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Ionic liquids have generated interest in applications as lubricants and as additives to conventional lubricants due to their unique physical properties. In these applications, the liquid thin film can be subjected simultaneously to extremely high shear and loads in addition to nanoconfinement effects. Here, we use molecular dynamics simulations with a coarse grained model to study a nanometric film of an ionic liquid confined between two planar solid surfaces both at equilibrium and at several shear rates. The strength of the interaction between the solid surface and the ions was changed by simulating three different surfaces with enhanced interactions with different ions. The increase of the interaction with either the cation or the anion leads to the formation of a solid-like layer that moves alongside the substrates, but this layer can exhibit different structures and stability. An increase in interaction with the high symmetry anion produces a more regular structure that is more resistant to the effects of shear and viscous heating. Two definitions were proposed and used for the calculation of the viscosity: a local definition based on microscopic characteristics of the liquid and an engineering definition based on the forces measured at the solid surfaces, with the former displaying a correlation with the layered structure induced by the surfaces. Because of the shear thinning behavior of the ionic liquids as well as the temperature rise brought on by viscous heating, both the engineering and the local viscosities decrease as the shear rate increases., 32 pages, 8 figures

Strong ionic interactions, as well as the consequent correlations between cation and anion dynamics, give ionic liquids various physical features that set them apart from ordinary organic solvents. In particular, they result in larger viscosities and larger densities than mixtures of neutral compounds with similar molecular structures. However, both the direct effect of electrostatic interactions and the increase of liquid density contribute to the high viscosity and so far no experimental or computational work enabled a clear quantification of those effects. Also, the effects over the shear thinning behavior, which may have important consequences for application as lubricants, were not considered yet. Here, these questions were tackled by performing non-equilibrium molecular dynamics (NEMD) simulations changing both the strength of ionic interactions and liquid density at several shear rates using a coarse grained model. The relative dielectric constant was adjusted to reproduce viscosity data from all-atoms simulations on both zero shear and high shear conditions. Elimination of ionic interactions results in a reduction of density and zero shear viscosity and also delays the beginning of shear thinning to higher shear rates. Restoring density to the ionic liquid's value only partially reverses the alterations. Correlations of the non-newtonian behavior and changes in the intermolecular structure and contact lifetimes were also explored.

Two polymerizable ionic liquids (or monomeric ionic liquids, mILs) namely 1-butyl-3-methylimidazolium and choline acrylates ([C4mim]A and ChA, respectively) were synthesized using the modified Fukumoto method from corresponding chlorides. The chemical structure of the prepared mILs was confirmed with FTIR and NMR study. Investigation of the thermal properties with DSC demonstrates that both mILs have a Tg temperature of about 180 K and a melting point around 310 K. It was shown that the temperature dependence of FTIR confirm the Tg to be below 200. Both mILs exhibited non-Newtonian shear thinning rheological behavior at shear rates >4 s−1. It was shown that [C4mim]A is able to dissolve bacterial cellulose (BC) leading to a decrease in its degree of polymerization and recrystallisation upon regeneration with water; although in the ChA, the crystalline structure and nanofibrous morphology of BC was preserved. It was demonstrated that the thixotropic and rheological properties of cellulose dispersion in ChA at room temperature makes this system a prospective ink for 3D printing with subsequent UV-curing. The 3D printed filaments based on ChA, containing 2 wt% of BC, and 1% of N,N′-methylenebisacrylamide after radical polymerization induced with 1% 2-hydroxy-2-methylpropiophenone, demonstrated Young’s modulus 7.1 ± 1.0 MPa with 1.2 ± 0.1 MPa and 40 ± 5% of strength and ultimate elongation, respectively.

Deep eutectic solvents (DESs) are similar to ionic liquids (IL) in terms of physicochemical properties and technical uses. In ILs, far-infrared (FIR) spectroscopy has been utilized to reveal ionic interactions and even to produce a signature of the strengthening of the cation-anion hydrogen bond. However, for the situation of the DES, where the mixing of a salt and a molecular species makes the interplay between multiple intermolecular interactions even more complex, a full investigation of FIR spectra is still absent. In this work, the FIR spectrum of the DES, often referred to as ethaline, which is a 1:2 mixture of choline chloride and ethylene glycol, is calculated using classical molecular dynamics (MD) simulations and compared to experimental data. To explore the induced dipole effect on the computed FIR spectrum, MD simulations were run with both nonpolarizable and polarizable models. The calculation satisfactorily reproduces the position of the peak at ∼110 cm

Binary mixtures of the ionic liquids 1-ethyl-3-methylimidazolium dicyanamide and tributyl(methyl)ammonium dicyanamide, [C2C1im]x[N4441](1-x)[N(CN)2], are studied by means of their excess properties (enthalpy and volume), their viscosity, and their Raman spectra. The mixtures exhibit positive values of excess volume VE and excess enthalpy HE. The plot of the logarithm of viscosity as a function of composition is consistent with the finding HE > 0. The excess thermodynamic properties of the ionic liquid mixtures are compared with well-known results for binary mixtures of (high-temperature) molten salts. The asymmetry of the curve HE versus composition is reproduced by considering the volumetric fraction of the [C2C1im]x[N4441](1-x)[N(CN)2] mixtures according to basic results of the theory of regular solutions. The anion totally symmetric stretching mode νs(C≡N) was used to probe the local environment around the anion in a Raman spectroscopy investigation of the [C2C1im]x[N4441](1-x)[N(CN)2] mixtures as a function of the composition. The Raman band shape (peak frequency and bandwidth) of the νs(C≡N) mode changes as expected from the fluctuation of concentration around the probe oscillator according to models for vibrational spectroscopy of liquid mixtures. This thermodynamic and spectroscopic study leads to the classification of the [C2C1im]x[N4441](1-x)[N(CN)2] mixture as a regular solution. The application of the regular solution equation for HE to other ionic liquid binary mixtures is discussed.

