Fragility, Stokes-Einstein violation, and correlated local excitations in a coarse-grained model of an ionic liquid

TL;DR

This study uses molecular dynamics simulations to analyze the dynamics of a coarse-grained ionic liquid model, revealing fragile behavior, local excitations, and Stokes-Einstein violation at near-room temperatures.

Contribution

It demonstrates the emergence of correlated local excitations and Stokes-Einstein violation in a coarse-grained ionic liquid model, linking microscopic dynamics to macroscopic properties.

Findings

Model exhibits fragile supercooled liquid behavior.

Local excitations drive structural relaxation.

Correlation of excitations increases at lower temperatures.

Abstract

Dynamics of a coarse-grained model for the room-temperature ionic liquid, 1-ethyl-3-methylimidazolium hexafluorophosphate, couched in the united-atom site representation are studied via molecular dynamics simulations. The dynamically heterogeneous behavior of the model resembles that of fragile supercooled liquids. At or close to room temperature, the model ionic liquid exhibits slow dynamics, characterized by nonexponential structural relaxation and subdiffusive behavior. The structural relaxation time, closely related to the viscosity, shows a super-Arrhenius behavior. Local excitations, defined as displacement of an ion exceeding a threshold distance, are found to be mainly responsible for structural relaxation in the alternating structure of cations and anions. As the temperature is lowered, excitations become progressively more correlated. This results in the decoupling of exchange and persistence times, reflecting a violation of the Stokes-Einstein relation.