Visual Evidence for Formation of Water-in-Ionic Liquid Microemulsions

Formulation of a stable diesel microemulsion using eco-friendly ionic liquids and investigation of particle size and fuel properties as an alternative fuel

Ecofriendly ionic liquids (ILs) were synthesized through amidation of ricinoleic acid, the main fatty acid in castor oil, followed by a quaternization reaction to solubilize ethanol in IL/diesel blends at different ratios. As a result, stable and highly renewable, low viscous microemulsion biofuels with high oxygen content were prepared. The prepared fuel samples combine the advantages of green ionic liquids and microemulsion properties. The chemical structures of ILs were confirmed with the aid of NMR and FTIR spectroscopy. DLS analysis revealed that the ethanol particles ranged in size from 8 to 18.1 nm in all samples. As ILs ratios decrease in microemulsion from 37 to 69%, the ethanol particle sizes increase from 10 to 25%. Ethanol shows good solubilization in diesel and IL-1 is more effective than IL-2 in ethanol solubilization at low percentages of ethanol due to more oxygen atoms besides three hydroxyl groups. The ternary phase diagram indicated that the microemulsion area in the case of using IL-1 is larger than that of IL-2. The fuel properties of the prepared microemulsions are nearly close to those of neat diesel and fall within the permitted range of ASTM D975. The viscosity and density values at low ratios of ILs are found to be very close to the values of the neat diesel at different temperatures. The prepared samples show a slight decrease in cetane number and heating value compared to diesel. However, they have improved flash points, cloud points, sulfur content, and acid value. The particle sizes were checked every week and the prepared samples showed high stability with the aid of the synthesized ILs. Moreover, the prepared microemulsions stayed in a transparent appearance for more than a year and no phase separation was observed.

Ionic liquids: environmentally sustainable materials for energy conversion and storage applications

Ionic liquids (ILs), often known as green designer solvents, have demonstrated immense application potential in numerous scientific and technological domains. ILs possess high boiling point and low volatility that make them suitable environmentally benign candidates for many potential applications. The more important aspect associated with ILs is that their physicochemical properties can be effectively changed for desired applications just by tuning the structure of the cationic and/or anionic part of ILs. Furthermore, these eco-friendly designer materials can function as electrolytes or solvents depending on the application. Owing to the distinctive properties such as low volatility, high thermal and electrochemical stability, and better ionic conductivity, ILs are nowadays immensely used in a variety of energy applications, particularly in the development of green and sustainable energy storage and conversion devices. Suitable ILs are designed for specific purposes to be used as electrolytes and/or solvents for fuel cells, lithium-ion batteries, supercapacitors (SCs), and solar cells. Herein, we have highlighted the utilization of ILs as unique green designer materials in Li-batteries, fuel cells, SCs, and solar cells. This review will enlighten the promising prospects of these unique, environmentally sustainable materials for next-generation green energy conversion and storage devices. Ionic liquids have much to offer in the field of energy sciences regarding fixing some of the world's most serious issues. However, most of the discoveries discussed in this review article are still at the laboratory research scale for further development. This review article will inspire researchers and readers about how ILs can be effectively applied in energy sectors for various applications as mentioned above.

Formation of water-in-oil microemulsions within a hydrophobic deep eutectic solvent

Hydrophobic deep eutectic solvents (DESs) as neoteric, non-toxic, and inexpensive media have the potential to replace organic solvents in various aggregation processes. Conventional water-in-oil microemulsions are formed using mostly environmentally unfavorable toxic organic solvents as the bulk oil phase. Evidence of formation of water-in-DES microemulsions is presented. These novel assemblies are formed using a hydrophobic DES constituted of n-decanoic acid (DA) and tetra-n-butylammonium chloride (TBAC) in 2 : 1 mole ratio, termed TBAC-DA, as the bulk oil phase. It is observed that in the presence of a common and popular non-ionic surfactant Triton X-100 (TX-100), water pools are formed within TBAC-DA under ambient conditions with maximum water loading (w0 = [water]/[TX-100]) of 60 ± 3 for [TX-100] = 300 mM. The formation of the microemulsions is established by using fluorescence probe pyranine, which exhibited the appearance of a band characterizing the un-protonated form of the probe clearly implying onset of water-in-TBAC-DA microemulsion formation. The UV-vis absorbance of CoII further corroborates TX-100-assisted water pool formation within TBAC-DA via the appearance of the band that is assigned to the response of the probe in water. Dynamic light scattering (DLS) measurement suggests average aggregate sizes to be in the range of 72(±4) to 122(±7) nm. These unprecedented water-in-DES microemulsions may have far reaching implications due to their benign nature.

