We carried out ab initio molecular dynamics simulations for the three cyano-based ionic liquids, 1-ethyl-3-methylimidazolium tetracyanoborate (C2C1ImB(CN)4), 1-ethyl-3-methyl-imidazolium ...dicyanamide (C2C1ImN(CN)2), and 1-ethyl-3-methylimidazolium thiocyanate (C2C1ImSCN). We found that the SCN-based ionic liquid is much more prone to π–π stacking interactions as opposed to the other two ionic liquids, contrary to the fact that all liquids bear the same cation. Hydrogen bonding is strong in the dicyanamide- and the thiocyanate-based ionic liquids and it is almost absent in the tetracyanoborate liquid. The anion prefers to stay on-top of the imidazolium ring with the highest priority for the N(CN)2− anion followed by the B(CN)4− anion. We find that experimental viscosity trends cannot be correlated to the hydrogen bond dynamics which is fastest for B(CN)4− followed by SCN− and N(CN)2−. For the dynamics of the cation on-top of itself, we find the order of B(CN)4− followed by N(CN)2− and finally by SCN−. Interestingly, this trend correlates well with the viscosity, suggesting a relation between the cation–cation dynamics and the viscosity at least for these cyano-based ionic liquids. These findings, especially the apparent correlation between cation–cation dynamics and the viscosity, might be useful for the suggestion of better ionic liquids in electrolyte applications.
In the present computational work, we develop a new tool for our trajectory analysis program TRAVIS to analyze the well‐known behavior of liquids to separate into microphases. The dissection of the ...liquid into domains of different subsets, for example, in the case of fluorinated ionic liquids with anionic and cationic head groups (forming together the polar domain), fluorous, and alkyl subsets is followed by radical Voronoi tessellation. This leads to useful average quantities of the subset neighbor count, that is, the domain count that gives the amount of particular domains in the liquid, the domain volume and surface, as well as the isoperimetric quotient, which provides a measure of the deviation of the domains from a spherical shape. Thus, the newly implemented method allows analysis of the domains in terms of their numbers and shapes on a qualitative and also quantitative basis. It is a simple, direct, and automated analysis that does not need evaluation of the structure beforehand in terms of, for example, first solvent shell minima. In the microheterogeneous ionic liquids that we used as examples, the polar subsets always form a single domain in all investigated liquids. Although the fluorous side chains are also more or less connected in one or, maximally, two domains, the alkyl phases are dispersed.
Master of your domain: Our direct and simple domain analysis based on radical Voronoi tessellation allows the nanostructure of liquids to be analyzed. In ionic liquids, many nonpolar domains can form depending on the size of the side chain and the nature of the anion, whereas the polar part always only exists in one domain.
We present here the possibility of forming triphilic mixtures from alkyl‐ and fluoroalkylimidazolium ionic liquids, thus, macroscopically homogeneous mixtures for which instead of the often observed ...two domains—polar and nonpolar—three stable microphases are present: polar, lipophilic, and fluorous ones. The fluorinated side chains of the cations indeed self‐associate and form domains that are segregated from those of the polar and alkyl domains. To enable miscibility, despite the generally preferred macroscopic separation between fluorous and alkyl moieties, the importance of strong hydrogen bonding is shown. As the long‐range structure in the alkyl and fluoroalkyl domains is dependent on the composition of the liquid, we propose that the heterogeneous, triphilic structure can be easily tuned by the molar ratio of the components. We believe that further development may allow the design of switchable, smart liquids that change their properties in a predictable way according to their composition or even their environment.
Going through a phase: Three microphases can be achieved within a homogenous liquid by making use of mixtures from fluorinated and parent ionic liquids. Changing the molar ratio allows this triphilic behavior to be switched.
