Strong Microheterogeneity in Novel Deep Eutectic Solvents.

With the increasing application of template assisted syntheses in deep eutectic solvents and successful application of hydrophobic deep eutectic solvents in extraction processes, where microheterogeneity plays a major role, suggestions for novel deep eutectic solvents which exhibit strong microheterogeneity are desirable. Therefore, classical molecular dynamics simulations were carried out on deep eutectic solvent systems constructed of choline chloride and some of its derivatives mixed with ethylene glycol in a molar composition of 1 : 2 since this is the optimal parent composition. The derivatives consisted of a series of elongated alkyl side chains and elongated alcohol side chains. Of these series only choline chloride ethylene glycol has been investigated experimentally, the other systems are suggested and theoretically investigated as possible target for synthesis. Our domain analysis supported by the clear distinction of polar and nonpolar parts from the electrostatic potentials reveals that strong microheterogeneity within these novel hypothetical deep eutectic solvents exists. Rather stretched than crumbled side chains maximize possible interaction sites for both polar and nonpolar parts which make the suggested compounds valuable objectives for experiments in order to exploit the microheterogeneity in deep eutectic solvents.

Finding the best density functional approximation to describe interaction energies and structures of ionic liquids in molecular dynamics studies.

Ionic liquids raise interesting but complicated questions for theoretical investigations due to the fact that a number of different inter-molecular interactions, e.g., hydrogen bonding, long-range Coulomb interactions, and dispersion interactions, need to be described properly. Here, we present a detailed study on the ionic liquids ethylammonium nitrate and 1-ethyl-3-methylimidazolium acetate, in which we compare different dispersion corrected density functional approximations to accurate local coupled cluster data in static calculations on ionic liquid clusters. The efficient new composite method B97-3c is tested and has been implemented in CP2K for future studies. Furthermore, tight-binding based approaches which may be used in large scale simulations are assessed. Subsequently, ab initio as well as classical molecular dynamics simulations are conducted and structural analyses are presented in order to shed light on the different short- and long-range structural patterns depending on the method and the system size considered in the simulation. Our results indicate the presence of strong hydrogen bonds in ionic liquids as well as the aggregation of alkyl side chains due to dispersion interactions.

On the origin of ionicity in ionic liquids. Ion pairing versus charge transfer.

In this paper we show by using static DFT calculations and classical molecular dynamics simulations that the charge transfer between ionic liquid ions plays a major role in the observed discrepancies between the overall mobility of the ions and the observed conductivities of the corresponding ionic liquids, while it also directly suppresses the association of oppositely charged ions, thus the ion pairing. Accordingly, in electrochemical applications of these materials it is important to consider this reduction of the total charges on the ions, which can greatly affect the performance of the given process or device in which the ionic liquid is used. By slightly shifting from the salt-like to a molecular liquid-like system via the decreased charges, the charge transfer also fluidizes the ionic liquid. We believe that this vital information on the molecular level structure of ionic liquids offers a better understanding of these materials, and allows us to improve the a priori design of ionic liquids for any given purpose.

Substitution effect and effect of axle's flexibility at (pseudo-)rotaxanes.

This study investigates the effect of substitution with different functional groups and of molecular flexibility by changing within the axle from a single C-C bond to a double C=C bond. Therefore, we present static quantum chemical calculations at the dispersion-corrected density functional level (DFT-D3) for several Leigh-type rotaxanes. The calculated crystal structure is in close agreement with the experimental X-ray data. Compared to a stiffer axle, a more flexible one results in a stronger binding by 1-3 kcal/mol. Alterations of the binding energy in the range of 5 kcal/mol could be achieved by substitution with different functional groups. The hydrogen bond geometry between the isophtalic unit and the carbonyl oxygen atoms of the axle exhibited distances in the range of 2.1 to 2.4 Å for six contact points, which shows that not solely but to a large amount the circumstances in the investigated rotaxanes are governed by hydrogen bonding. Moreover, the complex with the more flexible axle is usually more unsymmetrical than the one with the stiff axle. The opposite is observed for the experimentally investigated axle with the four phenyl stoppers. Furthermore, we considered an implicit continuum solvation model and found that the complex binding is weakened by approximately 10 kcal/mol, and hydrogen bonds are slightly shortened (by up to 0.2 Å).

Understanding the evaporation of ionic liquids using the example of 1-ethyl-3-methylimidazolium ethylsulfate.

