Invariant dynamics in a united-atom model of an ionic liquid

Area of Science:

• Condensed matter physics
• Computational chemistry
• Materials science

Background:

• Ionic liquids (ILs) exhibit complex dynamics.
• Understanding ILs requires simplified models for efficient simulation.
• Reduced units can reveal universal dynamic behaviors.

Purpose of the Study:

• Investigate invariant dynamic curves (isodynes) in ionic liquid phase diagrams.
• Determine the extent of microscopic dynamics invariance in reduced form.
• Identify conditions supporting coarse-grained descriptions of ILs.

Main Methods:

• Utilized a united-atom model for 1-butyl-1-methylpyrrolidinium bis(trifluoromethyl)sulfonylamide.
• Performed simulations to identify isodynes based on coinciding reduced viscosity and self-diffusion contours.
• Analyzed invariance of mean-squared displacement, shear-stress autocorrelation, and rotational correlation functions.

Main Results:

• Isodynes were identified as contours of reduced shear viscosity and self-diffusion.
• Most investigated dynamic properties, including mean-squared displacement and correlation functions, showed good invariance along isodynes.
• Anion rotation about its long axis was a notable exception to the observed invariance.

Conclusions:

• Invariant dynamics along isodynes support coarse-grained modeling of ionic liquids.
• Simplification of the potential energy landscape by removing microscopic degrees of freedom facilitates isodyne existence.
• Isodynes offer a pathway to understanding universal dynamic behaviors in ILs.