Ten C8C1Im+ (1‐methyl‐3‐octylimidazolium)‐based ionic liquids with anions Cl−, Br−, I−, NO3−, BF4−, TfO−, PF6−, Tf2N−, Pf2N−, and FAP− (TfO=trifluoromethylsulfonate, ...Tf2N=bis(trifluoromethylsulfonyl)imide, Pf2N=bis(pentafluoroethylsulfonyl)imide, FAP=tris(pentafluoroethyl)trifluorophosphate) and two C8C1C1Im+ (1,2‐dimethyl‐3‐octylimidazolium)‐based ionic liquids with anions Br− and Tf2N− were investigated by using X‐ray photoelectron spectroscopy (XPS), NMR spectroscopy and theoretical calculations. While 1H NMR spectroscopy is found to probe very specifically the strongest hydrogen‐bond interaction between the hydrogen attached to the C2 position and the anion, a comparative XPS study provides first direct experimental evidence for cation–anion charge‐transfer phenomena in ionic liquids as a function of the ionic liquid’s anion. These charge‐transfer effects are found to be surprisingly similar for C8C1Im+ and C8C1C1Im+ salts of the same anion, which in combination with theoretical calculations leads to the conclusion that hydrogen bonding and charge transfer occur independently from each other, but are both more pronounced for small and more strongly coordinating anions, and are greatly reduced in the case of large and weakly coordinating anions.
Charges in charged systems: Anion and cation interactions in ionic liquids strongly modify the charges on the ions as proven by XPS, NMR spectroscopy and DFT calculations. The smaller, more basic and, thus, more strongly coordinating the anion, the more pronounced the charge transfer to the cation (see graphic). Hydrogen‐bonding‐type interactions between anion and cation do not significantly influence the amount of charge transferred.
•A Fermi resonance of a combination band and a CN stretching normal mode is found in the dicyanamide anion.•Ab initio molecular dynamics (AIMD) simulations allow the determination of spectra of ...individual ions.•The combination of experimental spectroscopy and advanced AIMD simulations is a very promising approach to gain new physical insights.
The vibrational spectra of two cyano-based ionic liquids, 1-ethyl-3-methylimidazolium dicyanamide (C2C1Im DCA) and 1-ethyl-3-methylimidazolium tricyanomethanide (C2C1Im TCM) are revisited experimentally and computationally using IR spectroscopy and ab initio molecular dynamics simulations, respectively. In the experimental spectrum of C2C1Im DCA, a number of interesting vibrational signatures are found. Aside from the expected symmetric and anti-symmetric stretching bands of the cyano groups, a Fermi resonance of a combination band and a CN stretching normal mode is identified. The assignment is confirmed by a normal mode analysis of the AIMD based IR spectrum. The AIMD does not only predict the existence of the combination band but also seems to capture the frequency shift and intensity change caused by Fermi resonance. The interpretation of the experimental C2C1Im TCM spectrum is more straightforward as no resonance effects are found. The combination of experimental spectroscopy and advanced AIMD simulations is a very promising approach to gain new insights into the behavior of room-temperature ionic liquids.
We present a short overview of our results which have been obtained from theoretical investigations during the period of the SPP-1191 priority program. The liquid state of imidazolium-based ionic ...liquids and protic ionic liquids has been studied intensively in our group not only mainly from ab initio molecular dynamics simulations, but also from classical molecular dynamics simulations. Hydrogen bonds and intermolecular forces have also been examined by quantum chemical methods, and their influence on molecular and macroscopic properties is reviewed here. We have extended our investigations to the gas phase which has been studied from both classical and ab initio molecular dynamics simulations. More insights into water(-impurities) ionic liquid interactions have been gained, and mixtures of ionic liquids have been explored. The mechanism of reactions in ionic liquids and solute ionic liquid relationships as well as cation and anion effects — which is reflected in either an active or a passive role of the ionic liquid — have been considered.
•We have simulated the structural behaviour of ionic liquids from ab initio and from traditional molecular dynamics simulations.•Hydrogen bonds have been analyzed for ionic liquids.•The gas phase of ionic liquids has been studied.•Impurities like water in ionic liquids have been explored.•CO2 absorption in ionic liquids have been studied.