In this work we present a comprehensive temperature-dependence analysis of both the structural and the dynamic properties of a vaporized ionic liquid (1-ethyl-3-methylimidazolium ethylsulfate). This particular ionic liquid is known to be distillable from experimental studies and thus enables us to deepen the understanding of the evaporation mechanism of ionic liquids. We have used ab initio molecular dynamics of one ion pair at three different temperatures to accurately describe the interactions present in this model ionic liquid. By means of radial and spatial distribution functions a large impact on the coordination pattern at 400 K is shown which could explain the transfer of one ion pair from the bulk to the gas phase. Comparison of the free energy surfaces at 300 K and 600 K supports the idea of bulk phase-like and gas phase-like ion pairs. The different coordination patterns caused by the temperature, describing a loosening of the anion side chains, are also well reflected in the power spectra. The lifetime analysis of typical conformations for ionic liquids shows a characteristic behavior at 400 K (temperature close to the experimental evaporation temperature), indicating that conformational changes occur when the ionic liquid is evaporated.

Substituent effects on axle binding in amide pseudorotaxanes: comparison of NMR titration and ITC data with DFT calculations.

The binding behaviour of differently substituted diamide axle molecules to Hunter/Vögtle tetralactam macrocycles was studied with a combination of NMR titration, isothermal titration calorimetry (ITC) experiments and calculations employing density functional theory (DFT), along with dispersion-corrected exchange-correlation functionals. Guests with alkyl or alkenyl chains attached to the diamide carbonyl groups have a significantly higher binding affinity to the macrocycle than guests with benzoyl amides and their substituted analogues. While the binding of the benzoyl and alkenyl substituted axles is enthalpically driven, the alkyl-substituted guest binds mainly because of a positive binding entropy. The electronic effects of para-substituents at the benzoyl moieties have an influence on the binding affinities. Electron donating substituents increase, while electron-withdrawing substituents decrease the binding energies. The binding affinities obtained from both NMR titration and ITC experiments correlate well with each other. The substituent effects observed in the experimental data are reflected in adiabatic interaction energies calculated with density functional methods. The calculated structures also agree well with pseudorotaxane crystal structures.

The bulk and the gas phase of 1-ethyl-3-methylimidazolium ethylsulfate: dispersion interaction makes the difference.

Ab initio molecular dynamics simulations were carried out on systems representing the gas and the bulk phase of 1-ethyl-3-methylimidazolium ethylsulfate [C(2)C(1)im][C(2)SO(4)]. The power spectra and cation-anion spatial distribution revealed different interactions of the anion and cation in the bulk phase versus the gas phase. In the bulk phase, all oxygen atoms of the anions are involved and interaction via the rear hydrogen atoms is possible, forming a closer packed system. The alkyl groups of cations and anions governed by dispersion interaction stick together in the bulk but are relatively free in the gas phase.

On the ideality of binary mixtures of ionic liquids.

In this work, structural and dynamical properties of the binary mixture of 1-ethyl-3-methyl-imidazolium chloride and 1-ethyl-3-methyl-imidazolium thiocyanate are investigated from ab initio molecular dynamics simulations and compared to the pure ionic liquids. Furthermore, the binary mixture is simulated with two different densities to gain insight into how the selected density affects the different properties. In addition, a simple NMR experiment is carried out to investigate the changes of the chemical shifts of the hydrogen atoms due to the composition of the mixture.

Are There Magic Compositions in Deep Eutectic Solvents? Effects of Composition and Water Content in Choline Chloride/Ethylene Glycol from Ab Initio Molecular Dynamics.

Ab initio molecular dynamics simulations at elevated temperature are carried out to investigate the microscopic structure of liquid mixtures (deep eutectic solvents) composed of 1:1 and 1:2 choline chloride:ethylene glycol ([Ch]Cl:EG) and 1:2:1 choline chloride:ethylene glycol:water ([Ch]Cl:EG:water). In the present study, we aim to understand the composition effect on the choline chloride:ethylene glycol deep eutectic solvent and whether there is a specific composition in these solvents with marked special behavior at the microscopic level. The role of hydrogen bonds between all components was investigated through distribution functions. The structures are governed by the balance of hydrogen bond networks and the different populations of the EG molecule conformations. In the water-containing system, water competes for association with the anion. Also, the charge distribution analysis, which is consistent with structural analysis, indicates that the results are not impacted by changing composition. In addition, the charge transfer observed between ions, EG, and water molecules appears to be significant.

