Dielectric relaxation in ionic liquid/dipolar solvent binary mixtures: A semi-molecular theory

Area of Science:

• Physical Chemistry
• Materials Science
• Dielectric Phenomena

Background:

• Dielectric relaxation (DR) in binary mixtures of ionic liquids (ILs) and dipolar solvents is crucial for understanding molecular dynamics.
• Existing models often struggle to accurately capture the complex interplay between ion translation, rotation, and solvent effects on DR.
• Ionic liquids present unique challenges due to their complex structures and interactions with common solvents.

Purpose of the Study:

• To develop and validate a semi-molecular theory for dielectric relaxation in binary mixtures of ionic liquids with dipolar solvents.
• To investigate the influence of ion translation and rotation on DR and conductivity relaxation time scales.
• To explore the applicability of separate and effective medium approaches for modeling these mixtures.

Main Methods:

• Development of semi-molecular expressions for diffusive DR times, incorporating orientational correlations, dynamic structure factors, and ion translation.
• Application of two models: separate medium and effective medium approaches.
• Comparison of theoretical predictions with experimental DR data for [Bmim][BF4]/water and [Bmim][BF4]/acetonitrile mixtures, and guanidinium chloride solutions.

Main Results:

• The theory shows good agreement with experimental DR times at low ionic liquid mole fractions (x(IL)).
• At higher x(IL), the theory overestimates relaxation times, with deviations increasing significantly for neat ILs.
• The discrepancy is attributed to the assumption of full molecular volume for rotating dipoles, suggesting a reduction in effective rotational volume at high IL concentrations.

Conclusions:

• The semi-molecular theory provides valuable insights into DR mechanisms in IL-solvent mixtures, particularly at low IL concentrations.
• The concept of an effective rotational volume, which decreases with increasing IL concentration, is crucial for accurate modeling.
• The findings suggest a crossover in orientational relaxation mechanisms from viscosity-coupled diffusion to large-angle jumps as IL concentration increases.