with Different Oil-Water Ratios

The effects of oil-water ratios (α) on the composition of the balanced interfacial layer and the optimum solubilization of microemulsions C16mimBr/butan-1-ol/oil (octane, decane, dodecane)/5% NaCl solution were investigated. As α increases, the alcohol solubility in water and oil phases (Sa) in the microemulsions also tends to increase. The reason is the relatively high solubility of the alcohol in the oil phase compared to the solubility in water. When α increases, the number of surfactant ( n s s ${\rm{n}}_{\rm{s}}^{\rm{s}}$ ) and co-surfactant ( n a s ${\rm{n}}_{\rm{a}}^{\rm{s}}$ ) molecules required to balance the interface layer and the proportion of alcohol in the interfacial layer (AS) increase but the solubilization capacity (SP*) decreases. When the total mass of water and oil is unchanged, the solubilization capacity of the microemulsion systems is lower the higher the relative content of oil is. The effects of different oils, salinity and temperature on the composition of the balanced interfacial layer and the solubilization of microemulsions were also investigated.

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).

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.

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.

Transport properties of microemulsions with ionic liquid apolar domains as a function of ionic liquid content

The conductivity, dynamic viscosity and diffusion coefficient of aqueous ionic liquid microemulsions were measured as a function of ionic liquid content. ​The conclusions from transport properties were supported by UV-Vis as well as FTIR measurements.

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.

Light induced micelle to vesicle transition in an aqueous solution of a surface active ionic liquid

A new simple surface active ionic liquid displayed reversible micelle–vesicle transition under alternative UV/vis irradiation without additives.

Polymerization in ionic liquid-based microemulsions

Recent developments of polymerization in ionic liquid-based microemulsions and its applications are reviewed, along with the perspectives and challenges.

Ionic liquids as diluents in solvent extraction: first evidence of supramolecular aggregation of a couple of extractant molecules

Ionic liquids have many favorable properties over conventional diluents in solvent extraction.

Ionic liquids entrapped in reverse micelles as nanoreactors for bimolecular nucleophilic substitution reaction. Effect of the confinement on the chloride ion availability

In this work was explored how the confinement of two ionic liquids (ILs), 1-butyl-3-methylimidazolium chloride (bmimCl) and 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4), inside toluene/benzyl-n-hexadecyldimethylammonium chloride (BHDC) reverse micelles (RMs) affects the Cl(-) nucleophilicity on the bimolecular nucleophilic substitution (SN2) reaction between this anion and dimethyl-4-nitrophenylsulfonium trifluoromethanesulfonate. The results obtained show that, upon confinement, the ionic interactions between the ILs with the cationic surfactant polar head group and the surfactant counterion modify substantially the performance of both ILs as solvents. In toluene/BHDC/bmimCl RMs, the Cl(-) interacts strongly with bmim(+) (and/or BHD(+)) in such a way that its nucleophilicity is reduced in comparison with neat IL. In toluene/BHDC/bmimBF4 RMs, an ionic exchange equilibrium produces segregation of bmim(+) and BF4(-) ions, changing the composition of the RMs interface and affecting dramatically the Cl(-) availability. These results show the versatility of this kind of organized system to alter the ionic organization and influence on reaction rate when used as nanoreactors.

Phase Behavior and Solubilization of Microemulsion Systems Containing Imidazolium Type Surfactant CnmimBr and Butyric Acid as Cosurfactant

The phase behavior and solubilization of two microemulsion systems 1-alkyl-3-methylimidazolium bromide (CnmimBr, n = 12,14,16)/butyric acid/hexane/brine (5% NaCl) and CnmimBr/1-butanol/hexane/brine (5% NaCl) were studied and compared. The physico-chemical parameters, such as the solubility (∊ B ) of the cosurfactant, the mass fraction of the cosurfactant in the interfacial layer (AS ), and the solubilization ability (SP*) of the two microemulsion systems were calculated and discussed. As the carbon chain length of CnmimBr molecules increases, SP* increases, and the difference of SP* values between the two microemulsion systems is reduced. The effects of aqueous and oleic phases and temperatures on the phase behavior and solubilization of C14mimBr/butyric acid/alkane/aqueous phase microemulsion systems were also discussed.

