Molecular Dynamics Simulation of the Ionic Liquid N Ethyl N,N -dimethyl- N -(2-methoxyethyl)ammonium Bis(trifluoromethanesulfonyl)imide (original) (raw)
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Molecular dynamics simulation of imidazolium-based ionic liquids. II. Transport coefficients
Journal of Chemical Physics, 2009
A systematic molecular dynamics study is performed to determine the dynamics and transport properties of 12 room-temperature ionic liquids family with 1-alkyl-3-methylimidazolium cation, ͓amim͔ + ͑alkyl= methyl, ethyl, propyl, and butyl͒, with counterions, PF 6 − , NO 3 − , and Cl −. The goal of the work is to provide molecular level understanding of the transport coefficients of these liquids as guidance to experimentalists on choosing anion and cation pairs to match required properties of ionic liquid solvents. In the earlier paper ͑Part I͒, we characterized the dynamics of ionic liquids and provided a detailed comparison of the diffusion coefficients for each ion using the Einstein and Green-Kubo formulas. In this second part, other transport properties of imidazolium salts are calculated, in particular, the electrical conductivity is calculated from the Nernst-Einstein and Green-Kubo formulas. The viscosity is also determined from the Stokes-Einstein relation. The results of the calculated transport coefficients are consistent with the previous computational and experimental studies of imidazolium salts. Generally, the simulations give electrical conductivity lower than experiment while the viscosity estimate is higher than experiment. Within the same cation family, the ionic liquids with the NO 3 − counterion have the highest electrical conductivities: ͓NO 3 ͔ − Ͼ ͓PF 6 ͔ − Ͼ ͓Cl͔ −. The ͓dmim͔͓X͔ series, due to their symmetric cationic structure and good packing and the ͓bmim͔͓X͔ series due to higher inductive van der Waals interactions of ͓bmim͔ + , have the highest viscosities in these ionic liquid series. Our simulations show that the major factors determining the magnitude of the self-diffusion, electrical conductivity, and viscosity are the geometric shape, ion size, and the delocalization of the ionic charge in the anion.
The Journal of Chemical Physics, 2008
Molecular dynamics simulations are used to study the dynamics and transport properties of 12 room-temperature ionic liquids of the 1-alkyl-3-methylimidazolium ͓amim͔ + ͑alkyl= methyl, ethyl, propyl, and butyl͒ family with PF 6 − , NO 3 − , and Cl − counterions. The explicit atom transferable force field of Canongia Lopes et al. ͓J. Phys. Chem. B 108, 2038 ͑2004͔͒ is used in the simulations. In this first part, the dynamics of the ionic liquids are characterized by studying the mean-square displacement ͑MSD͒ and the velocity autocorrelation function ͑VACF͒ for the centers of mass of the ions at 400 K. Trajectory averaging was employed to evaluate the diffusion coefficients at two temperatures from the linear slope of MSD͑t͒ functions in the range of 150-300 ps and from the integration of the VACF͑t͒ functions at 400 K. Detailed comparisons are made between the diffusion results from the MSD and VACF methods. The diffusion coefficients from the integration of the VACFs are closer to experimental values than the diffusion coefficients calculated from the slope of MSDs. Both methods can show good agreement with experiment in predicting relative trends in the diffusion coefficients and determining the role of the cation and anion structures on the dynamical behavior of this family of ionic liquids. The MSD and self-diffusion of relatively heavier imidazolium cations are larger than those of the lighter anions from the Einstein results, except for the case of ͓bmim͔͓Cl͔. The cationic transference number generally decreases with temperature, in good agreement with experiments. For the same anion, the cationic transference numbers decrease with increasing length of the alkyl chain, and for the same cation, the trends in the cationic transference numbers are ͓NO 3 ͔ − Ͻ ͓Cl͔ − Ͻ ͓PF 6 ͔ − . The trends in the diffusion coefficient in the series of cations with identical anions are ͓emim͔ + Ͼ ͓pmim͔ + Ͼ ͓bmim͔ + and those for anions with identical cations are ͓NO 3 ͔ − Ͼ ͓PF 6 ͔ − Ͼ ͓Cl͔ − . The ͓dmim͔ + has a relatively low diffusion coefficient due to its symmetric structure and good packing in the liquid phase. The major factor for determining the magnitude of the self-diffusion is the geometric shape of the anion of the ionic liquid. Other important factors are the ion size and the charge delocalization in the anion.
