The Solubility of Gases in Ionic Liquids: A Chemoinformatic Predictive and Interpretable Approach.

This work comprises the study of solubilities of gases in ionic liquids (ILs) using a chemoinformatic approach. It is based on the codification, of the atomic inter-component interactions, cation/gas and anion/gas, which are used to obtain a pattern of activation in a Kohonen Neural Network (MOLMAP descriptors). A robust predictive model has been obtained with the Random Forest algorithm and used the maximum proximity as a confidence measure of a given chemical system compared to the training set. The encoding method has been validated with molecular dynamics. This encoding approach is a valuable estimator of attractive/repulsive interactions of a generical chemical system IL+gas. This method has been used as a fast/visual form of identification of the reasons behind the differences observed between the solubility of CO2 and O2 in 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM PF6 ) at identical temperature and pressure (TP) conditions, The effect of variable cation and anion effect has been evaluated.

Ionic Liquids and Water: Hydrophobicity vs. Hydrophilicity.

Many chemical processes rely extensively on organic solvents posing safety and environmental concerns. For a successful transfer of some of those chemical processes and reactions to aqueous media, agents acting as solubilizers, or phase-modifiers, are of central importance. In the present work, the structure of aqueous solutions of several ionic liquid systems capable of forming multiple solubilizing environments were modeled by molecular dynamics simulations. The effect of small aliphatic chains on solutions of hydrophobic 1-alkyl-3-methylimidazolium bis(trifluoromethyl)sulfonylimide ionic liquids (with alkyl = propyl [C3C1im][NTf2], butyl [C4C1im][NTf2] and isobutyl [iC4C1im][NTf2]) are covered first. Next, we focus on the interactions of sulphonate- and carboxylate-based anions with different hydrogenated and perfluorinated alkyl side chains in solutions of [C2C1im][CnF2n+1SO3], [C2C1im][CnH2n+1SO3], [C2C1im][CF3CO2] and [C2C1im][CH3CO2] (n = 1, 4, 8). The last system considered is an ionic liquid completely miscible with water that combines the cation N-methyl-N,N,N-tris(2-hydroxyethyl)ammonium [N1 2OH 2OH 2OH]+, with high hydrogen-bonding capability, and the hydrophobic anion [NTf2]-. The interplay between short- and long-range interactions, clustering of alkyl and perfluoroalkyl tails, and hydrogen bonding enables a wealth of possibilities in tailoring an ionic liquid solution according to the needs.

C13 - a new empirical force field to characterize the mechanical behavior of carbyne chains.

An accurate prediction of the mechanical behavior of long carbyne chains depends on the suitable modeling of bond alternation in these chains. While first-principles methods are a good approach, less computationally demanding empirical potentials are preferable for large carbyne-containing systems. AIREBO and Reax empirical potentials have extensively and successfully been used for simulating the mechanical behavior of graphene and carbon nanotubes. However, it remains unclear if these potentials can be directly applied in the accurate mechanical modeling of carbon nanostructures with sp hybridization, without re-parameterization. Here, a new force-field for carbyne, designated as C13 potential, that takes bond alternation into account, is presented. This new empirical potential was parameterized from ab initio calculations. Molecular dynamics (MD) simulations using the developed force-field are then conducted to determine the mechanical properties of carbyne chains under tensile loading, namely to assess their dependence on chain length and temperature. The bending stiffness of carbyne and its persistence length are also calculated. The results obtained are validated through comparison with results available in the literature. Lastly, the C13 potential is employed to model, for the first time, the tensile and the compressive behaviors of the hybrid system composed of carbon nanotubes infilled with carbyne chains.

Tuning the miscibility of water in imide-based ionic liquids.

