The Coiling Effect in Ether Ionic Liquids: Exploiting Acetate as a Probe for Transport Properties and Microenvironment Analysis

The inherently high viscosity of ionic liquids (ILs) can limit their potential applications. One approach to address this drawback is to modify the cation side chain with ether groups. Herein, we assessed the structure-property relationship by focusing on acetate (OAc), a strongly coordinating anion, with 1,3-dialkylimidazolium cations with different side chains, including alkyl, ether, and hydroxyl functionalized, as well as their combinations. We evaluated their viscosity, thermal stabilities, and microstructure using Raman and infrared (IR) spectroscopies, allied to density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations. The viscosity data showed that the ether insertion significantly enhances the fluidity of the ILs, consistent with the coiling effect of the cation chain. Through a combined experimental and theoretical approach, we analyzed how the OAc anion interacts with ether ILs, revealing a characteristic bidentate coordination, particularly in hydroxyl functionalized ILs due to specific hydrogen bonding with the OH group. IR spectroscopy showed subtle shifts in the acidic hydrogens of imidazolium ring C(2)-H and C(4,5)-H, suggesting weaker interactions between OAc and the imidazolium ring in ether-functionalized ILs. Additionally, spatial distribution functions (SDF) and dihedral angle distribution obtained via AIMD confirmed the intramolecular hydrogen bonding due to the coiling effect of the ether side chain.

Effect of Oil on Cellulose Dissolution in the Ionic Liquid 1-Butyl-3-methyl Imidazolium Acetate

While ionic liquids (ILs) are well known to be excellent solvents for cellulose, the exact mechanism of dissolution has been a much disputed topic in recent years and is still not completely clear. In this work, we add to the current understanding and highlight the importance of hydrophobic interactions, through studying cellulose dissolution in mixtures of 1-butyl-3-methyl imidazolium acetate (BmimAc) and medium-chain triglyceride (MCT) oil. We demonstrate that the order in which constituents are mixed together plays a key role, through nuclear magnetic resonance (NMR) spectroscopic analysis. When small quantities of MCT oil (0.25-1 wt %) were introduced to BmimAc before cellulose, the effect on BmimAc chemical shift values was much more significant compared to when the cellulose was dissolved first, followed by oil addition. Rheological analysis also showed small differences in the viscosities of oil-cellulose-BmimAc solutions, depending on the order the constituents were added. On the other hand, no such order effect on the NMR results was observed when cellulose was replaced with cellobiose, suggesting that this observation is unique to the macromolecule. We propose that a cellulose-oil interaction develops but only when the cellulose structure has a sufficient degree of order and not when the cellulose is molecularly dispersed, since the hydrophobic cellulose plane is no longer intact. In all cases, cellulose-BmimAc-oil solutions were stable for at least 4 months. To our knowledge, this is the first work that investigates the effect of oil addition on the dissolving capacity of BmimAc and highlights the need for further re-evaluation of accepted mechanisms for cellulose dissolution in ILs.

Why Do Liquids Mix? The Mixing of Protic Ionic Liquids Sharing the Same Cation Is Apparently Driven by Enthalpy, Not Entropy

We study hydrogen bond (HB) redistribution in mixtures of two protic ionic liquids (PILs) sharing the same cation: triethylammonium methanesulfonate ([TEA][OMs]) and triethylammonium trifluoromethanesulfonate ([TEA][OTf]). The mixtures exhibit large negative energies of mixing. Based on results obtained from atomic detail molecular dynamics (MD) simulations, we derive a lattice model, discriminating between HB and nonspecific intermolecular interactions. We demonstrate that due to the ordered structure of the PILs, mostly the HB interactions contribute to the mixing energy. This allows to us to connect the equilibrium of HBs to each of the two anion species with the corresponding excess energies and entropies. The entropy associated with HB redistribution is shown to be negative, and even overcompensating the positive entropy associated with a statistical distribution of the ions in the mixture. This is strongly suggesting that the mixing process is driven by enthalpy, not entropy.

Hydrogen Bond Redistribution Effects in Mixtures of Protic Ionic Liquids Sharing the Same Cation: Non-ideal Mixing with Large Negative Mixing Enthalpies

We report a joint experimental and theoretical study characterising the hydrogen bond (HB) redistribution in mixtures of two different protic ionic liquids (PILs) sharing the same cation: triethylammonium-methanesulfonate ([TEA][OMs]) and...

