How do external forces related to mass and charge affect the structures and dynamics of an ionic liquid?

Ionic liquids (ILs) are novel promising materials widely used in various fields. Their structures and properties can be tuned by means of external perturbations, thus further broadening their applications. Herein, forces proportional to atomic mass (mass-related field) and atomic charge (electric field) are applied in molecular dynamics simulations to the IL 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide to investigate the origin of the resulting changes in structures and dynamics. The results show that both electric and mass-related fields cause the ion cages to expand and deform, eventually leading to their breakdown to produce a transformation of ILs from the cage structure to a channel-like structure, which results in faster self-diffusion of ions in the directions of the applied force and to a lesser extent other directions. Further comparison of electric and mass-related fields demonstrates that only the electric fields reorientate cations to produce a hydrodynamically favored conformation in the force direction, which shows faster diffusion. The cis isomer of the anion is preferred in the presence of the electric fields, whereas applying the forces proportional to mass does not change the anion conformer equilibrium significantly. The results presented in this work aid in the understanding of how ions adjust their structures to adapt to external perturbations and facilitate the application of ILs as electrolytes. (c) 2022 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/5.0091322.

Automated summary

This study investigated the structure and dynamics changes of ionic liquids under mass-related and electric fields using molecular dynamics simulations. The results showed that both fields caused the expansion and deformation of ion cages, leading to a transformation of the structure and faster self-diffusion in the direction of the applied force. Electric fields reoriented cations and promoted faster diffusion, while mass-related fields had a lesser effect on anion conformer equilibrium. These findings contribute to understanding how ions adapt to external perturbations and the application of ionic liquids as electrolytes.