Liquid-liquid phase behavior measurements were performed for binary mixtures of water and ionic liquids (ILs) containing the same 1-ethyl-3-methylimidazolium ([C2mim]+) cation and different imide-based anions, having symmetric (bis(fluorosulfonyl)imide ([FSI]-)) or asymmetric structures (2,2,2-trifluoromethylsulfonyl-N-cyanoamide ([TFSAM]-) and 2,2,2-trifluoro-N-(trifluoromethylsulfonyl)acetamide ([TSAC]-)). An inversion of phase behavior was observed: while below ∼298 K, the miscibility of water in the studied ILs increases according to the order [C2mim][TSAC] < [C2mim][FSI] < [C2mim][NTf2], for temperatures above ∼303 K, the reverse trend is observed [C2mim][NTf2] < [C2mim][FSI] < [C2mim][TSAC]. In turn, above ∼306 K the [C2mim][TFSAM] is completely miscible with H2O in all ranges of concentrations. The obtained results also revealed an unusual water solubility variation of 11% in [C2mim][FSI], and 20% in [C2mim][TSAC], when the system temperature was changed by less than 1 K, around 298 K and 301 K, respectively. Molecular Dynamics (MD) simulations were used to understand the IL-water interactions and rationalize the experimental observations. These results suggested that the miscibility trends are mainly related to the ability of the water molecules to form water-anion and water-water aggregates in solution.

Neat ionic liquids versus ionic liquid mixtures: a combination of experimental data and molecular simulation.

Simple mixtures of ionic liquids (IL-IL mixtures) can become a promising approach for the substitution of task-specific ILs. Such a concept was explored in this article by comparison of the thermophysical properties of neat 1-ethyl-3-methylimidazolium 2,2,2-trifluoromethylsulfonyl-N-cyanoamide, [C2mim][TFSAM], and equimolar mixtures of two structurally similar ILs having more common ions. Molecular dynamics (MD) simulations were additionally used to further highlight structural aspects of these systems at a molecular level.

Probing the Surface Tension of Ionic Liquids Using the Langmuir Principle.

At 298 K, the surface tension of ionic liquids (ILs) of the 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide series, [C nC1Im][NTf2], ranges from around 35 mN·m-1 for [C2C1Im][NTf2] to just below 30 mN·m-1 for [C12C1Im][NTf2]. However, the decrease rate along the series is not constant: a large decrease from [C2C1Im][NTf2] to [C8C1Im][NTf2] is followed by almost constant values from [C8C1Im][NTf2] to [C12C1Im][NTf2]. Such behavior is hard to interpret from a molecular point of view without suitable information about the free-surface structure of the different ILs. In this work, we have successfully used the Langmuir principle in combination with structural data obtained from angle-resolved X-ray photoelectron spectroscopy experiments and molecular dynamics simulations, to predict the correct surface tension trend along the IL series. The concepts unveiled for this particular homologous IL family can be easily extended to other systems.

Exploring the bulk-phase structure of ionic liquid mixtures using small-angle neutron scattering.

Small-angle neutron scattering experiments, supported by molecular dynamics simulations, have been performed on a range of compositions of the [C2mim]1-x[C12mim]x[Tf2N] ionic liquid mixture system. Isotopic contrast variation, through selective deuteration of both cations, has been used to assist in fitting the data to different scattering models. These data, and subsequent fitting, show that the structure of the ionic liquid mixtures changes substantially as a function of composition. Mixtures where x < 0.32 are dominated by aggregates of amphiphilic [C12mim]+ ions in the relatively polar [C2mim][Tf2N] solvent. Compositions where x > 0.32 can be described as bicontinuous, containing networks of both polar and non-polar domains, where the C12 chains of the [C12mim]+ ions percolate through the system to form a continuous non-polar sub-phase. Temperature-dependent scattering experiments suggest that there is relatively little change in bulk structure in these liquids between 20 and 60 °C. The presence of water, however, does influence some aspects of the liquid structure in a composition that is rich in [C2mim][Tf2N] (where x = 0.24).

ILs through the looking glass: electrostatics and structure probed using charge-inverted ionic liquid pairs.

