Nanostructure Of Ionic Liquids Research Articles

How Does Nanostructure in Ionic Liquids and Hybrid Solvents Affect Surfactant Self-Assembly?

Ionic liquids (ILs) have recently emerged as novel classes of solvents that support surfactant self-assembly into micelles, liquid crystals, and microemulsions. Their low volatility and wide liquid stability ranges make them attractive for many diverse applications, especially in extreme environments. However, the number of possible ion combinations makes systematic investigations both challenging and rare; this is further amplified when mixtures are considered, whether with water or other H-bonding components such as those found in deep eutectics. In this Perspective we examine what factors determine amphiphilicity, solvophobicity and solvophilicity, in ILs and related exotic environments, in what ways these differ from water, and how the underlying nanostructure of the liquid itself affects the formation and structure of micelles and other self-assembled materials.

Extremely slow dynamics of ionic liquid self-assembled nanostructures near a solid surface

Hypothesis: The dynamics of the self-assembled liquid nanostructure of the ionic liquids (ILs) near a mica surface can be determined from video-rate atomic force microscopy (AFM) data.Experiments: Video-rate AFM has been used to record the nanostructure dynamics of two most widely studied ILs, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BMIM TFSI) and ethylammonium nitrate (EAN), as well as EAN-water mixtures, above a model anode, mica. Diffusion coefficients were extracted from the AFM videos using dynamic differential microscopy and direct tracking.Findings: Video rate AFM is able to record the movement of the IL nanostructure. This is the first time that any liquid has been directly visualized at a scale of 10 nm × 10 nm in real-time. Diffusion coefficients determined from AFM videos reveal IL nanostructures near surfaces diffuse orders of magnitude more slowly than individual ions in the bulk. Thus, rather than free-flowing liquid, the near-surface nanostructure is better conceptualized as self-assembled aggregates of IL ions diffusing slowly over the cation-rich Stern layer, akin to adsorbed surfactant micelles in aqueous systems. This new and surprising insight affects wide-ranging processes involving the interfacial dynamics of concentrated electrolytes.

The amphiphilic nanostructure of ionic liquids affects the dehydration of alcohols.

The effect of the amphiphilic nanostructure of ionic liquids on the dehydration of secondary alcohols to alkenes has been investigated. The influence of these nanostructures was inverted when an acid catalyst was added to the reaction. This phenomenon was ascribed to a balance between ion-solute interactions and the formation of solute-catalyst hydrogen bonds, highlighting the complex interplay between interactions and reaction outcomes in these nanostructured solvent systems.

Recovering the electrochemical window by forming a localized solvation nanostructure in ionic liquids with trace water

Recovering the electrochemical window by forming a localized solvation nanostructure in ionic liquids with trace water

Ionic liquids: "normal" solvents or nanostructured fluids?

Ionic liquids (ILs) are a class of non-conventional solvents, which, for almost two decades, have continued to generate burgeoning interest in different fields of present-day chemical research with few similar precedents. Among the various aspects related to ILs, a topic worthy of in-depth analysis is their influence on organic reactivity and reaction rates. In light of this, the present short review aims to provide an overview of the literature from 2010 to the present day that addresses this issue. In particular, we herein present two main different viewpoints by which the solvent effect of ILs is explained: the first is mainly based on considering the bulk polarity of ILs and linear solvation energy relationships, while the other treats ILs as nanostructured fluids. In both cases, studies dealing with IL mixtures are also covered. Finally, literature addressing the area of supramolecular catalysis "by" or "in" ILs is also reported. This is one of the few reviews covering these specific aspects, aiming to provide a useful framework to guide future research into the effects of ILs on organic reactivity.

Structural investigations of molecular solutes within nanostructured ionic liquids.

