Size-Polydisperse Model Ionic Liquid in Bulk.

Area of Science:

• Physical Chemistry
• Materials Science
• Computational Chemistry

Background:

• Ionic liquids (ILs) are tunable solvents with diverse applications.
• Understanding the impact of structural variations, like size polydispersity, is crucial for designing ILs with specific properties.
• Previous studies often focused on monodisperse systems, leaving the effects of size distribution less explored.

Purpose of the Study:

• To investigate the static and dynamic properties of a size-polydisperse ionic liquid model.
• To determine how anion size distribution (polydispersity index, δ) influences thermodynamic transitions, ion ordering, and screening behavior.
• To elucidate the relationship between polydispersity and the dynamic properties, such as relaxation time and diffusion coefficients.

Main Methods:

• Molecular dynamics simulations were employed to model an ionic liquid with size-polydisperse anions and fixed-size cations.
• Systematic variation of the polydispersity index (δ) to observe property changes.
• Analysis of thermodynamic transition temperatures, radial distribution functions (RDFs), and mean-square displacement (MSD) to characterize static and dynamic properties.

Main Results:

• A nonmonotonic dependence of transition temperatures and screening length on the polydispersity index (δ) was observed.
• Cation spatial ordering was affected by varying δ, while anion ordering remained largely unchanged.
• Both inertial and diffusive regimes were present in ion motion, but relaxation times increased (diffusion decreased) with increasing δ.

Conclusions:

• Anion size polydispersity significantly influences the macroscopic properties of ionic liquids.
• The observed nonmonotonic behavior highlights complex structure-property relationships in polydisperse ILs.
• Increased polydispersity leads to slower dynamics in ionic liquids, contrasting with neutral systems.