A Differentiable Neural-Network Force Field for Ionic Liquids.

We present NeuralIL, a model for the potential energy of an ionic liquid that accurately reproduces first-principles results with orders-of-magnitude savings in computational cost. Built on the basis of a multilayer perceptron and spherical Bessel descriptors of the atomic environments, NeuralIL is implemented in such a way as to be fully automatically differentiable. It can thus be trained on ab initio forces instead of just energies, to make the most out of the available data, and can efficiently predict arbitrary derivatives of the potential energy. Using ethylammonium nitrate as the test system, we obtain out-of-sample accuracies better than 2 meV atom-1 (<0.05 kcal mol-1) in the energies and 70 meV Å-1 in the forces. We show that encoding the element-specific density in the spherical Bessel descriptors is key to achieving this. Harnessing the information provided by the forces drastically reduces the amount of atomic configurations required to train a neural network force field based on atom-centered descriptors. We choose the Swish-1 activation function and discuss the role of this choice in keeping the neural network differentiable. Furthermore, the possibility of training on small data sets allows for an ensemble-learning approach to the detection of extrapolation. Finally, we find that a separate treatment of long-range interactions is not required to achieve a high-quality representation of the potential energy surface of these dense ionic systems.

Al3+ solvation in solutions of aluminum nitrate in a protic ionic liquid: Nuclear magnetic resonance verification of the solvation shell composition.

The closest environment of Al3+ cations was analyzed in detail in solutions of aluminum nitrate in the prototypical protic ionic liquid ethyl ammonium nitrate (EAN) using 1 H and 14 N nuclear magnetic resonance (NMR) spectra. For Al (NO3 )3 -EAN mixtures with different water content, a quantitative analysis of the integral intensities of the 1 H and 14 N signals was carried out and the composition of the first solvation shell of the aluminum cation was refined.

Influence of Metal Salts Addition on Physical and Electrochemical Properties of Ethyl and Propylammonium Nitrate.

In this work, we deepen in the characterization of two protic ionic liquids (PILs), ethylammonium nitrate (EAN) and propylammonium nitrate (PAN). With this aim, we determined the influence of inorganic nitrate salts addition on their physical properties and their electrochemical potential window (EPW). Thus, experimental measurements of electrical conductivity, density, viscosity, refractive index and surface tension of mixtures of {EAN or PAN + LiNO3, Ca(NO3)2, Mg(NO3)2 or Al(NO3)3} at a temperature range between 5 and 95 °C are presented first, except for the last two properties which were measured at 25 °C. In the second part, the corresponding EPWs were determined at 25 °C by linear sweep voltammetry using three different electrochemical cells. Effect of the salt addition was associated mainly with the metal cation characteristics, so, generally, LiNO3 showed the lower influence, followed by Ca(NO3)2, Mg(NO3)2 or Al(NO3)3. The results obtained for the EAN + LiNO3 mixtures, along with those from a previous work, allowed us to develop novel predictive equations for most of the presented physical properties as functions of the lithium salt concentration, the temperature and the water content. Electrochemical results showed that a general order of EPW can be established for both PILs, although exceptions related to measurement conditions and the properties of the mixtures were found.

A New Solid-State Proton Conductor: The Salt Hydrate Based on Imidazolium and 12-Tungstophosphate.

We report the structure and charge transport properties of a novel solid-state proton conductor obtained by acid-base chemistry via proton transfer from 12-tungstophosphoric acid to imidazole. The resulting material (henceforth named Imid3WP) is a solid salt hydrate that, at room temperature, includes four water molecules per structural unit. To our knowledge, this is the first attempt to tune the properties of a heteropolyacid-based solid-state proton conductor by means of a mixture of water and imidazole, interpolating between water-based and ionic liquid-based proton conductors of high thermal and electrochemical stability. The proton conductivity of Imid3WP·4H2O measured at truly anhydrous conditions reads 0.8 × 10-6 S cm-1 at 322 K, which is higher than the conductivity reported for any other related salt hydrate, despite the lower hydration. In the pseudoanhydrous state, that is, for Imid3WP·2H2O, the proton conductivity is still remarkable and, judging from the low activation energy (Ea = 0.26 eV), attributed to structural diffusion of protons. From complementary X-ray diffraction data, vibrational spectroscopy, and solid-state NMR experiments, the local structure of this salt hydrate was resolved, with imidazolium cations preferably orienting flat on the surface of the tungstophosphate anions, thus achieving a densely packed solid material, and water molecules of hydration that establish extremely strong hydrogen bonds. Computational results confirm these structural details and also evidence that the path of lowest energy for the proton transfer involves primarily imidazole and water molecules, while the proximate Keggin anion contributes with reducing the energy barrier for this particular pathway.

