Molecular dynamics simulation of imidazolium-based ionic liquids. I. Dynamics and diffusion coefficient

Abstract

Molecular dynamics simulations are used to study the dynamics and transport properties of 12 room-temperature ionic liquids of the 1-alkyl-3-methylimidazolium [amim](+) (alkyl = methyl, ethyl, propyl, and butyl) 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 (2004)] is used in the simulations. In this first part, the dynamics of the ionic liquids are characterized by studying the mean-square displacement (MSD) and the velocity autocorrelation function (VACF) 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 MSD(t) functions in the range of 150-300 ps and from the integration of the VACF(t) 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 [bmim][Cl]. 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)](-) < [Cl](-) < [PF(6)](-). The trends in the diffusion coefficient in the series of cations with identical anions are [emim](+) > [pmim](+) > [bmim](+) and those for anions with identical cations are [NO(3)](-) > [PF(6)](-) > [Cl](-). The [dmim](+) 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.

Revealing the complex conduction heat transfer mechanism of nanofluids

“…The third and final test involved plotting the normalised velocity autocorrelation function (VACF) of a nanoparticle and a normal argon fluid atom (more information can be found in an earlier study performed [25]). All three tests indicated good agreement with the literature [26][27][28][29][30][31][32][33][34][35].…”

Improved United-Atom Force Field for 1-Alkyl-3-methylimidazolium Chloride

“…For example, even the simple point charge model for water yields very good predictions of both structural and dynamic properties after optimization of a few parameters. 38 Many recent studies 35,[39][40][41][42][43][44][45][46][47][48] have shown that force fields of ILs that do not contain a polarization term, and charges fitted via ab initio calculations of the isolated ions, usually result in substantially lower predictions of transport properties and overestimation of the heat of vaporization. For example, the self-diffusion coefficient of [C 2 mim][BF 4 ] is underestimated by about an order of magnitude by two widely used force fields.…”

Communication: Probing the existence of partially arrested states in ionic liquids

“…10,[13][14][15][16][17][18][19][20][21] Many researchers have previously observed a slower diffusion of the smaller anions in RTIL, either by experiments 21 or by molecular dynamics (MD) simulations. 10,[13][14][15][16][17][18][19][20] The most common explanation appeals to the dynamics along the direction of the carbon located between the two nitrogens in the imidazolium ring with a marginal dependence on the anion. 22,23 However, the present work offers an alternative explanation for this observation on RTIL, one that is supported by an analysis of the simulation results in light of the SCGLE theoretical predictions.…”