Effect of anions on static orientational correlations, hydrogen bonds, and dynamics in ionic liquids: A simulational study (original) (raw)
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Journal of Physical Chemistry B, 2007
Thermodynamics, structure, and dynamics of an ionic liquid based on a quaternary ammonium salt with ether side chain, namely, N-ethyl-N,N-dimethyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide, MOENM 2 E TFSI, are investigated by molecular dynamics (MD) simulations. Average density and configurational energy of simulated MOENM 2 E TFSI are interpreted with models that take into account empirical ionic volumes. A throughout comparison of the equilibrium structure of MOENM 2 E TFSI with previous results for the more common ionic liquids based on imidazolium cations is provided. Several time correlation functions are used to reveal the microscopic dynamics of MOENM 2 E TFSI. Structural relaxation is discussed by the calculation of simultaneous space-time correlation functions. Temperature effects on transport coefficients (diffusion, conductivity, and viscosity) are investigated. The ratio between the actual conductivity and the estimate from ionic diffusion by the Nernst-Einstein equation indicates that correlated motion of neighboring ions in MOENM 2 E TFSI is similar to imidazolium ionic liquids. In line with experiment, Walden plot of conductivity and viscosity indicates that simulated MOENM 2 E TFSI should be classified as a poor ionic liquid.
Room temperature ionic liquids containing low water concentrations—a molecular dynamics study
Physical Chemistry Chemical Physics, 2008
We have performed classical molecular dynamics to study the properties of a water-miscible and a water-immiscible room-temperature ionic liquid when mixed with small quantities of water. The two ionic liquids consist of the same 1-ethyl-3-methylimidazolium ([EMIM]) cation combined with either the boron tetrafluoride ([BF 4 ]) or bis(trifluoromethylsulfonyl)imide ([NTf 2 ]) anion. It is found that, in both ionic liquids, water clusters of varying sizes are typically hydrogen bonded to two anions with the cation playing a minor role. We also highlight the difficulties of obtaining dynamic quantities such as self-diffusion coefficients from simulations of such viscous systems.
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.
Internal Dynamics of Ionic Liquids over a Broad Temperature Range—The Role of the Cation Structure
Materials, 2021
1H and 19F spin-lattice relaxation experiments have been performed for a series of ionic liquids sharing the same anion: bis(trifluoromethanesulfonyl)imide but including cations of different alkyl chain lengths: butyltriethylammonium, triethyloctylammonium, dodecyltriethylammo-nium and hexadecyltriethylammonium. The studies have been carried out in the temperature range from 383 to 108 K at the resonance frequency of 200 MHz (for 1H). A quantitative analysis of the relaxation data has revealed two dynamical processes for both kinds of ions. The dynamics have been successfully modeled in terms of the Arrhenius law. The timescales of the dynamical processes and their temperature evolution have been discussed in detail, depending on the structure of the cation.
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.
The Journal of Chemical Physics, 2010
We use molecular dynamics simulations to study the structure, dynamics, and details of the mechanism of congruent melting of the equimolar mixture of 1-ethyl-3-methylimidazolium bis͑trifluoromethanesulfonyl͒ imide with benzene, ͓emim͔͓NTf 2 ͔ •C 6 H 6. Changes in the molecular arrangement, radial distribution functions, and the dynamic behavior of species are used to detect the solid to liquid transition, show an indication of the formation of polar islands by aggregating of the ions in the liquid phase, and characterize the melting process. The predicted enthalpy of melting ⌬H m =38Ϯ 2 kJ mol −1 for the equimolar inclusion mixture at 290 K is in good agreement with the differential scanning calorimetry experimental results of 42Ϯ 2 kJ mol −1. The dynamics of the ions and benzene molecules were studied in the solid and liquid states by calculating the mean-square displacement ͑MSD͒ and the orientational autocorrelation function. The MSD plots show strong association between ion pairs of the ionic liquid in the inclusion mixture. Indeed, the presence of a stoichiometric number of benzene molecules does not affect the nearest neighbor ionic association between ͓emim͔ + and ͓NTf 2 ͔ − , but increases the MSDs of both cations and anions compared to pure liquid ͓emim͔͓NTf 2 ͔, showing that second shell ionic associations are weakened. We monitored the rotational motion of the alkyl chain sides of imidazolium cations and also calculated the activation energy for rotation of benzene molecules about their C 6 symmetry axes in their lattice sites prior to melting.
Journal of Molecular Liquids, 2021
The properties of [EMIM][BF 4 ] + [EMIM][TFSI] double salt ionic liquid (DSIL) were studied as a function of mixtures composition and temperature. Experimental physicochemical properties combined with molecular simulation (quantum chemistry and classical molecular dynamics) were considered, thus providing a micro and macroscopic characterization of fluids' structuring, intermolecular forces and molecular aggregation. The results were analysed in thermodynamics terms considering deviations of ideality and mixing properties as well as from the solvation and interaction between the involved ionic liquids by the developed complex hydrogen bonding networks. Likewise, liquid [EMIM][BF 4 ] + [EMIM][TFSI] interfaces (x [EMIM][BF4] = x [EMIM][TFSI] = 0.5) were also studied using molecular dynamics methods to examine the diffusion of [BF 4 ]and [TFSI]anions in the [EMIM][BF 4 ] + [EMIM][TFSI] liquid interface, and the mechanism of interface crossing. The results allow a multiscale characterization of the considered ionic liquid mixtures thus providing another way of designing IL-type solvents for specific applications, by choosing not only the ion identity but also the ion ratio.