Influence of Small Quantities of Water on the Physical Properties of Alkylammonium Nitrate Ionic Liquids.

This paper presents a comprehensive study of two alkylammonium nitrate ionic liquids. As part of this family of materials, mainly ethylammonium nitrate (EAN) and also propylammonium nitrate (PAN) have attracted a great deal of attention during the last decades due to their potential applications in many fields. Although there have been numerous publications focused on the measurement of their physical properties, a great dispersion can be observed in the results obtained for the same magnitude. One of the critical points to be taken into account in their physical characterization is their water content. Thus, the main objective of this work was to determine the degree of influence of the presence of small quantities of water in EAN and PAN on the measurement of density, viscosity, electrical conductivity, refractive index and surface tension. For this purpose, the first three properties were determined in samples of EAN and PAN with water contents below 30,000 ppm in a wide range of temperatures, between 5 and 95 °C, while the last two were obtained at 25 °C. As a result of this study, it has been concluded that the presence of water is critical in those physical properties that involve mass or charge transport processes, resulting in the finding that the absolute value of the average percentage change in both viscosity and electrical conductivity is above 40%. Meanwhile, refractive index (≤0.3%), density (≤0.5%) and surface tension (≤2%) present much less significant changes.

Langevin behavior of the dielectric decrement in ionic liquid water mixtures.

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.

3D structure of the electric double layer of ionic liquid-alcohol mixtures at the electrochemical interface.

Mixtures of the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate with amphiphilic cosolvents, such as methanol and ethanol, nanoconfined between graphene walls are studied by means of molecular dynamics simulations and the results are compared with those of the pure ionic liquid and its mixtures with water confined in the same conditions. We investigate the adsorption of cosolvent molecules at the graphene walls as well as their distribution across the system. The results show that, due to a higher affinity of the polar groups to be close to the anions in combination with the electrostatic and excluded volume interactions, there exists a high tendency of the OH groups to lie close to the anode, inducing small changes in the first cation layer. The orientation of cosolvent molecules is found to be closely related to the alignment of the molecular dipole moment. We also investigate the lateral ionic distribution in the layers close to the electrodes, which shows a structural transition from liquid-like lamellar ordering to solid-like hexagonal patterns as the size of the cosolvent molecules increases leading to smaller position fluctuations of the ions. The dependence of the specific patterns on the nature of the electrodes is also studied. This study strongly suggests that the ionic patterns formed in the first ionic layers next to the charged interfaces are universal since their existence does not crucially depend on the atomic composition of the interfacial material, but only on the net charge density of the considered ionic layer, which significantly changes the ionic mobility in this region.

Molecular dynamics simulations of the structure of mixtures of protic ionic liquids and monovalent and divalent salts at the electrochemical interface.

We perform molecular dynamics simulations of mixtures of a prototypical protic ionic liquid, ethylammonium nitrate, with lithium or magnesium nitrate (LiNO3/Mg(NO3)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.

Solvation of Al3+ cations in bulk and confined protic ionic liquids: a computational study.

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 Al3+ 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(NO3)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 Al3+-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(NO3)6]3- octahedral complexes do not significantly change the microstructure of the mixtures relative to those of Mg2+-based ones.

Mesostructure and physical properties of aqueous mixtures of the ionic liquid 1-ethyl-3-methyl imidazolium octyl sulfate doped with divalent sulfate salts in the liquid and the mesomorphic states.

