On the example of 40 ion pairs (5 cations times 8 anions), this study demonstrates how the core‐level binding energy values can be calculated and used to plot theoretical spectra at low computational ...cost using density functional theory methods. Three approaches for obtaining the binding energy values are based on delta Kohn–Sham (ΔKS) calculations, 1s KS orbital energies, and atomic charges. The ΔKS results show reasonable agreement with the available experimental X‐ray photoelectron data. The 1s KS orbital energies correlate well with the ΔKS results. Atomic charge correlation with ΔKS is improved by accounting for the charges of neighboring atoms. Assignment of binding energies to atoms and the applicability of the mentioned methods to model systems of ionic liquids are discussed.
This study shows how the core‐level binding energy values can be calculated and used to plot theoretical spectra using three density functional theory‐based approaches: delta Kohn–Sham (ΔKS) calculations, 1s KS orbital energies, and atomic charges. Good agreement with the experimental X‐ray photoelectron data is established for the ΔKS results.
Full text
Available for:
BFBNIB, FZAB, GIS, IJS, IZUM, KILJ, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBMB, UL, UM, UPUK
In this Letter, we report the first observation of the capacitance–potential hysteresis at the ionic liquid | electrode interface in atomistic molecular dynamics simulations. While modeling the ...differential capacitance dependence on the potential scan direction, we detected two long-living types of interfacial structure for the BMImPF6 ionic liquid at specific charge densities of the gold Au(111) surface. These structures differ in how counterions overscreen the surface charge. The high barrier for the transition from one structure to another slows down the interfacial restructuring process and leads to the marked capacitance–potential hysteresis.
Full text
Available for:
IJS, KILJ, NUK, PNG, UL, UM
The complex nature of electrode charge screening is well-known for ionic liquids (ILs). Due to strong ion–ion correlations, these electrolytes form a distinctive layered structure at interfaces. ...Variations in electrode potential cause structural changes that are reflected in a peculiar shape of differential capacitance–potential dependence with characteristic peaks. Although the differential capacitance for various ILs in conjunction with metal electrodes accessed via molecular dynamics (MD) simulations has been reported in the literature, retrieving a capacitance-potential curve, C(U), from the MD trajectories is not a trivial task. In this work, we present the results of the MD simulations of the IL 1-butyl-3-methylimidazolium hexafluorophosphate at a single-crystalline Au (100) surface. The discussion focuses on the simulation data treatment for C(U) curve fitting. It is shown that the resulting C strongly depends on the fitting method used. Four capacitance peaks and three structural reorganization types were identified in the studied system. With the help of a semi-quantitative approach in the framework of the original bilayer model of electric double layer (EDL), it is argued that the ions’ reorientation is in the origin of the capacitance peaks. Also, it is shown that under the conditions of this study, the multilayer structure, characteristic of EDL in ILs on the whole, is far from the “lattice saturation” regime. The multilayer structure possesses a steric packing effect that impedes structural changes, decreasing the capacitance.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
We present large scale polarizable simulations of mixtures of the ionic liquids 1-ethyl-3-methylimidazolium trifluoromethanesulfonate and 1-ethyl-3-methylimidazolium dicyanamide with water, where the ...dielectric spectra, the ion hydration and the conductivity were evaluated. The dielectric decrement, the depression of the dielectric constant of water upon addition of ions, is found to follow a universal functional of Langevin type. Only three physical properties need to be known to describe the complete range of possible concentrations, namely the dielectric constant of pure water, of pure ionic liquid and the linear slope of the dielectric decrement at low ionic liquid concentrations. Both the generalized dielectric constant, as well as the water contribution to the dielectric permittivity follow the functional dependence. We furthermore find that a scaling of van der Waals parameters upon addition of polarizable forces to the force field is necessary to correctly describe the frequency dependent dielectric conductivity and its contribution to the dielectric spectrum, as well as the static electric conductivity, which is also treated in the framework of a pseudolattice theory.
Mixtures of alkylammonium-based protic ionic liquids and alkylmethylimidazolium-based aprotic ionic liquids near neutral and charged graphene interfaces were studied by means of molecular dynamics ...simulations. Effects previously observed for bulk mixtures were found to have a pronounced impact on the liquid's interfacial structure. Hydrogen bonding effects were found to affect the composition of the electric double layer (EDL). The lateral structure of the innermost layer of the EDL was found to be rich, showing transitions from solid-like to more fluid configurations, in which low mobility or high mobility anions give rise to hexagonal or striped patterns, respectively. Finally, two-dimensional nanoscale segregation, into polar/apolar domains but also into protic/aprotic domains, was seen at the graphene-liquid interface, resulting in a highly heterogeneous interfacial structure. The integral EDL capacitance vs. electrode potential was seen to nonlinearly increase with protic ionic liquid concentration due to the rearrangements of the three-dimensional ionic density distribution of species with different charge geometries close to the electrode.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Molecular dynamics simulations of mixtures of the protic ionic liquid ethylammonium nitrate (EAN) and the aprotic 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF
4
) are reported and the ...results are compared with experimental density and electrical conductivity measurements. Essentially ideal mixing of the ionic liquids is seen to take place by means of experimental and simulated excess molar volumes, whose very low values suggest a gradual transition between the structures of the two end constituents of the mixture. A weak dominance of the structure of the protic ionic liquid is nevertheless registered, due to a slight preferential formation of the network of hydrogen bonds, as reflected in the coordination number and the number of hydrogen bonds in the mixture. A novel conductivity curve showing pronounced deviations from the simple ideal mixing rule is reported, with three different regions defined by a local maximum - reflecting enhanced translational dynamics relative to ideal mixture behaviour - and a global minimum at intermediate concentrations. The physical origin of this behaviour is discussed along with the structure and single-particle dynamics of the mixture, and it is seen that these regions are defined by the onset of the formation of the EAN hydrogen bonded network (
x
EAN
= 0.2) and the virtual disappearance of the structure of the aprotic ionic liquid at
x
EAN
= 0.7. It is concluded that the delicate interplay between both networks has a deep effect on the placement and mobility of EMIM
+
cations in the mixture all throughout the different stages of the structural transition, which seems to be the driving force behind the reported transport properties of the mixture at intermediate to high EAN concentrations.