Ionic liquids (ILs) containing sufficiently long alkyl chains form amphiphilic nanostructures with well-defined polar and non-polar domains. Here we have explored the robustness of these amphiphilic nanostructures to added solutes and gained insight into how the nature of the solute and IL ions affect the partitioning of these solutes within the nanostructured domains of ILs. To achieve this, small angle X-ray scattering (SAXS) investigations were performed and discussed for mixtures of 9 different molecular compounds with 6 different ILs containing imidazolium cations. The amphiphilic nanostructure of ILs persisted to high solute concentrations, over 50 mol% of added solute for most 1-butyl-3-methylimidazolium ILs and above 80 mol% for most 1-decyl-3-methylimidazolium ILs. Solute partitioning within these domains was found to be controlled by the inherent polarity and size of the solute, as well as specific interactions between the solute and IL ions, with SAXS results corroborated with IR spectroscopy and molecular dynamics simulations. Molecular dynamics simulations also revealed the ability to induce π+-π+ stacking between imidazolium cations through the use of these added molecular compounds. Collectively, these results provide scope for the selection of IL ions to rationally influence and control the partitioning behaviour of given solutes within the amphiphilic nanostructure of ILs.

DTAB micelle formation in ionic liquid/water mixtures is determined by ionic liquid cation structure

HypothesisThe high CMCs and low aggregation numbers of ionic micelles in the extreme electrolyte environment of ionic liquids (ILs) seem to be at odds with the effect of dilute aqueous electrolytes, which lower CMCs and promote elongated micelles. We hypothesise that the driving force for micellisation in ILs is determined by their underlying amphiphilic nanostructure, and that this can be controlled by mixing with water. ExperimentsCMCs and micelle sizes of dodecyltrimethylammonium bromide (DTAB) are determined in mixed solvents comprising water and the ionic liquids ethylammonium nitrate (EAN), ethanolammonium nitrate (EtAN), and propylammonium nitrate (PAN) over a wide composition range. Their behaviour is compared with aqueous electrolytes up to their solubility limit. CMCs are determined by a variety of techniques, and their relative strengths critically evaluated. Micelle morphology is determined by small-angle neutron scattering. FindingsIn water-rich mixtures, ILs do behave like simple electrolytes. Counterion binding dominates, both lowering the aqueous CMC and favouring a sphere-rod transition. However, even at modest concentrations, IL cations become incorporated into the micelle, causing the CMC to pass through a minimum, and arresting the sphere-rod transition. The efficiency of the cation depends on its amphiphilicity. As the IL content increases further, its role as a component of the bulk solvent becomes dominant: Only here does IL nanostructure influence micellization, as it increases alkyl chain solubility (EAN, PAN) and hence raises the CMC.

Water Clathrates in Nanostructural Organization of Hydrated Ionic Liquids Manifest a Peculiar Density Trend.

Ionic liquid-water binary solutions have significantly expanded the applications of ionic liquids (ILs) in chemical and biological research. Therefore, considerable research has focused on measuring the thermophysical properties of these binary mixtures. From low-to-moderate concentrations of water, several IL/water mixtures exhibit deviations from expected trends in thermophysical behavior. One such example is a unique density trend observed for certain IL classes, which exhibit a characteristic increase in density with the addition of small amounts of water. Since water primarily interacts with the IL anion, such deviations have always been explained in the context of anion-water associations. Surprisingly, however, IL/water mixtures containing different cations but a common lactate anion exhibit similar peculiarities in density trends. Using atomistic level molecular dynamics simulations, we show that diverse density trends are caused by cation-mediated modulations in the IL nanostructure. Depending on its nature, the IL cation can play a dual role in modulating the IL nanostructure: (i) resist water-mediated breakdown of the nanostructure by interacting with the anion very strongly, (ii) further strengthen the nanostructure by incorporating water in the IL framework. The [emim] cation fails to play both roles resulting in the density decrease, while the [tmg] cation fulfills both roles leading to a density rise. The choline cation resists the density fall by inducing the formation of "water-clathrates" in the solution. Such occurrence of clathrates in IL/water binary mixtures, reported for the first time in this study, further emphasizes that the properties of ILs and its mixtures are not merely determined by the chemical nature of the component ions, but also by their unique nanostructural organizations. These unique nanostructural organizations also manifest in their unusual dynamics.

Nanostructuration of ionic liquids: impact on the cation mobility. A multi-scale study.