Ionic liquids combining potassium cations with 1-alkyl-3-methylcyclopentadienyl anions, K[CnC1Cp] (n = 4, 6) have been synthesized. Differential scanning calorimetry measurements have shown that K[C4C1Cp] and K[C6C1Cp] melt without decomposition at around 90 °C. These two ionic liquids are the charge-inverted counterparts of [C4C1Im]Cl and [C6C1Im]Cl, two common ionic liquids. The concept of charge-inverted ionic pairs is used to explore the nature of the interactions and structure in different ionic compounds, from simple alkali halide salts to ionic liquids based on complex molecular ions. Different sets of experimental data, empirical correlations and molecular dynamics simulations are used to that effect.

Solvation of alcohols in ionic liquids - understanding the effect of the anion and cation.

In this work, we studied the effect of anion and cation properties on the interaction of alcohols with ionic liquids (ILs), using propan-1-ol as a molecular probe. The enthalpies of solution at infinite dilution of propan-1-ol in several ILs were measured by isothermal titration calorimetry (ITC). The calorimetric results were analysed together with molecular dynamics simulation and quantum chemical calculations of the interaction of the hydroxyl group of propan-1-ol with the anions. The results evidenced the role of the anion's basicity in the intermolecular interactions of alcohols and ionic liquids and further revealed a secondary effect of the cation nature on the solvation process. The effect of the anion basicity on the strength of the interaction of alcohols with ionic liquids was evaluated by comparing the results obtained for ILs with the same cation and different anions, [C4C1im][anion] (anions NTf2, PF6, FAP, DCA and TFA). The effect of the cation (size, aromaticity, charge distribution, and acidity) was explored using five different cations of the NTf2 series, [cation][NTf2] (cations C4C1im, C4C1pirr, C4py, C4C1pip, and C3C1C1im).

Molecular dynamics studies on the structure and interactions of ionic liquids containing amino-acid anions.

Several molecular dynamics (MD) simulations have been performed in order to obtain structural information on ionic liquids (ILs) based on amino-acid anions. Six hydrophilic ILs containing cholinium or imidazolium cations combined with alaninate, glycinate or lysinate anions were modelled using the all-atom CL&P and OPLS-AA force fields. Both pure ILs and their aqueous solutions have been studied. The MD data have allowed us to analyse structure factors, S(q), and pair radial distributions functions, g(r), as well as aggregation patterns and specific interactions. The results have shown us that in neat amino-acid-based ILs the anions interact mainly through their carboxylate moiety with the charged centres of the cations. Both the lack of heavy atoms and the small size of the interacting centre in the anion contribute to the absence of a charge ordering peak in the structure factor functions of the corresponding ILs. In turn, their aqueous solutions reveal the existence of small ionic aggregates. The size distribution of these aggregates is strongly dependent on the solution's concentration. This fact points to the possibility of using amino-acid-based ILs as agents to promote hydrotrope effects, significant for the solubilisation and stabilization of organic molecules and macromolecules in aqueous solution.

Influence of Nanosegregation on the Surface Tension of Fluorinated Ionic Liquids.

We have investigated, both theoretically and experimentally, the balance between the presence of alkyl and perfluoroalkyl side chains on the surface organization and surface tension of fluorinated ionic liquids (FILs). A series of ionic liquids (ILs) composed of 1-alkyl-3-methylimidazolium cations ([CnC1im] with n = 2, 4, 6, 8, 10, or 12) combined with the perfluorobutanesulfonate anion was used. The surface tensions of the investigated liquid salts are considerably lower than those reported for non-fluorinated ionic liquids. The most surprising and striking feature is the identification, for the first time, of a minimum at n = 8 in the surface tension versus the length of the IL cation alkyl side chain. Supported by molecular dynamics (MD) simulations, it was found that this trend is a result of the competition between the two nonpolar domains (perfluorinated and aliphatic) pointing toward the gas-liquid interface, a phenomenon which occurs in ILs with perfluorinated anions. Furthermore, these ILs present the lowest surface entropy reported to date.

