Modeling Carbon Dioxide Vibrational Frequencies in Ionic Liquids: IV. Temperature Dependence

Area of Science:

• Physical Chemistry
• Materials Science
• Spectroscopy

Background:

• Previous research investigated carbon dioxide (CO2) vibrational spectroscopy in ionic liquids (ILs) at ambient conditions.
• Ionic liquids (ILs) offer unique solvent properties for gas dissolution and separation.
• Understanding gas behavior in ILs is crucial for developing advanced separation technologies.

Purpose of the Study:

• To investigate the temperature dependence of CO2 structure, dynamics, and thermodynamics in 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]).
• To analyze the CO2 solvent cage breakdown and reorganization processes within the IL.

Main Methods:

• Simulated infrared absorption spectra of the CO2 asymmetric stretch mode at various temperatures using spectroscopic mapping techniques.
• Utilized structural correlation functions to analyze CO2 solvent cage dynamics.
• Calculated caging times and activation parameters for solvent cage reorganization.

Main Results:

• Simulated spectra align with experimental findings for CO2 in [bmim][PF6] across different temperatures.
• Thermodynamics of CO2 solvent cage breakdown were elucidated.
• Enthalpy and entropy of activation for solvent cage reorganization were determined to be 6.9 and 7.6 (kcal/mol)/K, respectively.
• Short-time CO2 dynamics were found to be temperature-independent, while long-time dynamics exhibited strong temperature sensitivity.

Conclusions:

• The study provides insights into the temperature-dependent behavior of CO2 within ionic liquids.
• Findings reveal distinct temperature effects on short- and long-time CO2 dynamics, influencing solvent cage interactions.
• The determined thermodynamic parameters are comparable to those governing spectral, orientational, and translational diffusion of CO2.