Molecular Simulations of Anion and Temperature Dependence on Structure and Dynamics of 1-Hexyl-3-methylimidazolium Ionic Liquids

Abstract

In this study, we examine the effect of various anions and temperature on structure and dynamics of 1-hexyl-3-methylimidazolium ionic liquids (ILs) from molecular dynamics simulations. The structural properties show that ILs containing smaller anions like Cl(-) and Br(-) are relatively higher cation-anion interactions, compared to ILs containing larger anions like OTf(-) and NTf2(-). In all ILs, the spatial distribution of anions is closer to the acidic hydrogen atom of the cation compared to the two nonacidic hydrogen atoms of the cation. The diffusion coefficients of cations and anions (ionic conductivity) increase with anionic size. At each temperature, the cationic and anionic diffusions and ionic conductivity are lowest in ILs containing anions like Cl(-) and Br(-) and highest in ILs containing anions like BF4(-), OTf(-), and NTf2(-). Consistent with experiments, simulations predict that ILs with an intermediate size BF4(-) anion show the highest cationic and anionic diffusion (and ionic conductivity). At each temperature, the interactions between ion pairs of each IL show that a decrease in ion-pair lifetimes is directly related to the increase in diffusion coefficients and conductivity in ILs, suggesting that characterization of ion-pair lifetimes is sufficient to validate the trends seen in dynamical properties of ILs.

Communication: Anion-specific response of mesoscopic organization in ionic liquids upon pressurization

“…Recent MD simulations conducted on [C n mim][X], with [X] being different anions, showed that also in the liquid state, the [Tf2N] anion, due to its bulkiness, tends to lay above and below the imidazolium ring, while the [BF 4 ] anion prefers being localised equatorially with respect to the imidazolium ring, in the neighbourhood of the hydrogen atoms. 61,62 These computational observations agree with the mechanism that we are presently indicating to rationalise the experimental evidence.…”

Molecular Dynamics Analysis of Charge Transport in Ionic‐Liquid Electrolytes Containing Added Salt with Mono, Di, and Trivalent Metal Cations

“…In addition, the diffusive regime can be examined computing the time‐dependent parameter: β(t)=dLn(MSD(t))/dLn(t). When β(t) is close to the unity, the system is in a linear diffusive regime . The extracted β(t) values are shown in Figure S2 in the Supporting Information.…”

“…When b(t) is close to the unity, the system is in a linear diffusive regime. [52] The extracted b(t) values are shown in Figure S2 in the Supporting Information. After approximately 50 ns of simulation time, b(t) values corresponding to the [C 4 PYR] + cations and [Tf 2 N] À anions in all systems fluctuate close to the unity.…”

“…This time interval is analyzed by the first derivative of the mean squared displacement (Figure S2 in the Supporting Information). To ensure that the diffusive regime is attained we confirm that the first derivative of the logarithm of MSD with respect to the logarithm of time is close to the unity . We approximated the standard deviations of the calculated self‐diffusion coefficients by small increments in this interval.…”

“…To ensure that the diffusive regime is attained we confirm that the first derivative of the logarithm of MSD with respect to the logarithm of time is close to the unity. [52] We approximated the standard deviations of the calculated selfdiffusion coefficients by small increments in this interval. We estimated the ionic conductivity through the Nernst-Einstein equation, which links the conductivity with the density and diffusion properties.…”