Bulk nanostructure of the prototypical 'good' and 'poor' solvate ionic liquids [Li(G4)][TFSI] and [Li(G4)][NO3].

The bulk nanostructures of a prototypical 'good' solvate ionic liquid (SIL) and 'poor' SIL have been examined using neutron diffraction and empirical potential structure refinement (EPSR) simulated fits. The good SIL formed by a 1 : 1 mixture of lithium bis(trifluoromethylsulfonyl)imide (Li[TFSI]) in tetraglyme (G4), denoted [Li(G4)][TFSI], and the poor SIL formed from a 1 : 1 mixture of lithium nitrate (Li[NO3]) in G4, denoted [Li(G4)][NO3], have been studied. In both SILs there are strong Lewis acid-base interactions between Li(+) and ligating O atoms. However, the O atoms coordinated to Li(+) depend strongly on the counter anion present. LiO coordination numbers with G4 are 2-3 times higher for [Li(G4)][TFSI] than [Li(G4)][NO3], and conversely the LiO anion coordination number is 2-3 times higher in [Li(G4)][NO3]. In both solvates the local packing of Li around G4 O atoms are identical but these interactions are less frequent in [Li(G4)][NO3]. In both SILs, Li(+) has a distribution of coordination numbers and a wide variety of different complex structures are present. For [Li(G4)][NO3], there is a significant proportion uncoordinated G4 in the bulk; ∼37% of glyme molecules have no LiO contacts and each G4 molecule coordinates to an average of 0.5 Li(+) cations. Conversely, in [Li(G4)][TFSI] only ∼5% of G4 molecules lack LiO contacts and G4 molecules coordinates to an average of 1.3 Li(+) cations. Li(+) and G4 form polynuclear complexes, of the form [Lix(G4)y](x+), in both solvates. For [Li(G4)][TFSI] ∼35% of Li(+) and G4 form 1 polynuclear complexes, while only ∼10% of Li(+) and G4 form polynuclear complexes in [Li(G4)][NO3].

Ionic Liquid Films at the Water-Air Interface: Langmuir Isotherms of Tetra-alkylphosphonium-Based Ionic Liquids.

The behavior of ionic liquids trihexyl(tetradecyl)phosphonium bis(trifluoromethylsulfonyl)imide and trihexyl(tetradecyl)phosphonium dicyanamide, [P6 6 6 14][Ntf2] and [P6 6 6 14][N(CN)2], respectively, at the water-air interface was investigated using the Langmuir trough technique. The obtained surface pressure versus mean molecular area (MMA) isotherms, π-A, and surface potential versus MMA isotherms, ΔV-A, show distinct interfacial behavior between the two systems. The results were interpreted at a molecular level using molecular dynamics simulations: the different compression regimes along the [P6 6 6 14][Ntf2] isotherm correspond to the self-organization of the ions at the water surface into compact and planar monolayers that coalesce at an MMA value of ca. 1.85 nm(2)/ion pair to form an expanded liquidlike layer. Upon further compression, the monolayer collapses at around 1.2 nm(2)/ion pair to yield a progressively thicker and less organized layer. These transitions are much more subdued in the [P6 6 6 14][N(CN)2] system because of the more hydrophilic nature of the dicyanamide anion. The numerical density profiles obtained from the MD simulation trajectories are also able to emphasize the very unusual packing of the four long alkyl side chains of the cation above and below the ionic layer that forms at the water surface. Such a distribution is also different for the two studied systems during the different compression regimes.

Modeling the structure and thermodynamics of ferrocenium-based ionic liquids.

