What Determines CO₂ Solubility in Ionic Liquids? A Molecular Simulation Study.

Area of Science:

• Physical Chemistry
• Materials Science
• Chemical Engineering

Background:

• Ionic liquids (ILs) are promising solvents for CO2 capture.
• Understanding the molecular mechanisms governing CO2 solubility in ILs is crucial for designing efficient capture systems.

Purpose of the Study:

• To identify the key factors determining CO2 solubility in various ionic liquids (ILs).
• To elucidate the relationship between IL structure, ion interactions, and CO2 absorption capacity.

Main Methods:

• Molecular dynamics (MD) simulations were performed on 10 different pure and CO2-saturated ionic liquids.
• Simulations utilized an empirical force field based on liquid-phase charges.
• Analysis focused on partial molar volume of CO2, radial distribution functions, and unoccupied space.

Main Results:

• CO2 insertion requires slight ion displacements, with partial molar volumes of CO2 ranging from 30 to 40 cm³/mol.
• CO2 absorption does not alter the overall ion organization in ILs.
• CO2 solubility strongly correlates with the ratio of unoccupied space in pure ILs, not direct CO2-ion interactions.
• Weak ion cohesion, indicated by larger ion distances and more unoccupied space, facilitates CO2 absorption.
• Ion cohesion is primarily governed by ion density (ion sizes) and secondarily by local electrostatic interactions.

Conclusions:

• The pre-existing unoccupied space in ionic liquids is the primary determinant of CO2 solubility.
• Weak ion cohesion, driven by ion size and density, enhances CO2 absorption by creating more accessible empty volume.
• MD simulations provide a consistent framework for understanding CO2 solubility across diverse ionic liquids.