When probed at the macroscopic scale, Ionic Liquids (ILs) behave as highly dissociated (i.e. strong) electrolytes while, at the molecular scale, they show clear characteristics of weak ionic solutions. The multi-scale analysis we report in this paper reconciles these apparently at odds behaviors. We investigate by quasi-elastic neutron scattering (QENS) and neutron spin-echo (NSE), the nanometer/nanosecond dynamics of OMIM-BF4, an imidazolium-based IL showing strong nanostructuration. We also probe the same IL on the microscopic (μm and ms) scale by pulsed field gradient NMR. To interpret the neutron data, we introduce a new physical model to account for the dynamics of the side-chains and for the diffusion of the whole molecule. This model describes the observables over the whole and unprecedented investigated spatial ([0.15-1.65] Å-1) and time ([0.5-2000] ps) ranges. We arrive at a coherent and unified structural/dynamical description of the local cation dynamics: a localized motion within the IL nanometric domains is combined with a genuine long-range translational motion. The QENS, NSE and NMR experiments describe the same long-range translational process, but probed at different scales. The associated diffusion coefficients are more than one order of magnitude different. We show how this apparent discrepancy is a manifestation of the IL nanostructuration.

Morphology of Imidazolium-Based Ionic Liquids as Deposited by Vapor Deposition: Micro-/Nanodroplets and Thin Films.

The morphology of micro- and nanodroplets and thin films of ionic liquids (ILs) prepared through physical vapor deposition is presented. The morphology of droplets deposited on indium-tin-oxide-coated glass is presented for the extended 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Cn C1 im][Ntf2 ]; n=1-8) series, and the results show the nanostructuration of ILs. The use of in-vacuum energetic particles enhances/increases the nanodroplets mobility/coalescence mechanisms and can be a pathway to the fabrication of thin IL films.

Nanostructuration Effect on the Thermal Behavior of Ionic Liquids.

This work shows how the nanostructuration of ionic liquids (ILs) governs the glass and melting transitions of the bistriflimide imidazolium-based [Cn C1 im][NTf2 ] and [Cn Cn im][NTf2 ] series, which highlights the trend shift that occurs at the critical alkyl size (CAS) of n=6. An initial increase in the glass temperature (Tg ) with an increase in the alkyl side chain was observed due to the intensification of the dispersive interactions (van der Waals). Above the CAS, the -CH2 - increment has the same effect in both glass and liquid states, which leads to a plateau in the glass transition after nanostructuration. The melting temperature (Tm ) of the [Cn C1 im][NTf2 ] and [Cn Cn im][NTf2 ] series presents a V-shaped profile. For the short-alkyl ILs, the -CH2 - increment affects the electrostatic ion pair interactions, which leads to an increase in the conformational entropy. The -CH2 - increment disturbs the packing ability of the ILs and leads to a higher entropy value (ΔslSm○ ) and consequently a decrease in Tm . Above the CAS, the -CH2 - contribution to the melting temperature becomes more regular, as a consequence of the nanostructuration of the IL into polar and nonpolar domains. The dependence of the alkyl chain on the temperature, enthalpy, and entropy of melting in the ILs above the CAS is very similar to the one observed for the alkane series, which highlights the importance of the nonpolar alkyl domains on the ILs thermal behavior.

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 composed of 1-alkoxycarbonyl-3-alkyl-imidazolium cations ([C1COOCnC1im](+), n = 1, 2, 4) combined with [Br](-), [NO3](-), [BF4](-), [PF6](-), [TfO](-), and [Tf2N](-) were explored by using the molecular dynamics (MD) simulations and quantum theory of atoms in molecules (QTAIM) analysis at 400 K. Various thermodynamic properties of these ILs were extensively computed in our earlier work (Ind. Eng. Chem. Res., 2015, 54, 11678-11700). Nano-scale segregation analysis demonstrates the formation of a small spherical island-like hydrocarbon within the continuous ionic domain for ILs with short alkyl side chain ([C1COOC1C1im]), and a sponge-like nanostructure for the compound with long alkyl side chain ([C1COOC4C1im]). Ester-functionalized ILs with ethyl side chain ([C1COOC2C1im]) are the turning point between two different morphologies. Non-polar channels were observed for [C1COOC4C1im] ILs composed of smaller anions such as [Br] and [NO3], whereas clustering organization was found for the other anions. Formation of the spherical micelle-like nanostructure was seen for lengthened cations. Finally, the incorporation of an ester group into the alkyl side chain of the cation leads to stronger segregation between charged and uncharged networks, which consequently increased the possibility of self-assembly and micelle formation.