The low melting point of room temperature ionic liquids is usually explained in terms of the presence of bulky, low symmetry and flexible ions, with the first two factors related to the lattice energy while an entropic effect is attributed to the latter. By means of molecular dynamics simulations, the melting points of 1-ethyl-3-methylimidazolium hexafluorophosphate and 1-decyl-3-methyl-imidazolium hexafluorophosphate were determined and the effect of the molecular flexibility over the melting point was explicitly computed by restraining the rotation of dihedral angles in both the solid and the liquid phase. The rotational flexibility over the bond between the ring and the alkyl chain affects the relative ordering of the anions around the cations and results in substantial effects over both the enthalpy and the entropy of melting. For the other dihedral angles of the alkyl group, the contributions are predominantly entropic and an alternating behavior was found. The flexibility of some dihedral angles has negligible effects on the melting point, while others can lead to differences in the melting point as large as 20 K. This alternating behavior is rationalized by the different probabilities of conformation defects in the crystal., Article published in Journal of Chemical Physics in 2020. Contains 34 pages of the main article plus 23 pages of supporting information

Several calorimetric measurements have shown that 1-ethyl-3-methylimidazolium dicyanamide, [C2C1im][N(CN)2], is a glass-forming liquid, even though it is a low-viscous liquid at room temperature. Here we found slow crystallization during cooling of [C2C1im][N(CN)2] along Raman spectroscopy measurements. The low-frequency range of the Raman spectrum shows that the same crystalline phase is obtained at 210 K either by cooling or by reheating the glass (cold-crystallization). Another crystalline phase is formed at ca. 260 K just prior the melting at 270 K. X-ray diffraction and calorimetric measurements confirm that there are two crystalline phases of [C2C1im][N(CN)2]. The Raman spectra indicate that polymorphism is related to [C2C1im]+ with the ethyl chain on the plane of the imidazolium ring (the low-temperature crystal) or non-planar (the high-temperature crystal). The structural reason for the glass-forming ability of [C2C1im][N(CN)2], despite of the relatively simple molecular structures of the ions, was pursued by quantum chemistry calculations and molecular dynamics (MD) simulations. Density functional theory (DFT) calculations were performed for ionic pairs in order to draw free energy surfaces of the anion around the cation. The MD simulations using a polarizable model provided maps of occurrence of anions around cations. Both the quantum and classical calculations suggest that the delocalization of preferred positions of the anion around the cation, which adopts different conformations of the ethyl chain, is on the origin of the crystallization being hampered during cooling and the resulting glass-forming ability of [C2C1im][N(CN)2]., Author revised version of the paper published in J. Phys. Chem B in 2019. File contains 31 pages with 7 figures plus 8 pages of supporting information with 7 figures

The effect of the shear rate on the viscosity and the structure of 1-ethyl-3-methylimidazolium based ionic liquids with three different anions (tetrafluoroborate, dicyanamide, and bis(trifluoromethylsulfonyl)imide) was studied by means of reverse non-equilibrium molecular dynamics (RNEMD) simulations using a polarizable force field. The three liquids display a Newtonian plateau followed by a shear thinning regime at shear rates of the order of GHz. Even though the main features of the liquid structure remains under shear, systematic changes were noticed at the GHz rates, with coordination shells becoming more diffuse as noticed by the reduction in the difference between consecutive maxima and minima in the radial distribution function. Interestingly, these structural changes with the shear rate can be precisely fitted using the Carreau equation, which is a well-known expression for the shear rate dependence of the viscosity. The fitting parameters for different distributions can be used to explain qualitatively the shear thinning behavior of these liquids. In the GHz range, the cations and, in a minor extension, some anions, tend to assume preferentially a parallel orientation with the flux, which contributes to the shear thinning behavior and may have consequences for adhesion in applications as lubricants.

The structure and dynamics of two ionic liquids based on imidazolium cations with the common anion dicyanamide, [N(CN)2]-, were studied and compared: one with an aromatic side chain, 1-benzyl-3-methylimidazolium dicyanamide [BzC1Im][N(CN)2], and another with a normal alkyl chain, 1-n-heptyl-3-methylimidazolium dicyanamide [C7C1Im][N(CN)2]. We use classical molecular dynamics (MD) simulations in order to unveil subtle structural differences between these two systems. The collective dynamics were analyzed by considering the spectra of the mass current correlation functions, which were analyzed within the framework of the Zwanzig-Mori formalism. This allowed to compare the effect of aromatic or non-aromatic substituents on the high-frequency collective dynamics and its relationship to a low-frequency transport property such as viscosity. It is found that even though the aromatic-substituted system is a denser and more viscous liquid, the high-frequency dynamics at finite momentum transfers and frequencies, i.e. within the length and timescales of the intermediate-range order (IRO), is only slightly altered. In the low-wavevector limit, however, there are significant differences in the vibrational density of states and viscosity. The scenario put forward by the MD simulations is that even though the role of ionic packing dominates over specific interactions, the different structural motifs within the IRO range imply signatures on the macroscopic dynamics of aromatic and aliphatic substituted imidazolium ionic liquids.