Magnetic Microemulsions Stabilized by Alkyltrimethylammonium-Based Magnetic Ionic Liquids Surfactants (MILSs)

While traditional microemulsions are versatile media for nanoscience and nanotechnology, stimulus-responsive microemulsions are more challenging to realize, and only a handful of cases have been reported. We here introduce magnetic microemulsions (MMEs) stabilized by alkyltrimethylammonium-based magnetic ionic liquids surfactants (MILSs), paired with water as the polar phase, aliphatic oils as the nonpolar phase, and aliphatic alcohols as the cosurfactant. n-Hexane coupled with n(MILSs/1-butanol) = 1:4 showed the most excellent ability to form MMEs, and the range of the monophasic region was expanded with increasing alkyl chain length of MILSs cation. Classical oil-in-water (O/W), bicontinuous (BC) sponge structure, and inverse water-in-oil (W/O) subregions were clarified by conductivity method. Dynamic light scattering showed that the diameter of W/O microemulsions droplets were about 2-6 nm. Magnetic susceptibility and rheological measurements revealed that these MMEs are with high magnetic susceptibility and low viscosity, which show interesting potential applications based on a manipulation via external magnetic field. Moreover, these MMEs showed Newtonian-like flow behavior within respective subregions, and their magnetic susceptibility was not affected by the subregion structure but MILSs mass fraction.

Ionic Liquid Green Assembly-Mediated Migration of Piperine from Calf-Thymus DNA: A New Possibility of the Tunable Drug Delivery System

Biocompatible surface-active ionic liquid (SAIL) was used first to study the deintercalation process of a well-known natural compound piperine (PIP) as an anticancer drug, obtained from PIP-calf thymus DNA (ctDNA) complex under controlled experimental conditions. In this study, we have been exploring the interaction of PIP in SAIL (1-butyl-3-methylimidazolium octyl sulfate ionic liquid ([C₄mim][C₈OSO₃])), ctDNA, and deintercalation of PIP from the PIP-ctDNA complex through SAIL micelle using various spectroscopic techniques. Absorption, emission, and lifetime decay measurements provide strong evidence of the relocation of PIP molecules from ctDNA to SAIL micelle. Fluorescence quenching and steady-state fluorescence anisotropy were employed to examine the exact location of PIP in different media. Moreover, the surface tension technique was also employed to confirm the release of PIP molecules from the PIP-ctDNA complex in the presence of SAIL. Circular dichroism analysis suggested that SAIL micelle does not perturb the ctDNA structure, which supported the fact that SAIL micelle can be used as a safe vehicle for PIP. Overall, the study highlighted a novel strategy for deintercalation of drug using SAIL because the release of the drug can be controlled over a period by varying the concentration and composition of the SAIL.

Small-angle X-ray scattering study on nano-scale structures controlled by water content in a binary water/ionic liquid system

We report the water-in-ionic-liquid microemulsions (ME) formed in a binary water/ionic liquid system, without organic solvents, using a surfactant ionic liquid (SAIL) based on 1-butyl-3-methylimidazolium (C4mIm+) as the cation and dioctyl sulfosuccinate (AOT-) as the anion. Small-angle X-ray scattering (SAXS) revealed that MEs were stably formed in the binary water/SAIL solutions in the low water content region (water volume fraction, φw < 0.1), and the ME size systematically increased with increasing φw. We further investigated the nanostructures of the high φw region using a combination of SAXS and rheological measurements and found that the MEs changed to a stacked lamellar structure comprising SAIL bilayers and water phases at φw > 0.12. At the largest water content, φw = 0.99, vesicle structures were obtained.

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.

Study of Ionic Liquid Microemulsions: Ethylammonium Nitrate/TritonX-100/Cyclohexane

In this study, ionic liquid (IL), specifically ethylammonium nitrate (EAN), was used instead of water to form nonaqueous microemulsions with cyclohexane and the nonionic surfactant Triton X-100 (TX-100). The phase behavior of the ternary system was investigated, and the microemulsions of ionic liquid-in-oil (IL/O) and oil-in-ionic liquid (O/IL) and the bicontinuous microregion were identified through traditional electrical conductivity measurement. The micropolarities of the IL/O microemulsions were determined via UV–Vis spectroscopy with methyl orange as an absorption probe. Results indicated that the polarity of the reverse micelles remained constant but that of the IL/O microemulsions increased when IL pools were formed. Fourier transform infrared spectroscopy was used to study the interaction mechanism between TX-100 and EAN molecules in IL/O microemulsions. We demonstrated that IL/O microemulsions may be promising for application due to the unique features of ILs and microemulsions.