Understanding of Versatile and Tunable Nanostructuration of Ionic Liquids on Fluorinated Copolymer

A fundamental understanding of the versatile and tunable nanostructuration effect of two phosphonium ionic liquids (ILs), denoted octadecyltriphenylphosphonium iodide (IL-C18) and tributyl(methyl)phosphonium methylsulfate (IL-108), on a semicrystalline fluorinated copolymer poly(vinylidene fluoride-co-chlorotrifluoroethylene) (P(VDF-CTFE)) was elaborated. This research suggests that the nanostructuration mechanism is very dependent on the diffusion and interacting abilities of ILs in matrix, which are determined by the chemical structures of ILs. Both ILs appear to diffuse and regularly assemble in the rigid amorphous fraction (RAF) of P(VDF-CTFE) between crystalline lamellae thanks to the “template” confinement effect of 2D structure. The assembled structure shows a gradually regular 1D to 2D geometric evolution with increasing the ILs amount revealed by Porod exponents in SANS analyses. Simultaneously, these regularly assembled ILs in the RAF also act as “template” to efficiently induce a complete transition of nonpolar α-phase to polar γ- and/or β-phase with only a tiny amount of ILs (from 2 wt %) (by XRD and FTIR). Moreover, the crystallization displays a depression behavior with reduced Tc, Tm, crystallinity (by DSC), and crystallite size (by SANS) due to the existence of ILs in the RAF seen as obstacle for the regular chain-folding and the dipolar interaction with P(VDF-CTFE). We propose that the synergistic influence of mutual “templates” endues phosphonium ILs a versatile nanostructuration on the fluorinated matrix. Moreover, this versatile nanostructuration effect can be easily tuned by tailoring the chemical structures of ILs. The difference in the chemical structures of ILs endows them different diffusion rates upon crystallization from solution for preparing resulting blend films and strengths of dipolar interaction with P(VDF-CTFE), consequently affecting their dispersion state in the matrix to reside in the RAF with different extents and interacting abilities (confirmed by DEA and DMA). Thus, IL-108 with faster migration rate and stronger dipolar interaction than IL-C18 (due to the smaller steric hindrance and extra polar groups in the cation/anion combination of the IL structure) locates in the RAF much more to induce more trans-configuration (β-phase), more homogeneous dispersion morphology, and higher depression of crystallization.

Structure and nanostructure in ionic liquids.

Structure and nanostructure in ionic liquids.

Thermal and structural studies of imidazolium-based ionic liquids with and without liquid-crystalline phases: the origin of nanostructure.

To clarify the origin of the nanostructure of ionic liquids (ILs), we have investigated two series of ILs 1-alkyl-3-methylimidazolium hexafluorophosphate (CnmimPF6, n = 4-16, n is an alkyl-carbon number) and 1-alkyl-3-methylimidazolium chloride (CnmimCl, n = 4-14) using differential scanning calorimetry and X-ray diffraction techniques. The PF6 samples with n > 13 and the Cl samples with n > 10 exhibited the liquid-crystalline (LC) to liquid (L) phase transitions, as reported before. We found that both samples with smaller n also exhibited the LC to L transitions under supercooled states as far as the ionic motions were not frozen-in at the glass transition temperatures Tg. The Tg of the LC phase was close to that of the L phase, indicating that the characteristic length of the glass transition is shorter than that of the nanostructure. A low-Q peak due to the nanostructure in the L phase and a diffraction peak due to the layer structure in the LC phase appeared at almost the same Q positions in both samples. On the basis of the above results and some thermodynamic analysis, we argue that the nanostructures of ILs are essentially the same as the layer structures in the LC phases.

The effect of lithium salt doping on the nanostructure of ionic liquids.