This paper extends the study of the induced temperature change in the mesostructure and in the physical properties occurring in aqueous mixtures of the ionic liquid 1-ethyl-3-methyl imidazolium octyl-sulfate [EMIm][OSO4]. For some compositions, these mixtures undergo a phase transition between the liquid (isotropic in the mesoscale) and the mesomorphic state (lyotropic liquid crystalline) at about room temperature. The behavior of mixtures doped with a divalent metal sulfate was investigated in order to observe their applicability as electrolytes. Calcium sulfate salt is almost insoluble even in the 20 wt% water mixture. The magnesium salt, in contrast, can be dissolved up to concentrations of 730 ppm in the same mixture and it has a profound impact on its properties. Six aqueous mixtures (with water content from 10 wt% to 33 wt%) of [EMIm][OSO4] were saturated with magnesium sulfate salt, producing the ternary mixture [EMIm][OSO4] + H2O + MgSO4. Viscosity, density and ionic conductivity for these samples were measured from 10 °C to 90 °C. In addition, SAXS, FTIR, diffussion NMR and Raman spectroscopy of the most interesting samples have been performed, and structural data indicate a transition between a hexagonal lyotropic liquid crystalline phase below and an isotropic solution phase above room temperature. The octyl sulfate anions of the cylindrical micelles in the hexagonal phase are coordinated with water molecules through H-bonds (about four per sulfate anion), while the [EMIm] cations seem to be poorly coordinated and so free to move. Inorganic salt addition reinforces that network, increasing the phase transition temperature.

The effect of alkyl chain length on the structure and thermodynamics of protic-aprotic ionic liquid mixtures: a molecular dynamics study.

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.

Two-dimensional pattern formation in ionic liquids confined between graphene walls.

We perform molecular dynamics simulations of ionic liquids confined between graphene walls under a large variety of conditions (pure ionic liquids, mixtures with water and alcohols, mixtures with lithium salts and defective graphene walls). Our results show that the formation of striped and hexagonal patterns in the Stern layer can be considered as a general feature of ionic liquids at electrochemical interfaces, the transition between patterns being controlled by the net balance of charge in the innermost layer of adsorbed molecules. This explains previously reported experimental and computational results and, for the first time, why these pattern changes are triggered by any perturbation of the charge density at the innermost layer of the electric double layer (voltage and composition changes, and vacancies at the electrode walls, among others), which may help tuning electrode-ionic liquid interfaces. Using Monte Carlo simulations we show that such structures can be reproduced by a simple two-dimensional lattice model with only nearest-neighbour interactions, governed by highly screened ionic interactions and short-range and excluded volume interactions. We also show that the results of our simulations are consistent with those inferred from the Landau-Brazovskii theory of pattern formation in self-assembling systems. The presence of these patterns at the ionic liquid graphene-electrode interfaces may have a strong impact on the process of ionic transfer from the bulk mixtures to the electrodes, on the differential capacitance of the electrode-electrolyte double layer or on the rates of redox reactions at the electrodes, among other physicochemical properties, and is therefore an effect of great technological interest.

Nanostructured solvation in mixtures of protic ionic liquids and long-chained alcohols.

The structural and dynamical properties of bulk mixtures of long-chained primary and secondary alcohols (propanol, butanol, and 2-pentanol) with protic ionic liquids (ethylammonium and butylammonium nitrate) were studied by means of molecular dynamics simulations and small angle X-ray scattering (SAXS). Changes in the structure with the alcohol concentration and with the alkyl chain length of the alcohol moieties were found, showing variations in the radial distribution function and in the number of hydrogen bonds in the bulk liquids. Moreover, the structural behaviour of the studied mixtures is further clarified with the spatial distribution functions. The global picture in the local scale is in good agreement with the nanostructured solvation paradigm [T. Méndez-Morales et al. Phys. Chem. B 118, 761 (2014)], according to which alcohols are accommodated into the hydrogen bonds' network of the ionic liquid instead of forming clusters in the bulk. Indeed, our study reveals that the alcohol molecules are placed with their polar heads at the interfaces between polar and nonpolar nanodomains in the ionic liquid, with their alkyl chains inside the nonpolar organic nanodomains. The influence of alcohol chain length in the single-particle dynamics of the mixtures is also reported calculating the velocity autocorrelation function and vibrational densities of states of the different species in the ionic liquid-alcohol mixtures, and a weak caging effect for the ethylammonium cations independent of the chain size of the alcohols was found. However, the SAXS data collected for the studied mixtures show an excess of the scattering intensities which indicates that there are also some structural heterogeneities at the nanoscale.