Molecular dynamics simulations of mixtures of the protic ionic liquid EAN and the aprotic EMIMBF
4
are reported and the results are compared with experimental density and electrical conductivity measurements.
A molecular dynamics study of mixtures of 1-butyl-3-methylimidazolium tetrafluoroborate (BMImBF
4
) with magnesium tetrafluoroborate (MgBF
4
2
) confined between two parallel graphene walls is ...reported. The structure of the system is analyzed by means of ionic density profiles, lateral structure of the first layer close to the graphene surface and angular orientations of imidazolium cations. Free energy profiles for divalent magnesium cations are calculated using two different methods in order to evaluate the height of the potential barriers near the walls, and the results are compared with those of mixtures of the same ionic liquid and a lithium salt (LiBF
4
). Preferential adsorption of magnesium cations is analyzed using a simple model and compared to that of lithium cations, and vibrational densities of states are calculated for the cations close to the walls analyzing the influence of the graphene surface charge. Our results indicate that magnesium cations next to the graphene wall have a roughly similar environment to that in the bulk. Moreover, they face higher potential barriers and are less adsorbed on the charged graphene walls than lithium cations. In other words, magnesium cations have a more stable solvation shell than lithium ones.
A molecular dynamics study of graphene-confined mixtures of 1-butyl-3-methylimidazolium tetrafluoroborate (BMImBF
4
) with MgBF
4
2
is reported.
We perform molecular dynamics simulations of mixtures of a prototypical protic ionic liquid, ethylammonium nitrate, with lithium or magnesium nitrate (LiNO
3
/Mg(NO
3
)
2
) confined between two ...graphene walls. The structure of the system is analyzed by means of ionic density profiles, angular orientations of ethylammonium cations close to the wall and the lateral structure of the first layer close to the graphene wall. All these results are compared to those of the corresponding aprotic ionic liquid systems, analyzing the influence of the graphene wall charge in the structure of the protic and aprotic mixtures. Moreover, vibrational densities of states are calculated for the salt cations close to the walls. Finally, we investigate the structure of the mixture with Li salt near the interface using
ab initio
density functional theory, and the results are compared with those obtained by classical molecular dynamics simulations.
Hydrogen bonded protic ionic liquids improve the transport of electrochemically relevant cations to charged walls relative to aprotic ones.
Mixtures of alkylammonium based protic ionic liquids and alkylmethylimidazolium based aprotic ionic liquids were studied by means of molecular dynamics simulations. Close to ideal mixing is observed ...in most studied magnitudes; however, the effect of increasing alkyl chain length in each of the cations is markedly different, with longer protic cations showing larger deviations, especially with regards to mixing enthalpy, which exhibits a strong compound forming tendency. The compound forming nature of these protic ionic liquids is shown to induce sharp changes in their local environment upon mixing.
Despite the growing interest in the potential electrochemical applications of both aluminium and ionic liquids in batteries, the microstructure of mixtures of trivalent salts and these dense ionic ...environments is completely unknown. In this work, the solvation of Al
3+
cations in highly dense ionic solvents is investigated. For this purpose, molecular dynamics simulations of mixtures of a protic ionic liquid, ethylammonium nitrate (EAN), with aluminium nitrate (Al(NO
3
)
3
), both in bulk and confined between graphene walls, are performed. Several structural quantities of the system are calculated for different salt concentrations, such as densities, radial distribution functions, structure factors, coordination numbers and hydrogen bonds for the bulk mixture and ionic density profiles for the confined ones. Moreover, vibrational density of states is calculated for the salt cations, both in bulk and when close to the walls. The results obtained are analyzed and compared to those for mixtures of EAN with monovalent and divalent salts, in order to probe the influence of the salt cation charge on the system's properties. Finally,
ab initio
density functional theory calculations were performed in order to analyze the structure of the Al
3+
-ligand complexes, and their predictions for the Raman spectrum are compared both to the corresponding experimental one and the one coming from molecular dynamics simulations. According to our calculations, Al(NO
3
)
6
3−
octahedral complexes do not significantly change the microstructure of the mixtures relative to those of Mg
2+
-based ones.
Nanostructured solvation of Al
3+
in an EAN ionic liquid, forming octahedral complexes with nitrate anions.
PDF