Time scale of dynamic heterogeneity in model ionic liquids and its relation to static length scale and charge distribution

Abstract

We study how dynamic heterogeneity in ionic liquids is affected by the length scale of structural relaxation and the ionic charge distribution by the molecular dynamics simulations performed on two differently charged models of ionic liquid and their uncharged counterpart. In one model of ionic liquid, the charge distribution in the cation is asymmetric, and in the other it is symmetric, while their neutral counterpart has no charge with the ions. It is found that all the models display heterogeneous dynamics, exhibiting subdiffusive dynamics and a nonexponential decay of structural relaxation. We investigate the lifetime of dynamic heterogeneity, τ(dh), in these systems by calculating the three-time correlation functions to find that τ(dh) has in general a power-law behavior with respect to the structural relaxation time, τ(α), i.e., τ(dh) ∝ τ(α)(ζ(dh)). Although the dynamics of the asymmetric-charge model is seemingly more heterogeneous than that of the symmetric-charge model, the exponent is found to be similar, ζ(dh) ≈ 1.2, for all the models studied in this work. The same scaling relation is found regardless of interactions, i.e., with or without Coulomb interaction, and it holds even when the length scale of structural relaxation is long enough to become the Fickian diffusion. This fact indicates that τ(dh) is a distinctive time scale from τ(α), and the dynamic heterogeneity is mainly affected by the short-range interaction and the molecular structure.

The Effect of Phenyl Substitutions on Microstructures and Dynamics of Tetraalkylphosphonium Bis(trifluoro‐ methylsulfonyl)imide Ionic Liquids

“…In addition to microstructural quantities between neighboring ion species, the rotational dynamics of hexyl chains and phenyl groups in cations have been investigated by examining rotations of representative vectors ê fixed in cation frameworks . Hexyl chains are described by a vector connecting two terminal carbon atoms (one is labelled as CP and the other one is labelled as CT in Figure ), and phenyl rings are depicted by their normal vectors, respectively.…”

Heterogeneous dynamics and its length scale in simple ionic liquid models: a computational study

“…Fig.4(c) shows the ratio of τ p and τ e , which shows similar divergent behavior of Dτ at low temperatures. 6 In addition, there are power law relations, τ e ∼ τ ν p , between the two physical time scales as it can be seen in Fig.4 . The α-relaxation time, τ α , is defined at which Q(τ α )/N = 1/e.…”

“…The results is consistent with the previous study that demonstrates the cation moves faster than the anion. 6 For the ACM case, the distribution of the anion is more heterogeneous than the cation. This opposite result comes from the different charge distribution of the cation.…”

“…While the lifetime of the heterogeneous dynamics is calculated using the three-time correlation functions, the length scale was not examined. In the present study, we further investigate the heterogeneous This phenomenon, typically called dynamic heterogeneity, has been investigated through diverse theoretical 6,10,[22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37] and experimental studies.…”

“…In the previous study, Ref 6, we examined the structural and dynamic properties of RTILs thoroughly. Especially, the relation between the time scale of the heterogeneous dynamics and the length scale of the structural relaxation were studied.…”