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 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.

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.

Ionic Liquid-Based Optical and Electrochemical Carbon Dioxide Sensors

Due to their unusual physicochemical properties (e.g., high thermal stability, low volatility, high intrinsic conductivity, wide electrochemical windows and good solvating ability), ionic liquids have shown immense application potential in many research areas. Applications of ionic liquid in developing various sensors, especially for the sensing of biomolecules, such as nucleic acids, proteins and enzymes, gas sensing and sensing of various important ions, among other chemosensing platforms, are currently being explored by researchers worldwide. The use of ionic liquids for the detection of carbon dioxide (CO₂) gas is currently a major topic of research due to the associated importance of this gas with daily human life. This review focuses on the application of ionic liquids in optical and electrochemical CO₂ sensors. The design, mechanism, sensitivity and detection limit of each type of sensor are highlighted in this review.

Water-in-ionic liquid microemulsion formation in solvent mixture of aprotic and protic imidazolium-based ionic liquids

We report that water-in-ionic liquid microemulsions (MEs) are stably formed in an organic solvent-free system, i.e., a mixture of aprotic (aIL) and protic (pIL) imidazolium-based ionic liquids (ILs) containing the anionic surfactant dioctyl sulfosuccinate sodium salt (AOT). Structural investigations using dynamic light, small-angle X-ray, and small-angle neutron scatterings were performed for MEs formed in mixtures of aprotic 1-octyl-3-methylimidazolium ([C8mIm(+)]) and protic 1-alkylimidazolium ([CnImH(+)], n = 4 or 8) IL with a common anion, bis(trifluoromethanesulfonyl)amide ([TFSA(-)]). It was found that the ME structure strongly depends on the mixing composition of the aIL/pIL in the medium. The ME size appreciably increases with increasing pIL content in both [C8mIm(+)][TFSA(-)]/[C8ImH(+)][TFSA(-)] and [C8mIm(+)][TFSA(-)]/[C4ImH(+)][TFSA(-)] mixtures. The size is larger for the n = 8 system than that for the n = 4 system. These results indicate that the shell part of MEs is composed of both AOT and pIL cation, and the ME size can be tuned by pIL content in the aIL/pIL mixtures.

Evidence of water-in-ionic liquid microemulsion formation by nonionic surfactant Brij-35

Brij-35, a common and popular nonionic surfactant, is shown to form water-in-ionic liquid (w/IL) microemulsions with IL 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) as the bulk phase. The presence of w/[bmim][PF6] microemulsions is hinted by the significantly increased solubility of water in Brij-35 solution of [bmim][PF6]. The formation of w/[bmim][PF6] microemulsions by Brij-35 is confirmed using dynamic light scattering (DLS) and small-angle X-ray scattering (SAXS) measurements. Brij-35 forms reverse micelle-type aggregates within [bmim][PF6] in the absence of added-water. These reverse micelles become w/[bmim][PF6] microemulsions as the water is added to the system. As the water loading (w0) is increased, the average diameter of the aggregates increases. Fourier transform infrared (FTIR) absorbance data reveal the presence of both "bound" and "free" water within the system. The "bound" water is associated with the water pools of the w/[bmim][PF6] microemulsions. Excited-state proton transfer (ESPT) involving probe pyranine shows deprotonation of pyranine within the water pools of the w/[bmim][PF6] microemulsions.

Contrasting behavior of classical salts versus ionic liquids toward aqueous phase J-aggregate dissociation of a cyanine dye