Molecular Physics, 2012
Molecular dynamics simulations for liquid CaAl 2 Si 2 O 8 have been carried out at 72 state points spanning ranges in density (q: 2398-4327 kg/m 3 ), temperature (T: 3490-6100 K) and pressure (P: 0.84-120 GPa) relevant to geosystems. The atomic scale structure of the melt is determined by analysis of nearest neighbor coordination statistics as a function of T and P. Dramatic structural change occurs as pressure increases especially for 0 < P <20 GPa at all temperatures. Changes in structure are encapsulated by examining the coordination of Si, Al, Ca and O around oxygen and vice versa. Si and Al change from predominantly fourfold at low-P to dominantly sixfold for P >$ 20 GPa. Pentahedrally coordinated Si and Al in distorted trigonal bipyramids attain abundance maxima corresponding to 6060% of total (Si, Al)O n at 3-5 GPa and weakly depend on T. The coordination of Ca by oxygen increases from 7 to 10 for 0 < P < 20 GPa and changes slowly for P > 20 GPa at 3500 K. Similar behavior is seen at 6000 K except that the interval of rapid changes occurs at higher pressure. Oxygen with only one nearest Si or Al neighbor (i.e., non-bridging oxygen, NBO) decreases whereas oxygen with two or three nearest neighbors of Si, Al or Ca increases as pressure increases. Changes in melt structure are reflected in the variation of thermodynamic and transport properties of the liquid. Values of the self-diffusivities of Ca, Al, Si and O are fit to a modified Arrhenian expression and compare well to limited laboratory data. Self-diffusivities are best fit using 'low P' and 'high-P' expressions, identical in form but with different parameters, with activation energies and activation volumes in the range 150-200 kJ/mol and +5 to À1 cm 3 /mol, respectively. Green-Kubo calculations for liquid shear viscosity are presented and compare well with limited laboratory results. Application of the Eyring model to determine the characteristic size and number of atoms in the activated cluster based on independently computed D and g suggests that the activated cluster decreases from 608 to 3atomsfromlowtohighpressurewhileitscharacteristicsizeshrinksfrom3 atoms from low to high pressure while its characteristic size shrinks from 3atomsfromlowtohighpressurewhileitscharacteristicsizeshrinksfrom14 Å to $3 Å providing insight into dynamics of atom mobility and possible cooperative behavior. The equation of state and variation of internal energy with T and V are used in Part II to derive a comprehensive thermodynamic description of liquid CaAl 2 Si 2 O 8 . This is best accomplished by allowing for EOS expressions broken into high and low pressure intervals consistent with coordination statistics and MD-derived transport properties.
Journal of Molecular Liquids, 2017
This work presents a molecular dynamics simulation study of the vapor-liquid equilibrium curve for the 1-butyl-3methylimidazolium mesilate ionic liquid [C 4 mim][OMs] along with other thermodynamic properties in condensed phase, such as: densities, surface tension, heats of vaporization, constant pressure heat capacities, and the dielectric constant. The latter were obtained using molecular simulation calculations employing a new nonpolarizable classical force field developed in this work, for this ionic liquid experimental thermodynamic properties as well as calculated properties by molecular simulations are very scarce in the literature. The development of the force field for the [C 4 mim][OMs] ionic liquid involved first the parameterization of all-atom non-polarizable force fields for two well-studied imidazolium ionic liquids [C 4 mim][BF 4 ], in order to validate first against existing experimental and calculated thermodynamic properties using molecular simulations. Even though many transferable force fields for ionic liquids have been developed in the past years, some of these force fields predict extremely well structural properties such as densities and radial distribution functions etc., while other force fields fail to predict accurately heats of vaporization, condensed phases properties such as dielectric constant and transport properties such as viscosity, diffusivity, and vice versa. The force fields developed in this work are centered on the OPLS functional, and were parameterized using a simple and robust methodology focused mainly in electrostatic charges determination and in the refinement of the most representative dihedral angles. The calculation of the electrostatic charges follows the methodology proposed by Salas et al. (J. Chem. Theory Compute. 