In this work we report on the evolution of the structure of two model ionic liquid families, N-alkyl-N-methylpyrrolidinium (Pyr1n-TFSI) and 1-alkyl-3-methylimidazolium (CnMIm-TFSI) (n = 3, 4, 6 and 8) both containing the bis(trifluoromethanesulfonyl)imide (TFSI) anion, upon the addition of LiTFSI using small angle X-ray scattering (SAXS). The introduction of a lithium salt (Li-salt) tunes the interactions through the substitution of the large cation in the ionic liquid with the small and charge localized lithium ion, thus increasing the coulombic contribution from ion-ion interactions. We find that the introduction of lithium ions results in a restructuring of the polar groups in the ionic liquids. These changes are manifested as an increase in the correlation lengths related to charge alternation of the ions and a more disordered structure. This restructuring is interpreted as a reconfiguration of the anions as they coordinate to the small and ionic lithium. In contrast, the length scale of the mesoscopic heterogeneities related to the clustering of alkyl chains is virtually unchanged with lithium doping. Moreover, the correlation corresponding to alkyl chain domains becomes more well defined with increasing salt concentration, suggesting that Li-salt doping, i.e. an increased columbic interaction in the system, promotes clustering of the alkyl tails.

Nanostructure-thermal conductivity relationships in protic ionic liquids.

The thermal conductivities of nine protic ionic liquids (ILs) have been investigated between 293 and 340 K. Within this range, the thermal conductivities are between 0.18 and 0.30 W · m(-1) · K(-1). These values are higher than those typically associated with oils and aprotic ILs, but lower than those of strongly hydrogen bonding solvents like water. Weak linear decreases in thermal conductivity with temperature are noted, with the exception of ethanolammonium nitrate (EtAN) where the thermal conductivity increases with temperature. The dependence of thermal conductivity on IL type is analyzed with use of the Bahe-Varela pseudolattice theory. This theory treats the bulk IL as an array of ordered domains with intervening domains of uncorrelated structure which enable and provide barriers to heat propagation (respectively) via allowed vibrational modes. For the protic ILs investigated, thermal conductivity depends strongly on the IL cation alkyl chain length. This is because the cation alkyl chain controls the dimensions of the IL bulk nanostructure, which consists of charged (ordered domains) and uncharged regions (disordered domains). As the cation alkyl chain controls the dimensions of the disordered domains, it thus limits the thermal conductivity. To test the generality of this interpretation, the thermal conductivities of propylammonium nitrate (PAN) and PAN-octanol mixtures were examined; water selectively swells the PAN charged domain, while octanol swells the uncharged regions. Up to a certain concentration, adding water increases thermal conduction and octanol decreases it, as expected. However, at high solute concentrations the IL nanostructure is broken. When additional solvent is added above this concentration the rate of change in thermal conductivity is greatly reduced. This is because, in the absence of nanostructure, the added solvent only serves to dilute the salt solution.

Vapor pressures of 1,3-dialkylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquids with long alkyl chains.

This work presents the vapor pressure at several temperatures for the 1,3-dialkylimidazolium bis(trifluoromethylsulfonyl)imide series, [CN/2CN/2im][NTf2] (N = 14, 16, 18, and 20), measured by a Knudsen effusion method combined with a quartz crystal microbalance. The thermodynamic properties of vaporization of the ionic liquids under study are analysed together with the results obtained previously for the shorter alkyl chain length [CN/2CN/2im][NTf2] (N = 2, 4, 6, 8, 10, and 12), in order to evaluate the effect of the alkyl side chains of the cation and to get additional insights concerning the nanostructuration of ionic liquids. The symmetry effect is explored, based on the comparison with the asymmetric imidazolium based ionic liquids, [CN-1C1im][NTf2]. A trend shift on the thermodynamic properties of vaporization along the alkyl side chains of the extended symmetric ionic liquids, around [C6C6im][NTf2], was detected. An intensification of the odd-even effect was observed starting from [C6C6im][NTf2], with higher enthalpies and entropies of vaporization for the odd numbered ionic liquids, [C7C7im][NTf2] and [C9C9im][NTf2]. Similar, but less pronounced, odd-even effect was found for the symmetric ionic liquids with lower alkyl side chains length, [CN/2CN/2im][NTf2] (with N = 4, 6, 8, 10, and 12). This effect is related with the predominant orientation of the terminal methyl group of the alkyl chain to the imidazolium ring and their influence in the cation-anion interaction. The same Critical Alkyl length at the hexyl, (C6C1and C6C6) was found for both asymmetric and symmetric series indicating that the nanostructuration of the ionic liquids is related with alkyl chain length.