Amphiphile self-assemblies in supercritical CO2 and ionic liquids

Supercritical (sc) CO2 and ionic liquids (ILs) are very attractive green solvents with tunable properties. Using scCO2 and ILs as alternatives of conventional solvents (water and oil) for forming amphiphile self-assemblies has many advantages. For example, the properties and structures of the amphiphile self-assemblies in these solvents can be easily modulated by tuning the properties of solvents; scCO2 has excellent solvation power and mass-transfer characteristics; ILs can dissolve both organic and inorganic substances and their properties are designable to satisfy the requirements of various applications. Therefore, the amphiphile self-assemblies in scCO2 and ILs have attracted considerable attention in recent years. This review describes the advances of using scCO2 or/and ILs as amphiphile self-assembly media in the last decade. The amphiphile self-assemblies in scCO2 and ILs are first reviewed, followed by the discussion on combination of scCO2 and ILs in creating microemulsions or emulsions. Some future directions on the amphiphile self-assemblies in scCO2 and ILs are highlighted.

Aggregation behavior and electrical properties of amphiphilic pyrrole-tailed ionic liquids in water, from the viewpoint of dielectric relaxation spectroscopy

The self-aggregation behavior of amphiphilic pyrrole-tailed imidazolium ionic liquids (Py(CH₂)₁₂mim⁺Br⁻: Py = pyrrole, mim = methylimidazolium) in water is investigated by dielectric spectroscopy from 40 Hz to 110 MHz. Dielectric determination shows that the critical micelle concentration (CMC) is 8.5 mM, which is lower than that for traditional ionic surfactants. The thermodynamic parameter of the micellization, the Gibbs free energy ΔG, was calculated for Py(CH₂)₁₂mim⁺Br⁻ and compared to those of the corresponding C(n)mim⁺Br⁻ (n = 12, 14). It was found that the main driven forces of the Py(CH₂)₁₂mim⁺Br⁻ aggregation were hydrophobic interaction and π-π interactions among the adjacent Py groups. Further, the structure of aggregation was speculated theoretically that Py groups partially insert into the alkyl chains and the staggered arrangement in micelles is formed. When the concentration of Py(CH₂)₁₂mim⁺Br⁻ is higher than CMC, two remarkable relaxations which originated from diffusion of counterions and interfacial polarization between the micelles and solution, were observed at about 1.3 MHz and 55 MHz. The relaxation parameters representing the real properties of the whole system were obtained by fitting the experimental data with Cole-Cole equation. A dielectric model characterizing the structure and electrical properties of spherical micelles was proposed by which the conductivity, permittivity and the volume fraction of micelles as well as electrical properties of solution were calculated from the relaxation parameters. An intriguingly high permittivity of about 150 for the micelle was found to be a direct consequence of the strong orientational order of water molecules inside the core of micelle, and essentially is attributed to the special structure of the micelle. Furthermore, the calculation of the interfacial electrokinetic parameters of the micelles, i.e., the surface conductivity, surface charge density and zeta potential, were also achieved based on the relaxation parameters and phase parameters from higher frequency relaxation. On the basis of the results obtained, the aggregation behaviours and interfacial electrokinetic properties of the special micelles are discussed.

Shape and size controlled synthesis of MOF nanocrystals with the assistance of ionic liquid mircoemulsions

In this work, the La-metal-organic frameworks (La-MOFs) were synthesized using lanthanum(III) nitrate and 1,3,5-benzenetricarboxylic acid (BTC) in H2O-in-1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6), bmimPF6-in-water, and the bicontinuous microemulsions stabilized by surfactant TX-100. The MOFs prepared were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), powder X-ray diffraction (XRD), thermal gravimetric analysis (TGA), and FT-IR methods, and the microstructures of the microemulsions in the H2O/bmimPF6/TX-100 system were studied by small-angle X-ray scattering (SXAS) technique. It was shown that the dispersed droplets in the water-in-bmimPF6, bicontinuous and bmimPF6-in-water microemulsions were spherical, lamellar, and cylindrical, respectively. The shapes of the La-MOFs synthesized were similar to that of the droplets in the corresponding microemulsions. This indicated that the morphology of MOFs could be controlled by the microstructures of the microemulsions. On the basis of the systematic experimental results, the mechanism for controlling the morphology of the MOFs was proposed.