Aqueous solutions of binary ionic liquids: insight into structure, dynamics, and interface properties by molecular dynamics simulations and DFT methods

The behavior of aqueous solutions of mixtures of ionic liquids (ILs) is of special interest because of their amphiphilic character, from both a fundamental and application viewpoint. In this work, we conducted molecular dynamics (MD) simulations and density functional theory (DFT) calculations to understand the effect of water on the intermolecular interactions in three IL binary mixtures [C4mim]/[Cl]/[BF4], [C4mim]/[Cl]/[PF6] and [C4mim]/[BF4]/[PF6] containing the well-characterized cation, 1-n-butyl-3-methylimidazolium [C4mim]+ and the anions chloride [Cl]-, tetrafluoroborate [BF4]-, and hexafluorophosphate [PF6]-. The perturbation of the structures in the binary IL mixture by water molecules was analyzed in the bulk and at the liquid/vacuum interface using distribution functions, hydrogen-bond statistics, and density profiles. Interactions between anions and cations change drastically when the IL mixtures are dissolved in water. In particular, anion-water interactions are stronger than anion-cation interactions. H-Bonds are the dominant interactions. They are prevalently electrostatic and strong for the two [Cl]-containing systems in both the water-free and the water-containing systems. The very hydrophobic [C4mim]/[BF4]/[PF6] system gains stability from dispersive interactions and consequently segregates water markedly when admixed. The most probable orientations of IL cations in the bulk and at the vicinity of the interface were examined using bivariate distribution calculations and show [PF6]- segregating to the surface in keeping with its highly hydrophobic nature. DFT calculated structures, energies, dipole moments, global hardness and solvation energies using model ion pairs [C4mim][X] or complexes [C4mim]2[X][Y], with [X/Y]- = [Cl]-, [BF4]-, or [PF6]- are completely consistent with the findings for the bulk.

Thorough studies of tricyanomethanide-based ionic liquids - the influence of alkyl chain length of the cation

The glassy, supercooled, and normal liquid states of the 1-alkyl-3-methylimidazolium tricyanomethanide series [CnC1im][TCM] (n = 2, 4, 6, 8, and 16) were investigated by dielectric and mechanical (rheological) experiments supplemented by X-ray diffraction. The conductivity relaxation was found to be accompanied by a pronounced secondary relaxation. However, based on ambient and high-pressure results as well as the coupling model, we assumed that the latter one can not be classified as Johari-Goldstein relaxation. Moreover, the studies on the nanoscale organization of ionic liquids indicated that 1-alkyl-3-methylimidazolium tricyanomethanide ILs begin to form nanoscale aggregates when the alkyl chain of the cation has six carbon atoms.

Probing the Reorganization of Ionic Liquids' Structure Induced by CO2 Sorption

The sorption of CO₂ is often used to modify the macroscopic properties of liquids and solids. In the particular case of ionic liquids, different from molecular liquids, the sorption of CO₂ may not induce volume expansions due to the strong Coulombic interactions between the ions of the fluid. However, a considerable viscosity decrease has been systematically observed. In order to understand the mechanisms of properties modifications in ionic fluids, herein we used Raman spectroscopy to probe the effect of CO₂ on the structure of ionic liquids. It is shown that CO₂ perturbs the electrostatic interactions between cations and anions, thus inducing a change in the polar domain of ionic liquids. It is observed that ionic liquids having bulkier ions are more prone to be perturbed by CO₂ in comparison to ionic liquids having smaller ions. These results reveal new means of controlling the electrostatic forces between the ions and contributes to the mechanistic understanding of the modification of the macroscopic properties of ionic liquids by CO₂ sorption.

Anions as Dynamic Probes for Ionic Liquid Mixtures

Ionic liquid (IL) mixtures have been proposed as a viable alternative to rationally fine-tune the physicochemical properties of ILs for a variety of applications. The understanding of the effects of mixing ILs on the properties of the mixtures is however only in the very early stages. Two series of ionic liquid mixtures, based on the 1-ethyl-3-methylimidazolium and 1-dodecyl-3-methylimidazolium cations, and having a common anion (tetrafluoroborate or bis(trifluoromethylsulfonyl)imide), have been prepared and deeply characterized via multiple NMR techniques. Diffusion and relaxation methods combined with 2D ion-ion correlation (nuclear Overhauser enhancement) experiments have been used for a better understanding of the interplay between dynamics and structure of IL mixtures. A crucial role of the anion in driving the mixture's behavior emerged, making them important "dynamic probes" for gaining information of the polar and nonpolar regions of ionic liquids and their mixtures.