Ionic liquids are called designer solvents because their physical properties can be tuned by the selection of different combinations cation and anion. Understanding the relation between the chemical structure and the viscosity and how the shear rate can affect the relative arrangements of the ions are important for practical applications specially as lubricants. Reverse non-equilibrium molecular dynamics (RNEMD) simulations were performed to study the effect of the shear rate over the viscosity and the structure at molecular level of four different imidazolium based ionic liquids. Since it is already well known that the absence of explicit electronic polarizability in usual classical force fields leads to artificially slow dynamics in ionic liquids, a Drude polarizable force field was employed in all simulations. Non-newtonian behavior is observed at shear rates at GHz scale, with a progressive reduction of the viscosity at the same time that the structure of second and further coordination shells are partially disrupted. The liquids that displayed the greater structural changes with increasing shear rate also displayed the strongest variation in the viscosities. At the highest shear rates studied, the imidazolium rings tends to align parallel to the induced flux, an effect similar to the well-known align of polymer chains under shear, despite becoming significant for ionic liquids only at extremely high rates.

The Raman band belonging to the totally symmetric stretching mode of the [BF4]− anion, νs(BF4), in 1-alkyl-3-methylimidazolium ionic liquids, [CnC1im][BF4], n = 4, 6, and 8, has been measured under high pressure using a diamond anvil cell. The pressure dependence of νs(BF4) frequency up to 2.0 GPa is converted to a function of density using high pressure extrapolations of equations of states as discussed in previous works. The density induced shift of the νs(BF4) frequency was calculated by a perturbed hard-sphere model by considering the probe oscillator as a pseudo-diatomic molecule. In contrast to previous findings for the stretching mode of [PF6]− in [CnC1im][PF6] ionic liquids, for which the calculation considered only the repulsive short-range forces, in the case of [CnC1im][BF4] the calculation indicates the role played by the attractive forces on the νs(BF4) frequency shift.

Poly(ionic liquid)s are an interesting class of compounds because of their unique chemical and physical properties gathering the characteristics of ionic liquids and polymers. Pressure and temperature have been demonstrated to be alternative parameters to obtain polymers from monomeric species using only physical tools. In this work, we investigate the reaction under high pressure and room temperature of the ionic liquid 1-allyl-3-methylimidazolium iodide by using the diamond anvil cell technique in combination with synchrotron X-ray diffraction and electronic and vibrational spectroscopies. The results indicate a chemical reaction happening through the terminal double bond of the allyl group both in crystalline and glassy phases with the onset of the reaction around ∼7 GPa. Vibrational spectra present evidence for an oligomerization reaction in both the phases. The reaction occurring both in glassy and crystal phases indicates a mechanism not driven by collective motions and likely related to local topological arrangements. The results presented herein extend our understanding of ionic liquid instability boundaries under high pressure and contribute to the development of alternative synthetic routes to achieve poly(ionic liquids).

A Raman spectroscopy study of 1-ethyl-3-methylimidazolium hydrogen sulfate, [C2C1im][HSO4], as a function of temperature, has been performed to reveal the role played by anion–anion hydrogen bond on the phase transitions of this ionic liquid. Anion–anion hydrogen bonding implies high viscosity, good glass-forming ability, and also moderate fragility of [C2C1im][HSO4] in comparison with other ionic liquids. Heating [C2C1im][HSO4] from the glassy phase results in cold crystallization at ∼245 K. A solid–solid transition (crystal I → crystal II) is barely discernible in calorimetric measurements at typical heating rates, but it is clearly revealed by Raman spectroscopy and X-ray diffraction. Raman spectroscopy indicates that crystal I has extended ([HSO4]−)n chains of hydrogen-bonded anions but crystal II has not. Raman spectra recorded at isothermal condition show the ultraslow dynamics of cold crystallization, solid–solid transition, and continuous melting of [C2C1im][HSO4]. A brief comparison is also provi...

The structure of choline salts containing the anions acetate, [Chol][Ac], and dihydrogen phosphate, [Chol][DHP], were investigated by infrared, Raman, and inelastic neutron scattering (INS). The chosen systems allow for the comparison of structural effects related to the bond acceptor characteristic of [Ac] and the simultaneous acceptor and donor characteristics of [DHP] in forming hydrogen bonds (H-bond) in salts of [Chol], which is itself prone to forming H-bonds. Different computational tools were used for the analysis of different spectral ranges. The calculation of the low-frequency range of Raman and INS spectra of the crystalline phases at low-temperatures by solid state DFPT (density functional perturbation theory) unveils the coupling between vibrations of the H-bonds and intramolecular modes. Changes observed in the spectral pattern of lattice and [DHP] modes upon heating crystalline [Chol][DHP] are analogous to the ferroelectric–paraelectric phase transition known in the potassium salt of [DHP]. The fingerprint region of the vibrational spectra provides information concerning the [Chol] conformation in the solid phase (gauche in [Chol][Ac] and anti in [Chol][DHP]) and in aqueous solution. DFT calculations of ionic pairs and ionic clusters unveil the interplay between [Chol] conformation and the [DHP] ability to form H-bonded dimers of anions. The high-frequency spectral range and the structures driven by H-bonds are discussed using classical molecular dynamics (MD) simulations. The MD simulations of aqueous solutions highlight the strong anion-cation H-bond in [Chol][Ac], in contrast to the strong anion–anion H-bond in [Chol][DHP] due to occurrence of dimers and larger clusters of [DHP].

