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

TL;DR

This computational study investigates how dynamic heterogeneity and its length scale evolve in simple ionic liquid models with different charge distributions, revealing charge-induced crossover behaviors and power-law relations at low temperatures.

Contribution

The paper introduces a detailed numerical analysis of dynamic heterogeneity in ionic liquid models with varied charge distributions, highlighting charge effects on heterogeneity and crossover phenomena.

Findings

Dynamic heterogeneity increases as temperature decreases.

Charged models exhibit unique crossover behaviors not seen in uncharged models.

Power-law relation between time and length scales at low temperatures.

Abstract

We numerically investigate the dynamic heterogeneity and its length scale found in the coarse-grained ionic liquid model systems. In our ionic liquid model systems, cations are modeled as dimers with positive charge, while anions are modeled as monomers with negative charge, respectively. To study the effect of the charge distributions on the cations, two ionic liquid models with different charge distributions are used and the model with neutral charge is also considered as a counterpart. To reveal the heterogeneous dynamics in the model systems, we examine spatial distributions of displacement and time distributions of exchange and persistence times. All the models show significant increase of the dynamic heterogeneity as the temperature is lowered. The dynamic heterogeneity is quantified via the well-known four-point susceptibility which measures the fluctuation of a time correlation function. The dynamic correlation length is calculated by fitting the dynamic structure factor, S4(k,t), with Ornstein-Zernike form at the time scale at which the dynamic heterogeneity reaches the maximum value. Obtained time and length scales exhibit a power law relation at the low temperatures, similar to various supercooled liquid models. Especially, the charged model systems show unusual crossover behaviors which are not observed in the uncharged model system. We ascribe the crossover behavior to the enhanced cage effect caused by charges on the particles.