Structural origin of proton mobility in a protic ionic liquid/imidazole mixture: insights from computational and experimental results.

The structure, dynamics, and phase behavior of a binary mixture based on the protic ionic liquid 1-ethylimidazolium bis(trifluoromethanesulfonyl)imide (C2HImTFSI) and imidazole are investigated by (1)H NMR spectroscopy, vibrational spectroscopy, diffusion NMR, calorimetric measurements, and molecular dynamics simulations. Particular attention is given to the nature of the H-bonds established and the consequent occurrence of the Grotthuss mechanism of proton transfer. We find that due to their structural similarity, the imidazolium cation and the imidazole molecule behave as interchangeable and competing sites of interaction for the TFSI anion. All investigated properties, that is the phase behavior, strength of ion-ion and ion-imidazole interactions, number of specific H-bonds, density, and self-diffusivity, are composition dependent and show trend changes at mole fractions of imidazole (χ) approximately equal to 0.2 and 0.5. Beyond χ = 0.8 imidazole is not miscible in C2HImTFSI at room temperature. We find that at the equimolar composition (χ ≈ 0.5) a structural transition occurs from an ionic network mainly stabilized by coulombic forces to a mixed phase held together by site specific H-bonds. The same composition also marks a steeper decrease in density and increase in diffusivity, resulting from the preference of imidazole molecules to H-bond to each other in a chain-like manner. As a result of these structural features the Grotthuss mechanism of proton transfer is less favored at the equimolar composition where H-bonds are too stable. By contrast, the Grotthuss mechanism is more pronounced in the low concentration range where imidazole acts as a base pulling the proton of the imidazolium cation. At high imidazole concentrations the contribution from the vehicular mechanism dominates.

Molecular origin of high free energy barriers for alkali metal ion transfer through ionic liquid-graphene electrode interfaces.

In this work we study mechanisms of solvent-mediated ion interactions with charged surfaces in ionic liquids by molecular dynamics simulations, in an attempt to reveal the main trends that determine ion-electrode interactions in ionic liquids. We compare the interfacial behaviour of Li(+) and K(+) at a charged graphene sheet in a room temperature ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate, and its mixtures with lithium and potassium tetrafluoroborate salts. Our results show that there are dense interfacial solvation structures in these electrolytes that lead to the formation of high free energy barriers for these alkali metal cations between the bulk and direct contact with the negatively charged surface. We show that the stronger solvation of Li(+) in the ionic liquid leads to the formation of significantly higher interfacial free energy barriers for Li(+) than for K(+). The high free energy barriers observed in our simulations can explain the generally high interfacial resistance in electrochemical storage devices that use ionic liquid-based electrolytes. Overcoming these barriers is the rate-limiting step in the interfacial transport of alkali metal ions and, hence, appears to be a major drawback for a generalised application of ionic liquids in electrochemistry. Some plausible strategies for future theoretical and experimental work for tuning them are suggested.

Molecular dynamics simulation of the structure and interfacial free energy barriers of mixtures of ionic liquids and divalent salts near a graphene wall.

A molecular dynamics study of mixtures of 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF4]) with magnesium tetrafluoroborate (Mg[BF4]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 (Li[BF4]). 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.

Molecular dynamics simulations of mixtures of protic and aprotic ionic liquids.

Molecular dynamics simulations of mixtures of the protic ionic liquid ethylammonium nitrate (EAN) and the aprotic 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]) 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 (xEAN = 0.2) and the virtual disappearance of the structure of the aprotic ionic liquid at xEAN = 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 analysis of the effect of electronic polarization on the structure and single-particle dynamics of mixtures of ionic liquids and lithium salts.