A local order-disorder transition has been disclosed in the thermophysical behavior of the ionic liquid 1-benzyl-3-methylimidazolium dicyanamide, [Bzmim][N(CN)2], and its microscopic nature revealed by spectroscopic techniques. Differential scanning calorimetry and specific heat measurements show a thermal event of small enthalpy variation taking place in the range 250-260 K, which is not due to crystallization or melting. Molecular dynamic simulations and X-ray diffraction measurements have been used to discuss the segregation of domains in the liquid structure of [Bzmim][N(CN)2]. Raman and NMR spectroscopy measurements as a function of temperature indicate that the microscopic origin of the event observed in the calorimetric measurements comes from structural rearrangement involving the benzyl group. The results indicate that the characteristic structural heterogeneity allow for rearrangements within local domains implying the good glass-forming ability for the low viscosity ionic liquid [Bzmim][N(CN)2]. This work sheds light on our understanding of the microscopic origin behind complex thermal behavior of ionic liquids.

The vibrational spectra of two cyano-based ionic liquids, 1-ethyl-3-methylimidazolium dicyanamide (C 2 C 1 Im DCA) and 1-ethyl-3-methylimidazolium tricyanomethanide (C 2 C 1 Im TCM) are revisited experimentally and computationally using IR spectroscopy and ab initio molecular dynamics simulations, respectively. In the experimental spectrum of C 2 C 1 Im 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 C 2 C 1 Im 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.

The terahertz spectrum encodes information about dynamics, structure, and intermolecular interactions of liquids being probed both experimentally and computationally by techniques such as Raman and far-infrared spectroscopies and molecular dynamics (MD) simulation. In the case of room temperature ionic liquids (RTILs), there has been a debate whether a mode observed at about 1.5-2.7 THz (50-90 cm-1) is due to a quasi-lattice structure or the formation of complex ions. Here we show through the analysis of Raman and far-infrared spectra and MD simulation of a typical RTIL that this mode has a collective optic-like character. Then, employing a simple model based on the theory for optical phonons in crystals, we show that a correlation between the frequency of this mode and material parameters holds for different RTILs. These results, which encompass a wide range of samples, reinforce a quasi-lattice view of the liquid phase.

This work discusses the remarkable differences in the mid- and far-infrared regions of the spectrum of the ionic liquid 1-ethyl-3-methylimidazolium hydrogen sulfate, [C2C1im][HSO4], in comparison to ionic liquids based on other anions. The infrared spectrum unveils the occurrence of anion–anion hydrogen bond in [C2C1im][HSO4]. The band related to the anion O H stretching mode, ν(OH), covers a large frequency range (2000–3400 cm−1) and exhibits the spectral pattern of strongly hydrogen bonded systems. The far-infrared (FIR) spectrum of [C2C1im][HSO4] also exhibits the signature of the anion–anion interaction, that is, the stretching mode of the hydrogen bond, ν(O…O), at ~170 cm−1. Hydrogen bond between [HSO4]− anions also occurs in ionic liquids containing imidazolium cation with longer alkyl chain ([C4C1im]+) or tetraalkylammonium cation. The vibrational frequencies of ν(OH) and ν(O…O) modes satisfy empirical correlations that characterize the anion–anion interaction in [C2C1Im][HSO4] as a strong hydrogen bond.

The intermolecular dynamics in the THz frequency range of the ionic liquids n-butyl-trimethylammonium bis(trifluoromethanesulfonyl)imide, [N1114][NTf2], and methyl-tributylammonium bis(trifluoromethanesulfonyl)imide, [N1444][NTf2], were investigated by a combined usage of inelastic neutron scattering (INS), Raman, and far-infrared (FIR) spectroscopies and the power spectrum calculated by molecular dynamics (MD) simulations. The collective dynamics of the simulated systems is also discussed by the calculation of time correlation functions of charge and mass currents that are projected onto acoustic- and optic-like motions. The INS and Raman measurements have been performed as a function of temperature in the glassy, crystalline, and liquid phases. The excess in the vibrational density of states over the expectation of the Debye theory, the so-called boson peak, is found in the INS and Raman spectra as a peak at ∼2 meV (∼16 cm-1) and also in the direct measurement of heat capacity at very low temperatures (4-20 K). This low-frequency vibration is incorporated into the curve fits of Raman, FIR, and MD data at room temperature. Fits of spectra from these different sources in the range below 100 cm-1 are consistently achieved with three components at ca. 25, 50, and 80 cm-1, but with distinct relative intensities among the different techniques. It is proposed as the collective nature of the lowest-frequency component and the anion-cation intermolecular vibration nature of the highest-frequency component. The MD results indicate that there is no clear distinction between acoustic and optic vibrations in the spectral range investigated in this work for the ionic liquids [N1114][NTf2] and [N1444][NTf2]. The analysis carried out here agrees in part, but not entirely, with other propositions in the literature, mainly from optical Kerr effect (OKE) and FIR spectroscopies, concerning the intermolecular dynamics of ionic liquids.

The competition between Coulomb and van der Waals interactions brings forth unique dynamic features and broad applications to ionic liquids. Herein, we present a combined calorimetric, X-ray diffraction, incoherent elastic, and quasi-elastic neutron scattering study, over a wide temperature range (180-340 K), of the relaxational dynamics of the liquid, supercooled liquid, crystalline, glassy, and glacial states of two model ionic liquids: tributylmethylammonium (a good glass-former) and butyltrimethylammonium (a good crystal-former) cations and the bis(trifluoromethanesulfonyl)imide anion. In both systems, we observed two distinct relaxation processes. The Q-dependence of the respective relaxation time shows that the α-process is diffusive, while the β-process is modulated by the structure of the liquids.

