Effect of Ion Rigidity on Physical Properties of Ionic Liquids Studied by Molecular Dynamics Simulation

Abstract

In this work, we have performed molecular dynamics (MD) simulations to compare the structural and dynamical properties of three ionic liquids (ILs), 1-ethyl-3-methyl-imidazolium tetrafluorborate ([EMI(+)][BF4(-)]), 1,1'-dimethyl-4,4'-bipyridinium bis(tetrafluorborate) ([VIO(2+)][BF4(-)]2), and 1,1'-dimethyl-4,4'-bipyridinium bis(trifluoromethylsulfonyl)imide (bistriflimide in short) ([VIO(2+)][Tf2N(-)]2), aiming to discover the influence of ion rigidity on the physical properties of ILs. [VIO(2+)] is more rigid than [EMI(+)], and [BF4(-)] is more rigid than [Tf2N(-)]. [VIO(2+)][BF4(-)]2 has an anion distribution different from the other two by the higher and sharper peaks in the cation-anion radial distribution functions, reflecting a close-packed local structure of anions around cations. [VIO(2+)][BF4(-)]2 and [VIO(2+)][Tf2N(-)]2 have similar dynamics much slower than [EMI(+)][BF4(-)], and [VIO(2+)][Tf2N(-)]2 shows a more isotropic molecular distribution than [VIO(2+)][BF4(-)]2 and [EMI(+)][BF4(-)]. Additionally, we have simulated two modified viologen-based ILs to reinforce our interpretations. We conclude from the above simulation results that the rigidity of anions influences the alignment of cations and that the rigidity of cations shows a large obstacle to their rotational capacity. Moreover, we have observed a slower diffusion of [VIO(2+)][BF4(-)]2 due to the electrostatic correlations, which stabilizes the ion-cage effect.

Competitive Microstructures Versus Cooperative Dynamics of Hydrogen Bonding and π-Type Stacking Interactions in Imidazolium Bis(oxalato)borate Ionic Liquids

“…The translational diffusion coefficients of representative ionic groups in imidazolium bis­(oxalato)­borate IL matrices at 323 K fall within an order of 10 –10 m 2 /s, which is a common magnitude for other ionic species at a similar temperature range. ,,,,,,− The scarcity of experimental diffusion coefficient data of these imidazolium bis­(oxalato)­borate ILs in the literature prevents a direct comparison with our computational results. Nevertheless, in present work, we mainly focus on exploring the effect of aliphatic chain length in imidazolium cations on translational diffusivity of these representative ionic groups in heterogeneous imidazolium bis­(oxalato)­borate IL matrices.…”

“…In concert with HB coordination, π-type interaction is related to favorable electrostatic contributions and preferential dispersion interactions between heteroaromatic ring planes, such as imidazolium, pyridinium, pyrazolium, triazolium, thiazolium cations, orthoborate anions, and their derivatives, despite of repulsive Coulombic forces between like charges. ,,− The π-type interaction has been recognized as a distinctive contributing factor in protein folding and ion selectivity, self-organized electronic materials, and organic nanodevices. ,,,,, In IL matrices, the cation–cation π–π stacking structures are less common but have been identified in representative imidazolium-based ILs. ,,,− The formation of π–π stacking imidazolium ring dimers was observed in 1-ethyl-3-methylimidazolium nitrate ([EMIM]­[NO 3 ]), 1-ethyl-3-methylimidazolium sulfate ([EMIM] 2 [SO 4 ]), dimethylimidazolium triflate ([MMIM]­[OTF]), and 1-ethyl-3-methylimidazolium bis­(trifluoromethylsulfonyl)­imide ([EMIM]­[NTF 2 ]) IL crystal structures because of a significant screening of cationic charge–charge repulsion forces that are mediated by anionic groups. Moreover, the observation of peculiar π–π stacking structures between imidazolium ring planes and benzene molecules in IL–benzene mixtures was rationalized by attractive arrangements of quadrupole moments associated with aromatic ring planes. , …”

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

“…30. The equilibrated sample was probed from 700 K to 250 K in temperature steps of ∆T = 50 K. We simulated the system for a duration of 10 ns (10 000 configurations) at each temperature for the calculation of the mean-square displacement (MSD), the diffusion coefficient, and the radial distribution functions (RDFs).…”

Charge Transport and Phase Behavior of Imidazolium-Based Ionic Liquid Crystals from Fully Atomistic Simulations

“…Mesophases have long been observed in molten salt systems [ 17 ], but tuning the molecules to reach liquid crystal phases closer to room-temperature has been of more recent interest. Molecules studied at the intersection of these two fields are often based around imidazolium or viologen groups [ 18 ], with elongated alkyl tails attached to one or more ions. The counter-ion can be either a “simple” ionic liquid species (such as [PF 6 ] − or [BF 4 ] − ) or another ion with a long alkyl chain.…”