We report a molecular dynamics study on the effect of electronic polarization on the structure and single-particle dynamics of mixtures of the aprotic ionic liquid 1-ethyl-3-methylimidazolium bis-(trifluoromethylsulfonyl)-imide ([EMIM][TFSI]) doped with a lithium salt with the same anion at 298 K and 1 bar. In particular, we analyze the effect of electron density fluctuations on radial distribution functions, velocity autocorrelation functions, cage correlation functions, mean-squared displacements, and vibrational densities of states, comparing the predictions of the quantum-chemistry-based Atomistic Polarizable Potential for Liquids, Electrolytes, & Polymers (APPLE&P) with those of its nonpolarizable version and those of the standard non-polarizable Optimized Potentials for Liquid Simulations-All Atom (OPLS-AA). We found that the structure of the mixture is scarcely modified by the fluctuations in electron charge of their constituents, but their transport properties are indeed quite drastically changed, with larger mobilities being predicted for the different species in the bulk mixtures with the polarizable force field. Specifically, the mean-squared displacements are larger for the polarizable potentials at identical time intervals and the intermediate subdiffusive plateaus are greatly reduced, so the transition to the diffusive regime takes place much earlier than in the non-polarizable media. Moreover, the correlations of the added cations inside their cages are weakened out earlier and their vibrational densities of states are slightly red-shifted, reflecting the weakening effect of the electronic polarization on the Coulomb coupling in these dense ionic media. The comparison of OPLS-AA with non-polarizable APPLE&P indicates that adding polarization to OPLS-AA is not sufficient to achieve results close to experiments.

Nanostructure of mixtures of protic ionic liquids and lithium salts: effect of alkyl chain length.

The bulk structure of mixtures of two protic ionic liquids, propylammonium nitrate and butylammonium nitrate, with a salt with a common anion, is analyzed at room temperature by means of small angle X-ray scattering and classical molecular dynamics simulations. The study of several structural properties, such as density, radial distribution functions, spatial distribution functions, hydrogen bonds, coordination numbers and velocity autocorrelation functions, demonstrates that increasing the alkyl chain length of the alkylammonium cation results in more segregated, better defined polar and apolar domains, the latter having a larger size. This increase, ascribed to the erosion of the H-bond network in the ionic liquid polar regions as salt is added, is confirmed by means of small angle X-ray scattering measurements, which show a clear linear increase of the characteristic spatial sizes of the studied protic ionic liquids with salt concentration, similar to that previously reported for ethylammonium nitrate (J. Phys. Chem. B, 2014, 118, 761-770). In addition, larger ionic liquid cations lead to a lower degree of hydrogen bonding and to more sparsely packed three-dimensional structures, which are more easily perturbed by the addition of lithium salts.

Molecular dynamics simulations of the structure and single-particle dynamics of mixtures of divalent salts and ionic liquids.

We report a molecular dynamics study of the structure and single-particle dynamics of mixtures of a protic (ethylammonium nitrate) and an aprotic (1-butyl-3-methylimidazolium hexaflurophosphate [BMIM][PF6]) room-temperature ionic liquids doped with magnesium and calcium salts with a common anion at 298.15 K and 1 atm. The solvation of these divalent cations in dense ionic environments is analyzed by means of apparent molar volumes of the mixtures, radial distribution functions, and coordination numbers. For the protic mixtures, the effect of salt concentration on the network of hydrogen bonds is also considered. Moreover, single-particle dynamics of the salt cations is studied by means of their velocity autocorrelation functions and vibrational densities of states, explicitly analyzing the influence of salt concentration, and cation charge and mass on these magnitudes. The effect of the valency of the salt cation on these properties is considered comparing the results with those for the corresponding mixtures with lithium salts. We found that the main structural and dynamic features of the local solvation of divalent cations in ionic liquids are similar to those of monovalent salts, with cations being localized in the polar nanoregions of the bulk mixture coordinated in monodentate and bidentate coordination modes by the [NO3](-) and [PF6](-) anions. However, stronger electrostatic correlations of these polar nanoregions than in mixtures with salts with monovalent cations are found. The vibrational modes of the ionic liquid (IL) are seen to be scarcely affected by the addition of the salt, and the effect of mass and charge on the vibrational densities of states of the dissolved cations is reported. Cation mass is seen to exert a deeper influence than charge on the low-frequency vibrational spectra, giving a red shift of the vibrational modes and a virtual suppression of the higher energy vibrational modes for the heavier Ca(2+) cations. No qualitative difference with monovalent cations was found in what solvation is concerned, which suggests that no enhanced reduction of the mobility of these cations and their complexes in ILs respective to those of monovalent cations is to be expected.