A new force-field for the description of ferrocenium-based ionic liquids is reported. The proposed model was validated by confronting Molecular Dynamics simulations results with available experimental data-enthalpy of fusion, crystalline structure and liquid density-for a series of 1-alkyl-2,3,4,5,6,7,8,9-octamethylferrocenium bis(trifluoromethylsulfonyl)imide ionic liquids, [CnFc][NTf2] (3 ≤ n ≤ 10). The model is able to reproduce the densities and enthalpies of fusion with deviations smaller than 2.6% and 4.8 kJ mol(-1), respectively. The MD simulation trajectories were also used to compute relevant structural information for the different [CnFc][NTf2] ionic liquids. The results show that, unlike other ILs, the alkyl side chains present in the cations are able to interact directly with the ferrocenium core of other ions. Even the ferrocenium charged cores (with relatively mild charge densities) are able to form small contact aggregates. This causes the partial rupture of the polar network and precludes the formation of extended nano-segregated polar-nonpolar domains normally observed in other ionic liquids.

Structural and aggregate analyses of (Li salt + glyme) mixtures: the complex nature of solvate ionic liquids.

The structure and interactions of different (Li salt + glyme) mixtures, namely equimolar mixtures of lithium bis(trifluoromethylsulfonyl)imide, nitrate or trifluoroacetate salts combined with either triglyme or tetraglyme molecules, are probed using Molecular Dynamics simulations. structure factor functions, calculated from the MD trajectories, confirmed the presence of different amounts of lithium-glyme solvates in the aforementioned systems. The MD results are corroborated by S(q) functions derived from diffraction and scattering data (HEXRD and SAXS/WAXS). The competition between the glyme molecules and the salt anions for the coordination to the lithium cations is quantified by comprehensive aggregate analyses. Lithium-glyme solvates are dominant in the lithium bis(trifluoromethylsulfonyl)imide systems and much less so in systems based on the other two salts. The aggregation studies also emphasize the existence of complex coordination patterns between the different species (cations, anions, glyme molecules) present in the studied fluid media. The analysis of such complex behavior is extended to the conformational landscape of the anions and glyme molecules and to the dynamics (solvate diffusion) of the bis(trifluoromethylsulfonyl)imide plus triglyme system.

The effect of the cation alkyl chain branching on mutual solubilities with water and toxicities.

The design of ionic liquids has been focused on the cation-anion combinations but other more subtle approaches can be used. In this work the effect of the branching of the cation alkyl chain on the design of ionic liquids (ILs) is evaluated. The mutual solubilities with water and toxicities of a series of bis(trifluoromethylsulfonyl)-based ILs, combined with imidazolium, pyridinium, pyrrolidinium, and piperidinium cations with linear or branched alkyl chains, are reported. The mutual solubility measurements were carried out in the temperature range from (288.15 to 323.15) K. From the obtained experimental data, the thermodynamic properties of the solution (in the water-rich phase) were determined and discussed. The COnductor like Screening MOdel for Real Solvents (COSMO-RS) was used to predict the liquid-liquid equilibrium. Furthermore, molecular dynamic simulations were also carried out aiming to get a deeper understanding of these fluids at the molecular level. The results show that the increase in the number of atoms at the cation ring (from five to six) leads to a decrease in the mutual solubilities with water while increasing their toxicity, and as expected from the well-established relationship between toxicities and hydrophobicities of ILs. The branching of the alkyl chain was observed to decrease the water solubility in ILs, while increasing the ILs solubility in water. The inability of COSMO-RS to correctly predict the effect of branching alkyl chains toward water solubility on them was confirmed using molecular dynamic simulations to be due to the formation of nano-segregated structures of the ILs that are not taken into account by the COSMO-RS model. In addition, the impact of branched alkyl chains on the toxicity is shown to be not trivial and to depend on the aromatic nature of the ILs.

Molecular Interactions between Ionic Liquid Lubricants and Silica Surfaces: An MD Simulation Study.