Vibrational spectroscopy has continued use as a powerful tool to characterize ionic liquids since the literature on room temperature molten salts experienced the rapid increase in number of publications in the 1990's. In the past years, infrared (IR) and Raman spectroscopies have provided insights on ionic interactions and the resulting liquid structure in ionic liquids. A large body of information is now available concerning vibrational spectra of ionic liquids made of many different combinations of anions and cations, but reviews on this literature are scarce. This review is an attempt at filling this gap. Some basic care needed while recording IR or Raman spectra of ionic liquids is explained. We have reviewed the conceptual basis of theoretical frameworks which have been used to interpret vibrational spectra of ionic liquids, helping the reader to distinguish the scope of application of different methods of calculation. Vibrational frequencies observed in IR and Raman spectra of ionic liquids based on different anions and cations are discussed and eventual disagreements between different sources are critically reviewed. The aim is that the reader can use this information while assigning vibrational spectra of an ionic liquid containing another particular combination of anions and cations. Different applications of IR and Raman spectroscopies are given for both pure ionic liquids and solutions. Further issues addressed in this review are the intermolecular vibrations that are more directly probed by the low-frequency range of IR and Raman spectra and the applications of vibrational spectroscopy in studying phase transitions of ionic liquids.

High energy X-ray diffraction has been combined with containerless techniques to determine the structure of a series of alkali and ammonium nitrate and nitrite liquids. The systems have been modelled using molecular dynamics simulation which allows for the flexibility of, and movement of charge within, the molecular anions. The model reproduces the experimentally-determined scattering functions in both the low- and high-Q regimes reflecting the inter- and intra-molecular length-scales. For ammonium nitrate the best fit to the diffraction data is obtained by assuming the NH4+ cation to have a radius closer to that for Cs+ rather than a smaller cation such as Rb+ as often previously assumed. The alkali nitrites show an emergent length scale, attributed to the nitrogen-nitrogen spatial correlations, that depends on both temperature and the identity of the alkali cation. The corresponding nitrates show a more subtle effect in the nitrogen-nitrogen correlations. As a result, the nature of this N-N length-scale appears different for the respective nitrites and nitrates.

Solubility parameters such as Hildebrand's solubility parameter or its square, the cohesive energy density ced are often not able to predict solubilities of organic compounds in ionic liquids accurately. It has been suggested that this is an effect of the structural heterogeneity that many ionic liquids exhibit due to the non-polar alkyl chains forming separate domains from the charged parts of the cation and the anion. We here calculated the partial volumes of the polar and non-polar domains in Molecular Dynamics simulations of several 1-alkyl-3-methylimidazolium nitrates and 1-octyl-3-methylimidazolium hexafluorophosphate. We used these partial volumes to calculate the ced of the polar and non-polar domains. For 1-octyl-3-methylimidazolium nitrate the internal pressures Pi of the domains were also calculated. Our results show that ced and Pi are indeed very different in the domains, and that the domains containing the hydrophobic alkyl chains have solvent parameters similar to alkanes. For the domains that contain the charged part of the ions, ced and Pi are much higher, and Pi is comparable to what is found for high temperature molten salts. The results provide a possible explanation why solubility parameters seem incorrect for many ionic liquids, and may be useful in estimating more useful values using computer simulations.

Melting of the ionic liquid choline acetate, [Chol][Ac], at ~335 K is manifest in the differential scanning calorimetry (DSC) curve as a broad band covering ~30 K of temperature range. The events taking place in the pre-melting of [Chol][Ac] are revealed by Raman spectroscopy and rheological measurements. The low-frequency range of the Raman spectrum exhibits changes in the intermolecular vibrational modes and the higher frequency range indicates conformational changes of the choline cation. Oscillatory rheological measurements show that the sample experiences a stiffening of its rheological characteristics within the temperature interval encompassing the melting process. Taking together the spectroscopic and rheological data, it is concluded that the sample experiences partial recrystallization during the pre-melting of [Chol][Ac].

X-ray diffraction and Raman scattering measurements of the ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesufonyl)imide, [Pyrr1,4][NTf2], were performed in a wide range of temperature and pressure. Pressure dependence of glass transition Tg(p), cold-crystallization Tcc(p), and melting Tm(p) temperatures were obtained from atmospheric pressure up to ca. 2.0 GPa. The temperature dependence of Raman spectra within the Tcc

The frequency range below ∼100 cm-1 of the Raman spectrum of a glass-forming liquid exhibits two features that characterize the short-time (THz) dynamics: the quasi-elastic scattering (QES) tail and the boson peak (BP). In this work, we follow temperature and pressure effects on the intermolecular dynamics of a typical ionic liquid, 1-butyl-1-methylpiperidinium bis(trifluoromethanesulfonyl)imide, [Pip14][[NTf2]. The glass transition temperature of [Pip14][[NTf2] at atmospheric pressure is Tg = 198 K, and the pressure of glass transition at room temperature is Pg = 1.1 GPa. Raman spectra obtained while cooling the liquid or heating the glass exhibit hysteresis in QES and BP intensities, IQES and IBP. The dependence of IQES, IBP, and the BP frequency, ωBP, with pressure up to the glass transition is steeper than the temperature dependence due to the stronger pressure effect on density within the GPa range. The temperature and pressure behaviors of the parameters IQES, IBP, and ωBP obtained here for [Pip14][[NTf2] are discussed in light of known results for other glass-formers.