The effect of addition of ionic liquids (ILs) on the aggregation behavior of a cyanine dye, 5,5',6,6'-tetrachloro-1,1'-diethyl-3,3'-di(4-sulfobutyl)-benzimidazolocarbocyanine (TDBC), was investigated. In basic aqueous buffer solutions (pH ≥ 10), TDBC preferably exists in its J-aggregated form. Addition of hydrophilic ILs > 5 wt % is observed to disrupt the TDBC J-aggregates, converting them to monomer form most likely because of the interaction between bulky IL cation and the J-aggregates in a time-dependent fashion. This is evidenced by the observed increase in monomer band absorbance at the expense of the absorbance band due to J-aggregates over time. Inorganic salts at similar molar concentrations do not cause this phenomenon but instead induce TDBC precipitation. At low concentrations (<5 wt %), the added IL acts similarly to the inorganic salts, reducing the overall absorbance of TDBC in the solution most likely due to cation exchange causing TDBC precipitation. Addition of a molecular solvent, ethanol, at 15 wt % results in an initial increase in monomer absorbance, albeit to a much lesser extent than for the corresponding molar fraction of IL, which then decreases over time with recovery of J-aggregate absorbance--quite opposite the time-dependent behavior seen for TDBC in PB at pH 12.0 with >5 wt % IL. The unique and dual behavior of ILs as an additive toward affecting cyanine dye aggregation is demonstrated.

Molecular dynamics simulations of nanoparticle self-assembly at ionic liquid-water and ionic liquid-oil interfaces

We have studied the self-assembly of hydrophobic nanoparticles at ionic liquid (IL)-water and IL-oil (hexane) interfaces using molecular dynamics (MD) simulations. For the 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF(6)])/water system, the nanoparticles rapidly approached the IL-water interface and equilibrated more into the IL phase although they were initially in the water phase. In contrast, when the nanoparticles were dispersed in the hexane phase, they slowly approached the IL-hexane interface but remained primarily in the hexane phase. Consequently, the IL-hexane interface was rather undisturbed by the nanoparticles whereas the IL-water interface changed significantly in width and morphology to accommodate the presence of the nanoparticles. The equilibrium positions of the nanoparticles were also supported and explained by potential of mean force (PMF) calculations. Interesting ordering and charge distributions were observed at the IL-liquid interfaces. At the IL-hexane interface, the [BMIM] cations preferentially oriented themselves so that they were immersed more in the hexane phase and packed efficiently to reduce steric hindrance. The ordering likely contributed to a heightened IL density and a slightly positive charge at the IL-hexane interface. In contrast, the cations at the IL-water interface were oriented isotropically unless in the presence of nanoparticles, where the cations aligned across the nanoparticle surfaces.

Particle self-assembly in ionic liquid-in-water Pickering emulsions

We report the self-assembly of a single species or a binary mixture of microparticles in ionic liquid-in-water Pickering emulsions, with emphases on the interfacial self-assembled particle structure and the partitioning preference of free particles in the dispersed and continuous phases. The particles form monolayers at ionic liquid-water interfaces and are close-packed on fully covered emulsion droplets or aggregated on partially covered droplets. In contrast to those at oil-water interfaces, no long-range-ordered colloidal lattices are observed. Interestingly, other than equilibrating at the ionic liquid-water interfaces, the microparticles also exhibit a partitioning preference in the dispersed and continuous phases: the sulfate-treated polystyrene (S-PS) and aldehyde-sulfate-treated polystyrene (AS-PS) microparticles are extracted to the ionic liquid phase with a high extraction efficiency, whereas the amine-treated polystyrene (A-PS) microparticles remain in the water phase.

A novel water-in-ionic liquid microemulsion and its interfacial effect on the activity of laccase

It is of great significance to develop an appropriate water-in-ionic liquid (W/IL) microemulsion suitable for the expression of the catalytic activity of a given enzyme. In this paper, the phase diagram of a new AOT/Triton X-100/H(2)O/[Bmim][PF(6)] pseudo ternary system is presented. With the aid of nonionic surfactant Triton X-100, AOT could be dissolved in hydrophobic ionic liquid [Bmim][PF(6)], forming a large single phase microemulsion region. The water-in-[Bmim][PF(6)] (W/IL) microemulsion domain was identified electrochemically by using K(3)Fe(CN)(6) as a probe. The existence of W/IL microemulsions was demonstrated spectrophotometrically by using CoCl(2) as a probe. New evidences from the FTIR spectroscopic study, which was first introduced to the W/IL microemulsion by substituting D(2)O for H(2)O to eliminate the spectral interference, demonstrated that there existed bulk water at larger ω(0) values (ω(0) was defined as the molar ratio of water to the total surfactant) in the W/IL microemulsion, which had remained unclear before. In addition to the inorganic salts, biomacromolecule laccase could be solubilized in the W/IL microemulsion. The laccase hosted in the microemulsion exhibited a catalytic activity and the activity could be regulated by the composition of the interfacial membrane.