2015, 11, 683−693) involving the inclusion of polarization effects in quantum mechanical calculations in order to represent implicitly the solvent by employing the experimental or calculated dielectric constant in conjunction with a restrain electrostatic potential fitted to an ionic liquid dimer, to account for the inclusion of solvation coordination effects. The dihedral angles were parameterized simultaneously from the energetic differences in molecular conformations between ab-initio calculations and the energies obtained with the classic force field. The force field validation for the ionic liquids 1-butyl-3-methylimidazolium Tetrafluoroborate, [C 4 mim][BF 4 ], and 1-butyl-3metilimidazolium Hexafluorophosphate [C 4 mim][PF 6 ], gave good agreement for the properties calculated compared with experimental and literature results. Our motivation for this work focused mostly in force field development for specific individual ionic liquids capable of reproducing thermodynamic properties in condensed phase, such as density, heats of vaporization, surface
Journal of Chemical and Engineering Data, 2010
The density and viscosity of synthesized 1-alkyl-3-methylimidazolium iodide ([C n mim]I, n ) 4, 6, 8) were measured in the wide range of temperature of (298 to 393) K. Using a vacuum line, measurements of the viscosity were made under a water-vapor free atmosphere. The viscosity decreases sharply with temperature and increases as the alkyl chain length increases. The molecular dynamics simulation was performed for the densities of these ionic liquids to remedy the lack of literature experimental data. The results are quite in agreement with the experiments, with a maximum deviation of 3.00 % due to [C 8 mim]I at 358 K. The viscosities fit best in the modified Arrhenius, Vogel-Fulcher-Tammann (VFT), and Litovitz equations. The viscosities also fit in the simple linear equation we proposed recently with accuracies comparable with Litovitz and VFT.
The Journal of Physical Chemistry B, 2011
The virtual laboratory allows for computer experiments that are not accessible via real experiments. In this work, three previously obtained charge sets were employed to study the influence of hydrogen bonding on imidazolium-based ionic liquids in molecular dynamics simulations. One set provides diffusion coefficients in agreement with the experiment and is therefore a good model for real-world systems. Comparison with the other sets indicates hydrogen bonding to influence structure and dynamics differently. Furthermore, in one case the total charge was increased and in another decreased by 0.1 e. Both the most acidic proton as well as the corresponding carbon atom were artificially set to zero, sequentially and simultaneously. In the final setup a negative charge was placed on the proton in order to introduce a barrier for the anion to contact the cation via this most acidic hydrogen atom. The following observations were made: changing the hydrogen bonding ability strongly influences the structure while the dynamic properties, such as diffusion and viscosity, are only weakly changed. However, the introduction of larger alterations (stronger hydrogen bonding and antihydrogen bonding) also strongly influences the diffusion coefficients. The dynamics of the hydrogen bond, ion pairing, and the ion cage are all affected by the level of hydrogen bonding. A change in total charges predominantly influences transport properties rather than structure. For ion cage dynamics with respect to transport porperties, we find a good correlation and a weak or no correlation for the ion pair or the hydrogen bond dynamics, respectively. Nevertheless, the hydrogen bond does influence ion cage dynamics. Therefore, we confirm that ionic liquids rather consist of loosely interacting counterions than of discrete ion pairs. Hydrogen bonding affects the properties only in a secondary or indirect manner.
Physical Chemistry Chemical Physics, 2011
A new, non-polarizable force field model (FFM) for imidazolium-based, room-temperature ionic liquids (RTILs), 1-ethyl-3-methyl-imidazolium tetrafluoroborate and 1-butyl-3-methylimidazolium tetrafluoroborate, has been developed. Modifying the FFM originally designed by Liu et al. (J. Phys. Chem. B, 2004, 108, 12978-12989), the electrostatic charges on interacting sites are refined according to partial charges calculated by explicit-ion density functional theory. The refined FFM reproduces experimental heats of vaporization, diffusion coefficients, ionic conductivities, and shear viscosities of RTILs, which is a significant improvement over the original model (Zh. Liu, Sh. Huang and W. Wang, J. Phys. Chem. B, 2004, 108, 12978-12989). The advantages of the proposed procedure include clarity, simplicity, and flexibility. Expanding the functionality of our FFM conveniently only requires modification of the electrostatic charges. Our FFM can be extended to other classes of RTILs as well as condensed matter systems in which the ionic interaction requires an account of polarization effects.
