Friedrich Malberg

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.

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.

Interactions and structure of ionic liquids on graphene and carbon nanotubes surfaces

Using quantum methods it was possible to build an atomistic force field for ionic liquids interacting with a graphene surface. Density functional calculations of 1-ethyl-3-methylimidazolium cation and thiocyanate anion interacting with a cluster of carbon atoms representing a graphene surface were performed, at a series of distances and orientations, using the BLYP-D functional. The DFT interaction energies were successfully fitted to a site–site potential function. The developed force field accounts also for the polarization of the graphene surface, therefore the use of induced dipoles to reproduce the interaction energy between charges and a conductor surface is not required. We report molecular dynamics results on the structure of the interfacial layer of the ionic liquid 1-ethyl-3-methylimidazolium thiocyanate at a flat graphene surface and inside single-wall carbon nanotubes of different diameters, including analyses of the positional and orientational ordering of the ions near the surface, and charge density profiles. Both anions and cations are found in the first ordered layer of ions near the surface, with the interfacial layer being essentially one ion thick.

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.

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.

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.

En route formation of ion pairs at the ionic liquid–vacuum interface

Abstract An increasing lack of single ion cation–anion associations (ion pairing) in ionic liquids suggests a structural motif that stands in contradiction to the single ion pair structure of their vapor phase, which was evidenced by different experimental and theoretical studies. Therefore, a structural rearrangement has to occur en route from the liquid to the vapor. In this study, we propose a detailed four-step evaporation mechanism for ionic liquids, providing a refined perspective on the theory of this process based on the connection between ion pairing and volatility. The process involves diffusion of ions from the bulk to the surface, where they float around until a well-defined ion pair is formed with a counterion, leading to the departure from the surface into the vacuum. To assess the validity of this scheme, we performed a series of classical and ab initio molecular dynamics simulations based on the most sophisticated methods and force fields available for ionic liquids.

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

Summary 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).

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.