Molecular dynamics (MD) simulations of five ionic liquids based on 1-alkyl-3-methylimidazolium cations, [CnC1im]+, have been performed in order to calculate high-frequency elastic moduli and to evaluate heterogeneity of local elastic moduli. The MD simulations of [CnC1im][NO3], n = 2, 4, 6, and 8, assessed the effect of domain segregation when the alkyl chain length increases, and [C8C1im][PF6] assessed the effect of strength of anion–cation interaction. Dispersion curves of excitation energies of longitudinal and transverse acoustic, LA and TA, modes were obtained from time correlation functions of mass currents at different wavevectors. High-frequency sound velocity of LA modes depends on the alkyl chain length, but sound velocity for TA modes does not. High-frequency bulk and shear moduli, K∞ and G∞, depend on the alkyl chain length because of a density effect. Both K∞ and G∞ are strongly dependent on the anion. The calculation of local bulk and shear moduli was accomplished by performing bulk and shea...

The phase diagram of the ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesufonyl)imide, [Pyrr1,4][NTf2], was explored by synchroton X-ray diffraction and Raman scattering measurements as a function of temperature and pressure. Glass transition Tg(p) and melting Tm(p) temperatures were obtained from atmospheric pressure up to ca. 2.0 GPa. We found that both the Tg(p) and Tm(p) curves follow essentially the same pressure dependence. The similarity of pressure coefficients, dTg/dp ≈ dTm/dp, is explained within the non-equilibrium thermodynamics approach for the glass transition by assuming that one of the Ehrenfest equations is appropriated for Tg(p), whereas Tm(p) follows the Clausius-Clapeyron equation valid for the first-order transitions. The results highlight that ionic liquids are excellent model systems to address fundamental questions related to the glass transition.

Collective dynamics within the THz frequency range has been investigated by molecular dynamics (MD) simulations of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, [C 2 C 1 im][TFSI], 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide, [C 2 C 1 im][FSI], and lithium solutions of Li[TFSI] and Li[FSI] in each of these liquids. We calculated time correlation functions of mass current fluctuations for longitudinal (LA) and transverse acoustic (TA) sound modes at different wave vectors, k . The dispersion curves of excitation energy, ω ( k ), calculated by MD simulations were compared with recent data for these systems reported by Fujii et al., J. Chem. Phys. 138, 151101 (2013), using inelastic X-ray scattering (IXS) spectroscopy. The reasonable agreement between experiment and simulation indicates that non-polarizable potential models for these ionic liquids provide satisfactory representation of the collective dynamics at least at high-( k , ω ) range. Even though Li + solution is more viscous than the pure ionic liquid, the high-frequency sound velocity of the solution is the same as the pure liquid.

The relationship between local structure and dynamics is explored for molten sodium carbonate. A flexible fluctuating-charge model, which allows for changes in the shape and charge distribution of the carbonate molecular anion, is developed. The system shows the evolution of highly temperature-dependent complex low-dimensional structures which control the dynamics (and hence the liquid fragility). By varying the molecular anion charge distribution, the key interactions responsible for the formation of these structures can be identified and rationalized. An increase in the mean charge separation within the carbonate ions increases the connectivity of the emerging structures and leads to an increase in the system fragility.

Differential scanning calorimetry, X-ray diffraction, and Raman spectroscopy were used to reveal the premelting events precursors of melting of the ionic liquid triethylsulfonium bis(trifluoromethanesufonyl)imide, [S222][NTf2]. On heating the crystalline phase of [S222][NTf2], melting occurs along a sequence of at least three steps. First, the crystalline long-range order breaks down, but local order is retained. The second step is characterized by conformational freedom of the ethyl chains of cations related to premelting of nonpolar domains, and the complete melting finally occurs when anions acquire conformational freedom. This work provides a microscopic view on the mechanism of melting of [S222][NTf2] in line with the picture of melting taking place as a sequence of structural changes. The results of this work shed light on the understanding of the complex melting process of ionic liquids.

When adjusting the pH of deposition baths, ammonia (NH 3 ) is often used instead of sodium hydroxide (NaOH), but its effects on the chemical and/or electrochemical processes are not usually considered. Here the influence of NH 3 on the deposition rate, composition and magnetic properties of amorphous cobalt-boron (CoB) alloys deposited from alkaline plating baths containing citrate is discussed. We have observed that when the pH of the solution was adjusted with NH 3 , CoB films were deposited at higher rates and the B content decreased with increasing pH. Opposite trends were observed when NaOH was used to adjust the bath pH. These observations were discussed in terms of the complex species distribution in solution and of the buffering characteristics of NH 3 . The reducing agent, dimethylaminoborane, has its electrooxidation also affected by the presence of NH 3 in solution. Amorphous, soft CoB magnetic films with coercivity values varying from 40 Oe to 80 Oe, inversely dependent on the B content of the film, were obtained.

The effect of water on the hydrophobic ionic liquid (IL) 1-n-butyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonylimide) and its Li(+) mixture was evaluated. The electrochemical stability, density, viscosity, and ionic conductivity were measured for both systems in different concentrations of water. The presence of Li(+) causes a large increase in the water absorption ability of the IL. The experimental results suggest a break of the interactions between Li(+) and Tf2N(-) anions in the strong aggregates formed in dried Li(+) mixtures, modifying the size and physicochemical nature of these aggregates. It is also observed that the size of the ions aggregates with formal charge increases at high temperature and decreases the mobility of the charge carrier, explaining the break in the Walden rules at high temperature. Raman spectroscopy and molecular dynamic simulations show the structural change of these systems. In neat ILs, the water molecules interact mainly among each other, while in the Li(+) mixtures, water interacts preferentially with the metallic cation, causing an important change in the aggregates present in this system.