Ion speciation: a key for the understanding of the solution properties of ionic liquid mixtures

Recently, combinations of two (or more) ionic liquids, known as ionic liquid mixtures, have become popular and have a broad range of applications. However, the fundamental knowledge on the molecular interactions that exist in ionic liquid mixtures is far from being understood. In this work, the experimental measurement of the water activity coefficient and computational modelling using Conductor-like Screening Model for Real Solvent (COSMO-RS) were carried out to get an insight into the molecular interactions that are present in ionic liquid mixtures in aqueous solution. The results show that the combination of two ionic liquids of different basicity in aqueous solution allows fine tuning of the water activities, covering a wide range of values that could replace several pure fluids. This is an important feature resulting from the unexpected ion speciation of the ionic liquid mixtures in aqueous solution.

On the structural origin of free volume in 1-alkyl-3-methylimidazolium ionic liquid mixtures: a SAXS and 129Xe NMR study

Ionic liquid (IL) mixtures enable the design of fluids with finely tuned structural and physicochemical properties for myriad applications. In order to rationally develop and design IL mixtures with the desired properties, a thorough understanding of the structural origins of their physicochemical properties and the thermodynamics of mixing needs to be developed. To elucidate the structural origins of the excess molar volume within IL mixtures containing ions with different alkyl chain lengths, 3 IL mixtures containing 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ILs have been explored in a joint small angle X-ray scattering (SAXS) and 129Xe NMR study. The apolar domains of the IL mixtures were shown to possess similar dimensions to the largest alkyl chain of the mixture with the size evolution determined by whether the shorter alkyl chain was able to interact with the apolar domain. 129Xe NMR results illustrated that the origin of excess molar volume in these mixtures was due to fluctuations within these apolar domains arising from alkyl chain mismatch, with the formation of a greater number of smaller voids within the IL structure. These results indicate that free volume effects for these types of mixtures can be predicted from simple considerations of IL structure and that the structural basis for the formation of excess molar volume in these mixtures is substantially different to IL mixtures formed of different types of ions.

Using Thermodynamics to Assess the Molecular Interactions of Tetrabutylphosphonium Carboxylate–Water Mixtures

Densities, ρ, viscosities, η, and enthalpies of mixing, , of binary [P4 4 4 4][CnCOO]–water mixtures (with n=1, 2 or 7) were determined at atmospheric pressure as a function of temperature. The excess, , apparent, , and partial, , molar volumes were deduced from experimental data, as well as fragilities, m*, and excess Gibbs free energies of activation of viscous flow, . exhibited predominantly negative deviation from ideality, with a minimum at approximately ~0.8 for all three systems, indicating strong hydrogen-bonding interactions. All three binary systems were found to be fragile, with [P4 4 4 4][C7COO] showing the smallest deviations in fragility with the addition of water. values of the systems were exothermic over the entire composition range, having the following trend: [P4 4 4 4][C2COO]>[P4 4 4 4][C7COO]>[P4 4 4 4][C1COO].

Using hydrogenated and perfluorinated gases to probe the interactions and structure of fluorinated ionic liquids

Ionic liquids with perfluorinated instead of hydrogenated alkyl chains dissolve larger quantities of perfluorocarbon gases that are solvated in their apolar domains.

Microheterogeneity in Ionic Liquid Mixtures: Hydrogen Bonding, Dispersed Ions, and Dispersed Ion Clusters

Mixtures of ionic liquids (ILs) having a common ion but differing in the identity of the anion or cation represent highly interesting media. By varying the composition, one can successfully modulate specific physicochemical, structural, and biological properties. The molecular interactions (coulombic, hydrogen-bonding, van der Waals, and π–π intermolecular forces) that determine the three-dimensional structure of pure ILs can indeed be modified by the addition of another IL. In this context, we present here a 1H NMR, Fourier transform (FT)-IR, thermogravimetric, and solvatochromic study of the structural features of IL binary mixtures based on a common imidazolium cation ([CnC1im]+) and anions of different size and hydrogen-bond acceptor ability. For each mixture, the analyses were carried out at different molar ratios of the two components.

A theoretical study on mixtures of amino acid-based ionic liquids

Ionic liquid mixtures containing amino acid anions are studied at the microscopic level using molecular dynamics simulations.

Can the tricyanomethanide anion improve CO2 absorption by acetate-based ionic liquids?

The chemical reaction of carbon dioxide with 1-alkyl-3-methylimidazolium acetate ionic liquids is not affected by presence of the C(CN)₃⁻ anion, which leads to a faster gas absorption and a higher solubility.