Effect of protic ionic liquid and surfactant structure on partitioning of polyoxyethylene non-ionic surfactants.

The partitioning constants and Gibbs free energies of transfer of poly(oxyethylene) n-alkyl ethers between dodecane and the protic ionic liquids (ILs) ethylammonium nitrate (EAN) and propylammonium nitrate (PAN) are determined. EAN and PAN have a sponge-like nanostructure that consists of interpenetrating charged and apolar domains. This study reveals that the ILs solvate the hydrophobic and hydrophilic parts of the amphiphiles differently. The ethoxy groups are dissolved in the polar region of both ILs by means of hydrogen bonds. The environment is remarkably water-like and, as in water, the solubility of the ethoxy groups in EAN decreases on warming, which underscores the critical role of the IL hydrogen-bond network for solubility. In contrast, amphiphile alkyl chains are not preferentially solvated by the charged or uncharged regions of the ILs. Rather, they experience an average IL composition and, as a result, partitioning from dodecane into the IL increases as the cation alkyl chain is lengthened from ethyl to propyl, because the IL apolar volume fraction increases. Together, these results show that surfactant dissolution in ILs is related to structural compatibility between the head or tail group and the IL nanostructure. Thus, these partitioning studies reveal parameters for the effective molecular design of surfactants in ILs.

Interplay Between Structure and Conductivity in Imidazolium-Based Ionic Liquids As Electrolytes for Magnesium Batteries

The search for new electrolytes is one of the most challenging aspects in the development of secondary batteries based on Li and Mg ions [1].The state of the art for Mg electrolytes is mainly constituted by organo-Mg compounds; in general, they show good Mg deposition and dissolution performance, high Coulombic efficiency and in some cases a wide electrochemical stability window [2-3]. The main drawback of this class of materials is associated with their dissolution in ethereal-based solvents characterized by a high vapor pressure and flammability. Nevertheless, the use of high-boiling solvents is known to lead to: (a) a decrease in the electrolyte conductivity; and (b) a decrease in the electrode reaction rate, which limits their practical applicability [2].Ionic liquids (ILs) are salts that are molten at room temperature [4]. ILs are also characterized by: (a) negligible vapor pressure; (b) high thermal stability; and (c) low flammability.Only a few examples of the applicability of ILs as electrolytes for secondary Mg batteries are reported in the literature [5]. Several issues regarding the conductivity mechanisms, formation of the solid-electrolyte interface (SEI) and long-term performance of these systems are still open questions.From a fundamental point of view, the interplay between the ILs nanostructure and the conductivity is a crucial point to clarify the effect of the IL matrix on the migration mechanism of Mg2+ ions.In this report, a comparison between two Mg electrolytes based on Ethyl-methylimidazolium tetrafluoroborate (EMImBF4) and Ethyl-methylimidazolium chloride/AlCl3 (EMImCl/(AlCl3)1.5) ILs is proposed. Both ionic liquids are doped with different amounts of δ-MgCl2, to achieve a sufficient conductivity of Mg2+ ions.The correlation between structure, thermal properties and conduction mechanism of the resulting EMImBF4/(δ-MgCl2)x and [EMImCl/(AlCl3)1.5]/(δ-MgCl2)x is investigated by a variety of techniques: (a) FT-MIR and FIR at different temperatures; (b) differential scanning calorimetry (DSC); and (c) broadband electrical spectroscopy (BES).Figure 1 shows a plot of σDC of EMImBF4 vs. 1/T. The observed trend of σDC is strongly correlated to the thermal features detected by DSC. A decrease in σDC of six orders of magnitude is evidenced at temperatures higher than the crystallization temperature (Tc). This phenomenon is investigated as a function of: (a) Mg concentration; and (b) the different anions in the ILs.BES measurements were undertaken to elucidate the electrical response of the electrolytes in terms of dielectric and polarization phenomena. At T < Tm (Tm = melting temperature) three dielectric relaxations are present, associated with the rotational motions of EMIm+ cations; at T > Tm, three and four interdomain polarization events are detected respectively for EMImBF4 and [EMImCl/(AlCl3)1.5]. These polarization events are associated with the presence of cation and anion nanocluster aggregates with different permittivities.