The interplay between crystallization and glass transition in the archetypal ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [C2C1im][NTf2], has been studied as a function of pressure up to ca. 12 GPa. Besides heterogeneous crystal nucleation, homogeneous nucleation in the sample inside the diamond anvil cell was also observed depending on compression/decompression rate. Amorphization of the crystal and glass formation under pressure has been followed by synchrotron X-ray diffraction. The characteristic Raman bands of the [NTf2]− anion provide a microscopic probe of the different phases. The crystalline phase is composed of the [NTf2]− cisoid conformer, but moisture implies formation of crystal with the transoid conformer. Raman spectra show that crystalline phases might become microscopically heterogeneous because of [NTf2]− conformational disorder. Raman mapping reveals the order–disorder evolution from crystal to glass. Crystals of [C2C1im][NTf2] formed under high pressure a...

Ionic liquids are an interesting class of materials due to their distinguished properties, allowing their use in an impressive range of applications, from catalysis to hypergolic fuels. However, the reactivity triggered by the application of high pressure can give rise to a new class of materials, which is not achieved under normal conditions. Here, we report on the high-pressure chemical instability of the ionic liquid 1-allyl-3-methylimidazolium dicyanamide, [allylC1im][N(CN)2], probed by both Raman and IR techniques and supported by quantum chemical calculations. Our results show a reaction occurring above 8 GPa, involving the terminal double bond of the allyl group, giving rise to an oligomeric product. The results presented herein contribute to our understanding of the stability of ionic liquids, which is of paramount interest for engineering applications. Moreover, gaining insight into this peculiar kind of reactivity could lead to the development of new or alternative synthetic routes to achieve, for example, poly(ionic liquids).

The light scattering (Raman) spectrum of ZnCl2 has been calculated in a computer simulation directly from molecular dynamics and by using an instantaneous normal modes (INM) approach. Good agreement between the spectra is reported. The calculations use a realistic model for the fluctuating polarizability of ZnCl2, derived from earlier work on simpler ionic melts. This contains several mechanisms which couple the radiation field to the ionic motion - short-range, dipole-induced dipole and hyperpolarization. INM analysis of ZnCl2 has previously shown how the character of the underlying vibrational modes changes across the density of states. Here it is shown that the efficiency of the coupling of a given mode to the radiation field depends strongly on its character and on the polarizability mechanism, so that the Raman spectra predicted for the different mechanisms differ markedly. A consequence is that the discrete Raman bands observed at high frequency in the polarized spectrum do not coincide with the spectrum of the localized, quasi-molecular ZnCl4 units of the network. Furthermore, the "light-vibration" coupling, relating the reduced Raman spectrum to the underlying vibrational density of states, is appreciably frequency dependent and different for each mechanism. © 1999 American Institute of Physics. ).

Assembling a diamond anvil cell for high-pressure measurements involves placing in a gasket hole the sample of interest, a pressure transmitting fluid, and a material for pressure calibration. In this communication, we propose the use of ionic liquids containing the bis(trifluoromethylsulfonyl)imide anion ([Tf2N]-), [(CF3SO2)2 N]-, as a simultaneous pressure transmitting and calibrant material for high-pressure Raman spectroscopy measurements of solid samples that are not soluble in ionic liquids. The position of the characteristic Raman band of the [Tf2N]- anion at 740 cm−1 exhibits linear frequency shift for pressures up to 2.5 GPa. High-pressure Raman spectra of different ionic liquids containing the same anion indicate that the actual magnitude of the pressure-induced frequency shift of the [Tf2N]- normal mode depends on the counterion, the typical shift being 4.2 cm−1/GPa. Ionic liquids based on the [Tf2N]- anion are also good pressure transmitting mediums because hydrostatic condition is kept at high pressure, and no crystallization is observed up to 4.0 GPa. Copyright © 2012 John Wiley & Sons, Ltd.

Silicon nitride, with a permittivity mid-way between SiO 2 and common high-k materials such as HfO 2 , is widely used in microelectronics as an insulating layer on top of oxides where it serves as an impurity barrier with the positive side effect of increasing the dielectric constant of the insulator when it is SiO 2 . It is also employed as charge storage in nonvolatile memory devices thanks to its high concentration of charge traps. However, in the case of memories, it is still unclear which defects are responsible for charge trapping and what is the impact of defect concentration on the structural and electronic properties of SiN x . Indeed, for the amorphous phase the band gap was measured in the range 5.1–5.5 eV, with long tails in the density of states penetrating the gap region. It is still not clear which defects are responsible for the tails. On the other hand, the K-center defects have been associated with charge trapping, though its origin is assigned to one Si back bond. To investigate the contribution of defect states to the band edge tails and band gap states, we adopted the β phase of stoichiometric silicon nitride (β-Si 3 N 4 ) as our model material and calculated its electronic properties employing ab initio DFT/LDA simulations with self-energy correction to improve the location of defect states in the SiN x band gap through the correction of the band gap underestimation typical of DFT/LDA. We considered some important defects in SiN x , as the Si anti-site and the N vacancy with H saturation, in two defect concentrations. The location of our calculated defect levels in the band gap correlates well with the available experimental data, offering a structural explanation to the measured band edge tails and charge trapping characteristics.

We performed ab initio calculations of the electronic structures of bulk CdSe and CdTe, and their interface band alignments on the CdSe in-plane lattice parameters. For this, we employed the LDA-1/2 self-energy correction scheme [L.G. Ferreira, M. Marques, L.K. Teles, Phys. Rev. B 78 (2008) 125116] to obtain corrected band gaps and band offsets. Our calculations include the spin–orbit effects for the bulk cases, which have shown to be of importance for the equilibrium systems and are possibly degraded in these strained semiconductors. Therefore, the SO showed reduced importance for the band alignment of this particular system. Moreover, the electronic structure calculated along the transition region across the CdSe/CdTe interface shows an interesting non-monotonic variation of the band gap in the range 0.8–1.8 eV, which may enhance the absorption of light for corresponding frequencies at the interface between these two materials in photovoltaic applications.

