Exploring the Relation among Free Volume, Electrostatic Interaction, and Side Chain Flexibility of Ether-Functionalized Ionic Liquids by Molecular Dynamics Simulations

Alkoxy chain effect on the viscosity of ionic liquids has been extensively studied by experiments in order to disclose the relationship between viscosity and structure. A free-volume model was used to interpret the mechanism of the alkoxy chain effect, suggesting that flexible alkoxy chains can improve the viscosity of ionic liquids owing to the increase of free volume. Computational studies revealed that the improvement of viscosity arises from the less effective packing between flexible alkoxy chains as well as the reduction in electrostatic interaction between cations and anions. However, few computational studies have been undertaken to address the relation among free volume, electrostatic interaction, and side chain flexibility of ether-functionalized ionic liquids. In this work, we develop a general amber force field (GAFF) for ether-functionalized ionic liquids by following the traditional procedure of parameterizing GAFF for ionic liquids. Our work demonstrates that the alkoxy chain effect on the viscosity of ionic liquids can be qualitatively captured by GAFF without any optimizations. Analysis of electrostatic interactions between cations and anions in molecular dynamics (MD) simulations advances our understanding of the alkoxy chain effect from various aspects. Finally, we propose for the first time that the total free volume can further be divided into two portions, which are associated with electrostatic interactions between cations and anions.