Ionic liquids (ILs) have become an established option for the use as electrolytes in dye‐sensitized solar cells. In the present study, the adsorption of a multitude of different ILs on a TiO2 surface ...is studied systematically, focusing on the energetic modifications of the semiconductor. The cation was found to generally cause an energetic downward shift of the TiO2 band levels by accepting electron density from the surface, and the anions were observed to function in the opposite direction, raising the energy levels by donating electron density. Both effects counterbalance each other, leaving the desired outcome dependent on the choice of the specific IL, i.e., the choice of the cation/anion combination. The correlation of the band levels with the properties of the IL was successfully achieved. The dipole moment of the adsorbed ionic liquid species showed little to no correlation with the semiconductor energetics, but the charge transfer calculated by radical Voronoi tessellation revealed a high correlation. The current findings contribute to a deeper understanding of the role of the electrolyte in dye‐sensitized solar cells, and ILs in general, and help with choosing and tuning of the electrolyte solutions in existing applications.
The electrolyte effect! The adsorption of cations and anions from ionic liquids is decisive for the band level alignment in dye‐sensitized solar cells. Electron deficiency typically caused by cations results in an energetic lowering of states, that is, valence and conduction bands. Consequently, electron donation from anions results in an energetic increase of states.
Structural properties of the ionic liquids 1-ethyl-3-methylimidazolium thiocyanate and tetracyanoborate at the anatase (101) surface are of crucial interest for energy harvesting and storage devices, ...but their investigation via molecular dynamics requires large simulation cells. Thus, two classical interaction potentials with and without polarization effects were parametrized in order to accurately model the interface between the liquid and solid. The parameters were fitted to match ab initio reference interaction energies. Application of the generated force field to model the interface is demonstrated. The adsorption profiles of the ionic liquid’s components reveal a very dense surface layer with an excess of cations, which has some possible implications for the use of the ionic liquids as electrolytes, for example in solar cells.
The adsorption behavior of the ionic liquids 1,3-dimethylimidazolium thiocyanate and 1,3-dimethylimidazolium tetracyanoborate on the anatase (101) surface is studied through theoretical methods. This ...includes a comparison of the PBE and PBE0 functionals as well as the D3 and D3M+ dispersion correction schemes for the calculation of energetic and structural properties. The PBE functional was found to underestimate interaction energies, and the D3 dispersion correction was observed to overbind. While the surface titanium atoms predominantly form contact with anion’s nitrogen atoms, the oxygen atoms of anatase interact in hydrogen-bond-like structures with the acidic hydrogen atoms of the imidazolium cation in addition to ubiquitous electrostatic and dispersive interactions. Ionic liquid ion pairs remain stable after adsorption and closely match the geometry of the isolated ions. A band shift of the density of states of TiO2, possibly resulting in higher values of the open-circuit voltage in complete solar cells, is attributed to the adsorption, which might be interesting for application of these ionic liquids in real dye-sensitized solar cell devices.
The ionic liquid 1-ethyl-3-methylimidazolium acetate C(2)C(1)ImOAc shows a great potential to dissolve strongly hydrogen bonded materials, related with the presence of a strong hydrogen bond network ...in the pure liquid. A first step towards understanding the solvation process is characterising the hydrogen bonding ability of the ionic liquid. The description of hydrogen bonds in ionic liquids is a question under debate, given the complex nature of this media. The purpose of the present article is to rationalise not only the existence of hydrogen bonds in ionic liquids, but also to analyse their influence on the structure of the pure liquid and how the presence of water, an impurity inherent to ionic liquids, affects this type of interaction. We perform an extensive study using ab initio molecular dynamics on the structure of mixtures of the ionic liquid 1-ethyl-3-methylimidazolium acetate with water, at different water contents. Hydrogen bonds are present in the pure liquid, and the presence of water modifies and largely disturbs the hydrogen bond network of the ionic liquid, and also affects the formation of other impurities (carbenes) and the dipole moment of the ions. The use of ab initio molecular dynamics is the recommended tool to explore hydrogen bonding in ionic liquids, as an explicit electronic structure calculation is combined with the study of the condensed phase.
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