The Journal of Chemical Physics, 2018
We study ionic liquids composed 1-alkyl-3-methylimidazolium cations and bis(trifluoromethylsulfonyl)imide anions ([CnMIm][NTf2]) with varying chain-length n = 2, 4, 6, 8 by using molecular dynamics simulations. We show that a reparametrization of the dihedral potentials as well as charges of the [NTf2] anion leads to an improvment of the force field model introduced by Köddermann et al. [ChemPhysChem, 8, 2464 (2007)] (KPL-force field). A crucial advantage of the new parameter set is that the minimum energy conformations of the anion (trans and gauche), as deduced from ab initio calculations and Raman experiments, are now both well represented by our model. In addition, the results for [CnMIm][NTf2] show that this modification leads to an even better agreement between experiment and molecular dynamics simulation as demonstrated for densities, diffusion coefficients, vaporization enthalpies, reorientational correlation times, and viscosities. Even though we focused on a better representation of the anion conformation, also the alkyl chain-length dependence of the cation behaves closer to the experiment. We strongly encourage to use the new NGKPL force field for the [NTf2] anion instead of the earlier KPL parameter set for computer simulations aiming to describe the thermodynamics, dynamics and also structure of imidazolium based ionic liquids.
Chemical Engineering Science, 2017
The viscosity and density of the ionic liquid (IL) 1-n-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C 6 mim][N T f 2 ]), the molecular solvent tetraethylene glycol dimethyl ether (tetraglyme or G4) and their binary mixtures were measured experimentally as a function of temperature. The same systems were also studied using classical molecular dynamics (MD) simulations. The viscosities of the [C 6 mim][N T f 2 ]/G4 mixtures decrease with increasing G4 concentration, though not as much as an ideal mixing model would predict. Detailed analysis of the MD results reveals that G4 preferentially solvates cations, leading to a reduction in the interaction energy between cations and anions and a subsequent enhancement in anion mobility. A similar effect has been reported when glymes are mixed with salts containing alkali metal cations, with the resulting mixtures being called "solvate" ionic liquids. The simulations predict that the ionic conductivity will be maximized when the G4 mole fraction is around 10-20%. The ability of G4 to effectively solvate the cations stems from localized charges on the oxygens. The simulations predict that solvents having large localized positive charges would preferentially solvate anions, leading to enhanced cation mobility.
Journal of Physical Chemistry B, 2016
All-atom molecular dynamics (MD) simulations of 1-hexyl-2,3-dimethylimidazolium bis(fluorosulfonyl)imide ([C 6 mmim][FSI]) ionic liquid (IL) and its binary mixtures with acetonitrile (ACN) are reported for the first time. The presence of ACN as a cosolvent, similar to the effect of increasing temperature, causes an enhancement to the ion translational motion and fluidity in the IL, leading to significant improvement of ionic conductivity and self-diffusion which is well explained by a microscopic structural analysis. In neat IL and concentrated IL mixture, self-diffusion of the cation is higher than that of corresponding anion; however, further adding of ACN into the diluted mixtures with the IL molar fraction (x IL) below 0.50 results in more weakened interactions of the nearest ACN-anion neighbors rather than those of ACNcation neighbors so that the number of isolated anions is more than that of isolated cations at this condition, and the anions diffuse faster than the cations as expected based of their relative sizes. The velocity autocorrelation function (VACF) analysis indicates the inverse relation between the x IL and the mean collision time of each species. Additionally, at a fixed x IL , both the mean collision time and the velocity randomization time of ACN are shorter than those of the ions. The gradual addition of ACN changes the morphology of nano-segregated domains and tends to disrupt ionic clusters (i.e., it scatters and decomposes both the polar and non-polar domains) compared to those of pure IL, whereas both the radial and spatial distribution functions show the stabilization role of ACN on the close contact ion pair association. On the other hand, increasing of ACN causes a weakening of the structural correlations of the cation-cation and anion-anion neighbors in the solutions. ACN molecules appeared as a bridge with balanced affinities between the polar and non-polar domains, and no indication was observed for aggregation of ACN