Transport coefficients have been measured as a function of the concentration of sulfur dioxide, SO(2), dissolved in 1-butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide, [BMMI][Tf(2)N], as well as in its lithium salt solution, Li[Tf(2)N]. The SO(2) reduces viscosity and density and increases conductivity and diffusion coefficients in both the neat [BMMI][Tf(2)N] and the [BMMI][Tf(2)N]-Li[Tf(2)N] solution. The conductivity enhancement is not assigned to a simple viscosity effect; the weakening of ionic interactions upon SO(2) addition also plays a role. Microscopic details of the SO(2) effect were unraveled using Raman spectroscopy and molecular dynamics (MD) simulations. The Raman spectra suggest that the Li(+)-[Tf(2)N] interaction is barely affected by SO(2), and the SO(2)-[Tf(2)N] interaction is weaker than previously observed in an investigation of an ionic liquid containing the bromide anion. Transport coefficients calculated by MD simulations show the same trend as the experimental data with respect to SO(2) content. The MD simulations provide structural information on SO(2) molecules around [Tf(2)N], in particular the interaction of the sulfur atom of SO(2) with oxygen and fluorine atoms of the anion. The SO(2)-[BMMI] interaction is also important because the [BMMI] cations with above-average mobility have a larger number of nearest-neighbor SO(2) molecules.

Heterogeneous dynamics within a time range of nanoseconds was investigated by molecular dynamics (MD) simulations of 1-butyl-3-methylimidazolium chloride ([bmim]Cl). After identifying groups of fast and slow ions, it was shown that the separation between the location of the center of mass and the center of charge of cations, dCMCC, is a signature of such difference in ionic mobility. The distance dCMCC can be used as a signature because it relaxes in the time window of the dynamical heterogeneity. The relationship between the parameter dCMCC and conformations of the side alkyl chain in [bmim] is discussed. Since the relatively slow relaxation of dCMCC is a consequence of coexisting polar and nonpolar domains in the bulk, the MD simulations reveal a subtle interplay between structural and dynamical heterogeneity in ionic liquids.

In this work two systems with total phase separation composed of alternated a-Si/SiO2 and a-Si/Si3N4 respectively along with silicon-rich SiOxNy films were deposited by the PECVD technique and characterized by XANES, Raman, TEM and PL techniques. The multilayers were used to analyse the effects of phase separation in the XANES and Raman spectra as well as to determine the phase contrast in TEM images in order to elucidate upon the existence of phase separation in the silicon-rich SiOxNy films and correlate it with the high photoluminescence intensity in the visible spectra region observed in the as deposited films. The results clearly indicate that the as deposited films present partial phase separation, which is crucial for the PL emission. The effect of high temperature annealing in the silicon-rich SiOxNy films is also studied, Raman and XANES results show that heat treatments promote silicon aggregation and, in some cases, crystallization strongly influencing the PL emission. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Raman spectra of dilute solutions of acetonitrile in ionic liquids reveal the characteristic features of ionic liquids’ polarity. This is accomplished by investigating the Raman bandshape of the ν (CN) band, corresponding to the CN stretching mode of CH3CN, which is a very sensitive probe of the local environment. The amphiphilic nature of the CH3CN molecule allows us to observe the effect of electron pair acceptor and electron pair donor characteristics on ionic liquids. It has been found that the overall polarity of nine different ionic liquids based on 1-alkyl-3-methylimidazolium cations is more dependent on the anion than cation. The observed wavenumber shift of the ν (CN) band of CH3CN in ionic liquids containing alkylsulfate anions agrees with the significant different values previously measured for the dielectric constant of these ionic liquids. The conclusions obtained from the analysis of the ν (CN) band were corroborated by the analysis of the symmetric ν1 (CD3 ) stretching mode of deuterated acetonitrile in different ionic liquids. Copyright c � 2010 John Wiley & Sons, Ltd.

Raman and electronic spectra of the [3,5-bis(dicyanomethylene)cyclopentane-1,2,4-trionate] dianion, the croconate violet (CV), are reported in solutions of ionic liquids based on imidazolium cations. Different normal modes of the CV anion, ν (CO), ν (CO) + ν (CC) + ν (CCN), and ν(C≡N), were used as probes of solvation characteristics of ionic liquids, and were compared with spectra of CV in common solvents. The spectra of CV in ionic liquids are similar to those in dichloromethane solution, but distinct from those in protic solvents such as ethanol or water. The UV–vis spectra of CV in ionic liquids strongly suggest π–π interactions between the CV anion and the imidazolium cation. Copyright © 2009 John Wiley & Sons, Ltd.

Molecular dynamics (MD) simulations of LiCl·6H2O showed that the diffusion coefficient D, and also the structural relaxation time , follow a power law at high temperatures, D−1 ∝ (T − To)−μ, with the same experimental parameters for viscosity (To = 207 K, μ = 2.08). Decoupling between D and occurs at Tx ∼ 1.1To. High frequency acoustic excitations for the LiCl·6H2O model were obtained by the calculation of time correlation functions of mass current fluctuations. The temperature dependence of the instantaneous shear modulus, G∞(T), was considered in the shoving model for supercooled liquids [J.C. Dyre, T. Christensen, N.B. Olsen, J. Non-Cryst. Solids 352 (2006) 4635] resulting in a linear relationship log (D−1) vs. G∞/T.