Ion pairing in ionic liquids.

In the present article we briefly review the extensive discussion in literature about the presence or absence of ion pair-like aggregates in ionic liquids. While some experimental studies point towards the presence of neutral subunits in ionic liquids, many other experiments cannot confirm or even contradict their existence. Ion pairs can be detected directly in the gas phase, but no direct method is available to observe such association behavior in the liquid, and the corresponding indirect experimental proofs are based on such assumptions as unity charges at the ions. However, we have shown by calculating ionic liquid clusters of different sizes that assuming unity charges for ILs is erroneous, because a substantial charge transfer is taking place between the ionic liquid ions that reduce their total charge. Considering these effects might establish a bridge between the contradicting experimental results on this matter. Beside these results, according to molecular dynamics simulations the lifetimes of ion-ion contacts and their joint motions are far too short to verify the existence of neutral units in these materials.

Bulk and liquid-vapor interface of pyrrolidinium-based ionic liquids: a molecular simulation study.

Using molecular dynamics simulations, we have studied the structure of three 1-butyl-1-methylpyrrolidinium ionic liquids whose anions are triflate, bis(trifluoromethanesulfonyl)imide, and tris(pentafluoroethyl)trifluorophosphate. The structure of the bulk phase of the three ionic liquids has been interpreted using radial and spatial distribution functions and structure factors that allows us to characterize the morphology of the polar and nonpolar domains present in this family of liquids. The size of the polar regions depends on the anion size, whereas the morphology of the nonpolar domains is anion-independent. Furthermore, the surface ordering properties of the ionic liquids and charge and density profiles were also studied using molecular simulations. The surface tension of the liquid-vapor interfaces of these ionic liquids was also predicted from our molecular simulations. In addition, microscopic structural analysis of orientational ordering at the interface and density profiles along the direction normal to the interface suggest that the alkyl chains of the cation tend to protrude toward the vacuum, and the presence of the interface leads to a strong organization of the liquid phase in the region close to the interface. In the interfacial area, the polar regions of the ionic liquids are more structured than those in the bulk phase, whereas the opposite behavior is observed for the nonpolar regions.

Short Time Dynamics of Ionic Liquids in AIMD-Based Power Spectra.

Power spectra of several imidazolium-based ionic liquids, 1,3-dimethylimidazolium chloride, 1-ethyl-3-methylimidazolium thiocyanate, 1-ethyl-3-methylimidazolium dicyanamide 5, 1-butyl-3-methylimidazolium chloride, 1-butyl-3-methylimidazolium thiocyanate, and 1-butyl-3-methylimidazolium dicyanamide, are presented based on ab initio molecular dynamics simulations. They provide an alternative tool of analysis of several electronic structure-based properties, in particular, those related to the strength of hydrogen bonding in liquids. Moreover, they can be employed to interpret experimental IR or Raman spectra, avoiding the additional calculations required for theoretical IR or Raman spectra. The obtained power spectra are shown to be in good agreement with experimental spectra, and electronic structure properties related to them are analyzed. Further, there are indications for a locality of the power spectra on a relatively short time scale of ≈10 ps or system size of about 8 ion pairs as already speculated in previous work.

Using molecular simulation to understand the structure of [C2C1im]+-alkylsulfate ionic liquids: bulk and liquid-vapor interfaces.

Using molecular dynamics simulations we have studied the structure of alkylsulfate-based ionic liquids: 1-ethyl-3-methylimidazolium n-alkylsulfate [C(2)C(1)im][C(n)SO(4)] (n = 2, 4, 6 and 8). The structure of the different ionic liquids have been interpreted taking into account radial and spatial distribution functions, and structure factors, that allowed us to characterize the morphology of the polar and nonpolar domains present in this family of liquids. The size of the nonpolar regions depends linearly on the anion alkyl chain length. Furthermore, properties of the surface of ionic liquids, such as surface tension, ordering, and charge and density profiles, were studied using molecular simulation. We were able to reproduce the experimental values of the surface tension with a maximum deviation of 10%, and it was possible to relate the values of the surface tension with the structure of the liquid-vacuum interfaces. Microscopic structural analysis of orientational ordering at the interface and density profiles along the direction normal to the interface suggest that the alkyl chains of the anions tend to protrude toward the vacuum, and the presence of the interface leads to a strong organization of the liquid phase in the region close to the interface, stronger when the side-chain length of the anions increases.