The unique physicochemical properties of ionic liquids (ILs) attracted interest in their application as lubricants of micro/nano-electromechanical systems. This work evaluates the feasibility of using the protic ionic liquids [4-picH][HSO4], [4-picH][CH3SO3], [MIMH][HSO4], and [MIMH][CH3SO3] and the aprotic ILs [C6mim][HSO4] and [C6mim][CH3SO3] as additives to model lubricant poly(ethylene glycol) (PEG200) to lubricate silicon surfaces. Additives based on the cation [4-picH]+ exhibited the best tribological performance, with the optimal value for 2% [4-picH][HSO4] in PEG200 (w/w). Molecular dynamics (MD) simulations of the first stages of adsorption of the ILs at the glass surface were performed to portray the molecular behavior of the ILs added to PEG200 and their interaction with the silica substrate. For the pure ILs at the solid substrates, the MD results indicated that weak specific interactions of the cation with the glass interface are lost to accommodate the larger anion in the first contact layer. For the PEG200 + 2% [4-picH][HSO4] system, the formation of a more compact protective film adsorbed at the glass surface is revealed by a larger trans population of the dihedral angle -O(R)-C-C-O(R)- in PEG200, in comparison to the same distribution for the pure model lubricant. Our findings suggest that the enhanced lubrication performance of PEG200 with [4-picH][HSO4] arises from synergistic interactions between the protic IL and PEG200 at the adsorbed layer.

Viscosity mixing rules for binary systems containing one ionic liquid.

In this work the applicability of four of the most commonly used viscosity mixing rules to [ionic liquid (IL)+molecular solvent (MS)] systems is assessed. More than one hundred (IL+MS) binary mixtures were selected from the literature to test the viscosity mixing rules proposed by 1) Hind (Hi), 2) Grunberg and Nissan (G-N), 3) Herric (He) and 4) Katti and Chaudhri (K-C). The analyses were performed by estimating the average (absolute or relative) deviations, AADs and ARDs, between the available experimental data and the predicted ideal mixture viscosity values obtained by means of each rule. The interaction terms corresponding to the adjustable parameters inherent to each rule were also calculated and their trends discussed.

Nano-segregation in ionic liquids: scorpions and vanishing chains.

The present study analyses the large structural differences, first observed using X-ray diffraction, between 1-alkyl-3-methylimidazolium-based ionic liquids, [Cnmim][Ntf2] (n = 3, 6, 9), and their counterparts with ether-substituted alkyl side chains, [(C1OC1)(n/3)mim][Ntf2] (n = 3, 6, 9). The MD simulations-obtained using a non-polarizable atomistic force-field to model the ionic liquids under discussion-demonstrate that the suppression of the nanostructured nature in the ionic liquids with ether chains is persistent along the entire series and it is not due to any modification of the polar network of the ionic liquid but rather due to the different morphologies of the non-polar regions that surround it. The modification of the non-polar regions-shift from bulky segregated domains in [Cnmim][Ntf2] to thin enveloping ones in [(C1OC1)(n/3)mim][Ntf2]-are caused by the inability of the oxygen-substituted alkyl side chains to pack effectively side by side, the existence of kinks along the chain that lead eventually to intra-molecular, scorpion-like interactions between the chains and the imidazolium ring, and by their stronger interactions with the cations of the polar network via the lone electron pairs of the ether oxygen atoms.

High ionicity ionic liquids (HIILs): comparing the effect of ethylsulfonate and ethylsulfate anions.

The subject of ionicity has been extensively discussed in the last decade, due to the importance of understanding the thermodynamic and thermophysical behaviour of ionic liquids. In our previous work, we established that ionic liquids' ionicity could be improved by the dissolution of simple inorganic salts in their milieu. In this work, a comparison between the thermophysical properties of two binary systems of ionic liquid + inorganic salt is presented. The effect of the ammonium thiocyanate salt on the ionicity of two similar ionic liquids, 1-ethyl-3-methylimidazolium ethylsulfonate and ethylsulfate, is investigated in terms of the related thermophysical properties, such as density, viscosity and ionic conductivity in the temperature range 298.15-323.15 K. In addition, spectroscopic (NMR and Raman) and molecular dynamic studies were conducted in order to better understand the interactions that occur at a molecular level. The obtained results reveal that although the two anions of the ionic liquid exhibit similar chemical structures, the presence of one additional oxygen in the ethylsulfate anion has a major impact on the thermophysical properties of the studied systems.